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Authors: Hideki Yoshida and Makoto Matsuoka.
Molecular Plant (2024)
Excerpts: "For example, brassinosteroids (BRs) positively affect plant growth and grain size but negatively regulate the grain number and plant density (Figure 1, top left; Deveshwar et al., 2020; Hwang et al., 2021). Therefore, a thorough understanding of the trade-off functions and regulatory mechanisms of phytohormones is a promising avenue for agricultural improvement."
"Zhang et al. (2024) revealed the molecular regulation of panicle branching by BRs through the cloning and functional analysis of a long-puzzling gene for the clustered spikelet (CL) phenotype. These observations demonstrated that the proper regulation of BR function resulted in higher rice yield."
"Zhang et al. (2024) successfully cloned a causal gene for CL using an elegant genomic analysis with both short- and long-read sequencing techniques. The CL trait is associated with complex structural variations in chromosomes that activate the expression of the BR catabolic gene BRASSINOSTEROID-DEFICIENT DWARF3 (BRD3) in secondary branch meristems."
"Zhang et al. (2024) also showed that introducing the genetic modification of the BRD3 promoter into modern rice varieties resulted in a higher rice yield with a high grain number due to CL and no other defects in field tests."
Authors: Suzanne Daignan Fornier, Antoinette Keita and François-Didier Boyer.
ChemBioChem (2024)
Abstract: "Today, the use of artificial pesticides is questionable and the adaptation to global warming is a necessity. The promotion of favorable natural interactions in the rhizosphere offers interesting perspectives for changing the type of agriculture. Strigolactones (SLs), the latest class of phytohormones to be discovered, are also chemical mediators in the rhizosphere. We present in this review the diversity of natural SLs, their analogs, mimics, and probes essential for the biological studies of this class of compounds. Their biosynthesis and access by organic synthesis are highlighted especially concerning noncanonical SLs, the more recently discovered natural SLs. Organic synthesis of analogs, stable isotope-labeled standards, mimics, and probes are also reviewed here. In the last part, the knowledge about the SL perception is described as well as the different inhibitors of SL receptors that have been developed."
Authors: Lin Du, Xingjia Li, Yimin Ding, Dengke Ma, Chunxin Yu, Hanqing Zhao, Ye Wang, Ziyan Liu and Liusheng Duan.
International Journal of Molecular Sciences (2024)
Abstract: "Strigolactones (SLs) are plant hormones that regulate several key agronomic traits, including shoot branching, leaf senescence, and stress tolerance. The artificial regulation of SL biosynthesis and signaling has been considered as a potent strategy in regulating plant architecture and combatting the infection of parasitic weeds to help improve crop yield. DL1b is a previously reported SL receptor inhibitor molecule that significantly promotes shoot branching. Here, we synthesized 18 novel compounds based on the structure of DL1b. We performed rice tillering activity assay and selected a novel small molecule, C6, as a candidate SL receptor inhibitor. In vitro bioassays demonstrated that C6 possesses various regulatory functions as an SL inhibitor, including inhibiting germination of the root parasitic seeds Phelipanche aegyptiaca, delaying leaf senescence and promoting hypocotyl elongation of Arabidopsis. ITC analysis and molecular docking experiments further confirmed that C6 can interact with SL receptor proteins, thereby interfering with the binding of SL to its receptor. Therefore, C6 is considered a novel SL receptor inhibitor with potential applications in plant architecture control and prevention of root parasitic weed infestation."
Authors: Jian You Wang, Guan-Ting Erica Chen, Justine Braguy and Salim Al-Babili.
Trends in Plant Science (2024)
Highlights: Strigolactones (SLs) are structurally diverse and divided into canonical and non-canonical subgroups. SLs are generally considered as plant hormones, best known for inhibiting shoot branching/tillering. SLs are rhizospheric signals important for arbuscular mycorrhizal symbiosis, which may be their ancestral function that is conserved from liverworts to flowering plants. Recent results have revealed functional specificity, indicating that canonical SLs are not the tillering/branching inhibitory hormone in rice or tomato. Increasing the content of 4-deoxyorobanchol in rice by interrupting its hydroxylation affects root, shoot, and panicle growth, suggesting that this canonical SL has hormonal functions. Reducing the levels of canonical SLs by genome editing or applying specific inhibitors is a promising strategy for reducing Striga parasitism.
Abstract: "Strigolactones (SLs) act as regulators of plant architecture as well as signals in rhizospheric communications. Reduced availability of minerals, particularly phosphorus, leads to an increase in the formation and release of SLs that enable adaptation of root and shoot architecture to nutrient limitation and, simultaneously, attract arbuscular mycorrhizal fungi (AMF) for establishing beneficial symbiosis. Based on their chemical structure, SLs are designated as either canonical or non-canonical; however, the question of whether the two classes are also distinguished in their biological functions remained largely elusive until recently. In this review we summarize the latest advances in SL biosynthesis and highlight new findings pointing to rhizospheric signaling as the major function of canonical SLs."
Authors: Ruijuan Yang, Zhenying Wu, Ying Sun, Yuchen Liu, Yuqing Hang, Min Liu, Yajun Liu, Xiaoshan Wang, Wenwen Liu and Chunxiang Fu.
The Plant Journal (2024)
Significance Statement: It is well known that excessive grass tiller number negatively impacts culm development. Suppressing CYTOKININ OXIDASE/DEHYDROGENASE4 in miR156-overexpressing background switchgrass plants reduced the negative impacts of miR156 on culm development while leaving tiller number unaffected. Finally, the double transgenic plants yielded 110% more biomass than the wild type. Thus, the trade-off between tiller number and culm development widely observed in grasses can be successfully altered through precise manipulation of the miR156-SPL2-CKX4 module."
Abstract: "Culm development in grasses can be controlled by both miR156 and cytokinin. However, the crosstalk between the miR156-SPL module and the cytokinin metabolic pathway remains largely unknown. Here, we found CYTOKININ OXIDASE/DEHYDROGENASE4 (PvCKX4) plays a negative regulatory role in culm development of the bioenergy grass Panicum virgatum (switchgrass). Overexpression of PvCKX4 in switchgrass reduced the internode diameter and length without affecting tiller number. Interestingly, we also found that PvCKX4 was always upregulated in miR156 overexpressing (miR156OE) transgenic switchgrass lines. Additionally, upregulation of either miR156 or PvCKX4 in switchgrass reduced the content of isopentenyl adenine (iP) without affecting trans-zeatin (tZ) accumulation. It is consistent with the evidence that the recombinant PvCKX4 protein exhibited much higher catalytic activity against iP than tZ in vitro. Furthermore, our results showed that miR156-targeted SPL2 bound directly to the promoter of PvCKX4 to repress its expression. Thus, alleviating the SPL2-mediated transcriptional repression of PvCKX4 through miR156 overexpression resulted in a significant increase in cytokinin degradation and impaired culm development in switchgrass. On the contrary, suppressing PvCKX4 in miR156OE transgenic plants restored iP content, internode diameter, and length to wild-type levels. Most strikingly, the double transgenic lines retained the same increased tiller numbers as the miR156OE transgenic line, which yielded more biomass than the wild type. These findings indicate that the miR156-SPL module can control culm development through transcriptional repression of PvCKX4 in switchgrass, which provides a promising target for precise design of shoot architecture to yield more biomass from grasses."
Source: Agroscope Newsletter (15.02.2024)
Excerpts: "Agroscope has been granted approval by the Federal Office for the Environment for a field trial with spring barley. The focus is on a barley gene that has been disabled by new breeding techniques. The trial, which will be launched in spring 2024 on the Protected Site in Zurich-Reckenholz and will run for three years, aims to determine whether yields can be increased in this manner. The CKX2 gene is involved in the regulation of seed formation. Disabling this gene by means of a new breeding methods (CRISPR/Cas9 genome editing) brings about increased yields in rice and oilseed rape (see, below, ‘From rice to barley’).
"From rice to barley - Crop yield formation is complex and involves many different genes. However, Japanese researchers have discovered that the mutation of the CKX2 gene in rice has an unexpectedly significant effect on yield. Results were so convincing that they are now used in rice breeding. Research results show that genes corresponding to the CKX2 gene from rice also play a role e.g. in oilseed-rape yield formation. Therefore, it is reasonable to study this effect in further crops. In the best-case scenario, at the end of these trials on the Protected Site it will be possible to issue a recommendation as to whether breeders should disable one or both CKX2 genes in order to boost yields. What is certain, however, is that important information will be provided on the function of the CKX2 genes in barley – and hence further pieces of the puzzle will be available for a better understanding of yield formation."
Authors: Shining Han, Weijie Yue, Anar Bao, Tongtong Jiao, Yanxi Liu, Hua Zeng, Kai Song, Ming Wu and Liquan Guo.
Functional & Integrative Genomics (2024)
Abstract: "The COP9 signalosome (CSN) is a conserved protein complex found in higher eukaryotes, consisting of eight subunits, and it plays a crucial role in regulating various processes of plant growth and development. Among these subunits, CSN2 is one of the most conserved components within the COP9 signalosome complex. Despite its prior identification in other species, its specific function in Oryza sativa L. (Rice) has remained poorly understood. In this study, we investigated the role of CSN2 in rice using gene editing CRISPR/Cas9 technology and overexpression techniques. We created two types of mutants: the oscsn2 mutant and the OsCSN2-OE mutant, both in the background of rice, and also generated point mutants of OsCSN2 (OsCSN2K64E, OsCSN2K67E, OsCSN2K71E and OsCSN2K104E) to further explore the regulatory function of OsCSN2. Phenotypic observation and gene expression analysis were conducted on plants from the generated mutants, tracking their growth from the seedling to the heading stages. The results showed that the loss and modification of OsCSN2 had limited effects on plant growth and development during the early stages of both the wild-type and mutant plants. However, as the plants grew to 60 days, significant differences emerged. The OsCSN2 point mutants exhibited increased tillering compared to the OsCSN2-OE mutant plants, which were already at the tillering stage. On the other hand, the OsCSN2 point mutant had already progressed to the heading and flowering stages, with the shorter plants. These results, along with functional predictions of the OsCSN2 protein, indicated that changes in the 64th, 67th, 71st, and 104th amino acids of OsCSN2 affected its ubiquitination site, influencing the ubiquitination function of CSN and consequently impacting the degradation of the DELLA protein SLR1. Taken together, it can be speculated that OsCSN2 plays a key role in GA and BR pathways by influencing the functional regulation of the transcription factor SLR1 in CSN, thereby affecting the growth and development of rice and the number of tillers."
Authors: Nana Fan, Liantai Su, Aimin Lv, Wuwu Wen, Li Gao, Xiangkai You, Peng Zhou and Yuan An.
Plant Physiology (2024)
One-sentence summary: A pectin acetylesterase participates in indole acetic acid signaling pathways and affects shoot branching in alfalfa.
Abstract: "Shoot branching is an important biological trait affecting alfalfa (Medicago sativa L.) production, but its development is complicated and the mechanism is not fully clear. In the present study, pectin acetylesterase 12 (MsPAE12) and NAM/ATAF/CUC-domain transcription factor gene (MsNAC73) were isolated from alfalfa. MsPAE12 was highly expressed in shoot apexes, and MsNAC73 was found to be a key transcriptional repressor of MsPAE12 by directly binding to SA and JA elements in the MsPAE12 promoter. The biological functions of MsPAE12 and MsNAC73 were studied through overexpression (OE) and down-expression (RNAi) of the two genes in alfalfa. The numbers of shoot branches increased in MsPAE12-OE lines but decreased in MsPAE12-RNAi and MsNAC73-OE plants, which was negatively related to their indole-3-acetic acid (IAA) accumulation in shoot apexes. Furthermore, the contents of acetic acid (AA) in shoot apexes decreased in MsPAE12-OE plants but increased in MsPAE12-RNAi and MsNAC73-OE plants. The changes of AA contents were positively related to the expression of TRYPTOPHAN AMINOTRANSFERASE 1 (MsTAA1), TRYPTOPHAN AMINOTRANSFERASE-RELATED 2 (MsTAR2) and YUCCA flavin monooxygenase (MsYUCC4) and the contents of tryptophan (Trp), indole-3-pyruvic acid (IPA) and IAA in shoot apexes of MsPAE12-OE, MsPAE12-RNAi and MsNAC73-OE plants. Exogenous application of AA to WT and MsPAE12-OE plants increased Trp, IPA and IAA contents and decreased branch number. Exogenous IAA suppressed shoot branching in MsPAE12-OE plants, but exogenous IAA inhibitors increased shoot branching in MsPAE12-RNAi plants. These results indicate that the MsNAC73-MsPAE12 module regulates auxin-modulated shoot branching via affecting acetic acid accumulation in shoot apexes of alfalfa."
Authors: Chen Zhang, Fanhao Wang, Peng Jiao, Jiaqi Liu, Honglin Zhang, Siyan Liu, Shuyan Guan and Yiyong Ma.
International Journal of Molecular Sciences (2024)
Abstract: "Strigolactones (SLs) represent a recently identified class of plant hormones that are crucial for plant tillering and mycorrhizal symbiosis. The D14 gene, an essential receptor within the SLs signaling pathway, has been well-examined in crops, like rice (Oryza sativa L.) and Arabidopsis (Arabidopsis thaliana L.), yet the research on its influence in maize (Zea mays L.) remains scarce. This study successfully clones and establishes Arabidopsis D14 gene overexpression lines (OE lines). When compared with the wild type (WT), the OE lines exhibited significantly longer primary roots during germination. By seven weeks of age, these lines showed reductions in plant height and tillering, alongside slight decreases in rosette and leaf sizes, coupled with early aging symptoms. Fluorescence-based quantitative assays indicated notable hormonal fluctuations in OE lines versus the WT, implying that D14 overexpression disrupts plant hormonal homeostasis. The OE lines, exposed to cold, drought, and sodium chloride stressors during germination, displayed an especially pronounced resistance to drought. The drought resistance of OE lines, as evident from dehydration–rehydration assays, outmatched that of the WT lines. Additionally, under drought conditions, the OE lines accumulated less reactive oxygen species (ROS) as revealed by the assessment of the related physiological and biochemical parameters. Upon confronting the pathogens Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), post-infection, fluorescence quantitative investigations showed a significant boost in the salicylic acid (SA)-related gene expression in OE lines compared to their WT counterparts. Overall, our findings designate the SL receptor D14 as a key upregulator of drought tolerance and a regulator in the biotic stress response, thereby advancing our understanding of the maize SL signaling pathway by elucidating the function of the pivotal D14 gene."
Authors: Yilong Yao, Denghao Xiang, Nai Wu, Yao Wang, Yu Chen, Yang Yuan, Ying Ye, Dan Hu, Chang Zheng, Yu Yan, Qingya Lv, Xiaokai Li, Guoxing Chen, Honghong Hu, Haiyan Xiong, Shaobing Peng and Lizhong Xiong.
Molecular Plant (2023)
Abstract: "Rice ratooning, the fast outgrowth of dormant buds on stubble, is an important cropping practice in rice production. However, the low ratooning ability (RA) of most rice varieties restricts the application of this cost-efficient system, and the genetic basis of RA remains unknown. In this study, we dissected the genetic architecture of RA by a genome-wide association study in a natural rice population. Rice ratooning ability 3 (RRA3), encoding a hitherto not characterized nucleoredoxin involved in reduction of disulfide bonds, was identified as the causal gene of a major locus controlling RA. Overexpression of RRA3 in rice significantly accelerated leaf senescence and reduced RA, whereas knockout of RRA3 significantly delayed leaf senescence and increased RA and ratoon yield. We demonstrated that RRA3 interacts with Oryza sativa histidine kinase 4 (OHK4), a cytokinin receptor, and inhibits the dimerization of OHK4 through disulfide bond reduction. This inhibition ultimately led to decreased cytokinin signaling and reduced RA. In addition, variations in the RRA3 promoter were identified to be associated with RA. Introgression of a superior haplotype with weak expression of RRA3 into the elite rice variety Guichao 2 significantly increased RA and ratoon yield by 23.8%. Collectively, this study not only uncovers an undocumented regulatory mechanism of cytokinin signaling through de-dimerization of a histidine kinase receptor—but also provides an eximious gene with promising value for ratoon rice breeding."
Authors: Xue Ha, Jinqing Zhang, Fenqi Chen, Yajun Li and Huiling Ma.
Agronomy (2023)
Abstract: "Poa pratensis is widely distributed in cold temperate regions and can be used as a species for stress restoration and as a forage for livestock. Studying the genetic characteristics of tillering occurrence in bluegrass provides a theoretical basis for studying plant yield formation, environmental adaptation, and improving survival competitiveness. The regulating effects of endogenous hormone IAA content and the expression of related genes ARF1, ARF12, ARF14, ZT content and the expression of related genes CKX2, CKX3, CKX4, SL content and the expression of related genes D14-like, D14.1-like and D14 in wild Kentucky bluegrass were investigated. Kentucky bluegrass from Sunan and Qingshui was used to evaluate the influence of hormone and gene expression on tillering behavior. Endogenous hormone contents and expression levels of related genes in stems and roots of both materials were measured at prophase, peak, and anaphase of tillering. The results showed that among the three materials, the Sunan material had a better tillering ability for Poa pratensis, while the Qingshui material had poorer tillering ability. The downregulation of CKX2, CKX3, and CKX4 gene expression levels promotes the synthesis of ZT, thereby improving the tillering ability of the germplasm. Upregulation of ARF14, D14, and D14.1-like gene expression levels enhances the synthesis of IAA and SL, thereby inhibiting tillering. More importantly, the interaction between hormones affects the tillering ability of bluegrass, and high levels of ZT/IAA, ZT/SL, and ZT/(IAA+SL) values promote tillering. In summary, this study reveals the mechanism by which hormones regulate the occurrence of tillering in Kentucky bluegrass, providing a theoretical basis for understanding the genetic characteristics of plant type, effectively regulating tillering, studying yield development, environmental adaptation, and improving survival rate."
Authors: José A. Abelenda and Javier Barrero-Gil.
Journal of Experimental Botany (2023)
Abstract: "Despite its prominent role as a stress hormone and its related importance for agriculture, abscisic acid (ABA) molecular signaling is poorly characterized in crops such as potato (Solanum tuberosum). Potato is the world’s third most produced food crop and the most prominent non-cereal in terms of production and consumption (FAOSTAT, 2021). Its importance as a staple food is increasing worldwide. With the immediate need for stress-tolerant and more resilient plants due to changing climate patterns, research on ABA functions in potato will be essential to maintain productivity. Moreover, multiple developmental and physiological aspects of ABA unrelated to abiotic stresses are often overlooked and are difficult to study in plant models. Investigating putative ABA signaling players in potato plants, Liu et al. (2023) have deciphered the negative role of the phosphatase StHAB1 on ABA perception and potato drought stress responses. In addition, the authors provide conclusive evidence that ABA signaling is required for maintaining dormancy in aerial lateral buds. It has long been known that ABA stimulates tuber formation, but the molecular details of such mechanisms remain unknown. It was recently shown that the development of aerial lateral buds leads to a reduction in underground tuber production (Nicolas et al., 2022). Therefore, the ABA-mediated blockage of lateral bud outgrowth reported by Liu et al. (2023) may provide a molecular explanation of the effects of ABA on tuber formation. The transgenic potato lines created in this study offer excellent genetic tools for further study of these connections and have potential biotechnological implications. This work underscores the importance of translational research from models to crops and how it is key not to circumscribe observations to obvious outputs in functional analysis in non-model plants."
Authors: Yundong Yuan, Said Khourchi, Shujia Li, Yanfang Du, and Pierre Delaplace.
Plants (2023)
Abstract: "Shoot branching is a complex and tightly regulated developmental process that is essential for determining plant architecture and crop yields. The outgrowth of tiller buds is a crucial step in shoot branching, and it is influenced by a variety of internal and external cues. This review provides an extensive overview of the genetic, plant hormonal, and environmental factors that regulate shoot branching in several plant species, including rice, Arabidopsis, tomato, and wheat. We especially highlight the central role of TEOSINTE BRANCHED 1 (TB1), a key gene in orchestrating bud outgrowth. In addition, we discuss how the phytohormones cytokinins, strigolactones, and auxin interact to regulate tillering/branching. We also shed light on the involvement of sugar, an integral component of plant development, which can impact bud outgrowth in both trophic and signaling ways. Finally, we emphasize the substantial influence of environmental factors, such as light, temperature, water availability, biotic stresses, and nutrients, on shoot branching. In summary, this review offers a comprehensive evaluation of the multifaced regulatory mechanisms that underpin shoot branching and highlights the adaptable nature of plants to survive and persist in fluctuating environmental conditions."
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Authors: Wanmin Wang, Zizhao Xie, Yuanyuan Wu, Ying Sun, Chenghang Zhan, Liang Jin and Junli Huang.
Environmental and Experimental Botany (2024)
Highlights: • OsJAZ6 modulates rice tillering and drought response by integrating JA with GA signaling. • OsJAZ6 controls the tiller bud growth but not formation. • OsJAZ6 interacts with SLR1 to promote its degradation, which further destabilizes MOC1. • OsJAZ6 and SLR1 have opposite functions in regulating rice tiller bud growth and drought tolerance.
Abstract: "Jasmonic acid (JA) plays crucial functions during plant growth and stress response, but its roles and regulatory mechanism in plant branching remain largely unknown. Rice basal branching (tillering) is an essential agronomic trait that affects crop production. Here, we report that OsJAZ6, the repressor of JA signaling, negatively modulates rice tillering and drought stress tolerance. Loss-of-function mutants of OsJAZ6 exhibit a significant increase in tiller number, while OsJAZ6ΔJas-overexpression lines produce fewer tillers than wild-type plants. Further investigations show that function loss of OsJAZ6 promotes the tiller bud growth rather than formation. Mechanistic studies show that OsJAZ6 interacts with rice DELLA/SLR1 (SLENDER RICE 1), a transcription repressor of gibberellin (GA) signaling, and the interaction promotes SLR1 degradation, which further facilitates the degradation of rice tillering regulator MOC1 (MONOCULM 1), thereby inhibiting the tiller bud growth. In agreement, the slr1 mutant exhibits fewer tillers than wild type. Consistently, application of JA promotes the growth of tiller bud and thus increases the tiller number, while GA treatment results in opposite result. Meanwhile, osjaz6 mutants display enhanced drought tolerance, coupled with increased JA sensitivity, while the slr1 mutant shows the reverse behavior. Collectively, our data demonstrate that OsJAZ6 negatively modulates rice tillering as well as drought stress tolerance by destabilizing SLR1 protein. Our data shed light on the regulatory mechanism of controlling the tiller development and drought stress response in rice by the JA-OsJAZ6-SLR1 module."
Authors: Jiacai Chen, Liu Liu, Guanghui Wang, Guangxin Chen, Xiaofeng Liu, Min Li, Lijie Han, Weiyuan Song, Shaoyun Wang, Chuang Li, Zhongyi Wang, Yuxiang Huang, Chaoheng Gu, Zhengan Yang, Zhaoyang Zhou, Jianyu Zhao and Xiaolan Zhang.
The Plant Cell (2024)
One-sentence summary: CsAGL16 positively regulates axillary bud outgrowth in cucumber by directly promoting CsCYP707A4-mediated ABA catabolism, and CsGRF1 interacts with CsAGL16 and antagonizes the CsAGL16-mediated CsCYP707A4 activation.
Abstract: "Lateral branches are important components of shoot architecture and directly affect crop yield and production cost. Although sporadic studies have implicated abscisic acid (ABA) biosynthesis in axillary bud outgrowth, the function of ABA catabolism and its upstream regulators in shoot branching remain elusive. Here, we showed that the MADS-box transcription factor AGAMOUS-LIKE 16 (CsAGL16) is a positive regulator of axillary bud outgrowth in cucumber (Cucumis sativus). Functional disruption of CsAGL16 led to reduced bud outgrowth, whereas overexpression of CsAGL16 resulted in enhanced branching. CsAGL16 directly binds to the promoter of the ABA 8'-hydroxylase gene CsCYP707A4 and promotes its expression. Loss of CsCYP707A4 function inhibited axillary bud outgrowth and increased ABA levels. Elevated expression of CsCYP707A4 or treatment with an ABA biosynthesis inhibitor largely rescued the Csagl16 mutant phenotype. Moreover, cucumber General Regulatory Factor 1 (CsGRF1) interacts with CsAGL16 and antagonizes CsAGL16-mediated CsCYP707A4 activation. Disruption of CsGRF1 resulted in elongated branches and decreased ABA levels in the axillary buds. The Csagl16 Csgrf1 double mutant exhibited a branching phenotype resembling that of the Csagl16 single mutant. Therefore, our data suggest that the CsAGL16–CsGRF1 module regulates axillary bud outgrowth via CsCYP707A4-mediated ABA catabolism in cucumber. Our findings provide a strategy to manipulate ABA levels in axillary buds during crop breeding to produce desirable branching phenotypes."
Authors: Yafan Han, Minghao Qu, Zhongchi Liu and Chunying Kang.
The Plant Cell (2024)
One-sentence summary: An MYB transcription factor directly regulates cytokinin homeostasis to repress axillary bud growth and consequently affect crown formation in woodland strawberry.
Abstract: "Shoot branching affects plant architecture. In strawberry (Fragaria L.), short branches (crowns) develop from dormant axillary buds to form inflorescences and flowers. While this developmental transition contributes greatly to perenniality and yield in strawberry, its regulatory mechanism remains unclear and understudied. In the woodland strawberry (Fragaria vesca), we identified and characterized two independent mutants showing more crowns. Both mutant alleles reside in FveMYB117a, a R2R3-MYB transcription factor gene highly expressed in shoot apical meristems, axillary buds and young leaves. Transcriptome analysis revealed that the expression of several cytokinin pathway genes was altered in the fvemyb117a mutant. Consistently, active cytokinins were significantly increased in the axillary buds of the fvemyb117a mutant. Exogenous application of cytokinin enhanced crown outgrowth in the wild type, whereas the cytokinin inhibitors suppressed crown outgrowth in the fvemyb117a mutant. FveMYB117a binds directly to the promoters of the cytokinin homeostasis genes FveIPT2 encoding an isopentenyltransferase and FveCKX1 encoding a cytokinin oxidase to regulate their expression. Conversely, the type-B Arabidopsis response regulators FveARR1 and FveARR2b can directly inhibit the expression of FveMYB117a, indicative of a negative feedback regulation. In conclusion, we identified FveMYB117a as a key repressor of crown outgrowth by inhibiting cytokinin accumulation and provide a mechanistic basis for bud fate transition in an herbaceous perennial plant."
Authors: Wenzhen Ku, Yi Su, Xiaoyun Peng, Ruozhong Wang, Haiou Li and Langtao Xiao.
Plants (2024)
Abstract: "Axillary bud outgrowth, a key factor in ratoon rice yield formation, is regulated by several phytohormone signals. The regulatory mechanism of key genes underlying ratoon buds in response to phytohormones in ratoon rice has been less reported. In this study, GR24 (a strigolactone analogue) was used to analyze the ratooning characteristics in rice cultivar Huanghuazhan (HHZ). Results show that the elongation of the axillary buds in the first seasonal rice was significantly inhibited and the ratoon rate was reduced at most by up to 40% with GR24 treatment. Compared with the control, a significant reduction in the content of auxin and cytokinin in the second bud from the upper spike could be detected after GR24 treatment, especially 3 days after treatment. Transcriptome analysis suggested that there were at least 742 and 2877 differentially expressed genes (DEGs) within 6 h of GR24 treatment and 12 h of GR24 treatment, respectively. Further bioinformatics analysis revealed that GR24 treatment had a significant effect on the homeostasis and signal transduction of cytokinin and auxin. It is noteworthy that the gene expression levels of OsCKX1, OsCKX2, OsGH3.6, and OsGH3.8, which are involved in cytokinin or auxin metabolism, were enhanced by the 12 h GR24 treatment. Taken overall, this study showed the gene regulatory network of auxin and cytokinin homeostasis to be regulated by strigolactone in the axillary bud outgrowth of ratoon rice, which highlights the importance of these biological pathways in the regulation of axillary bud outgrowth in ratoon rice and would provide theoretical support for the molecular breeding of ratoon rice."
Authors: Zoe Nahas, Fabrizio Ticchiarelli, Martin van Rongen, Jean Dillon and Ottoline Leyser.
New Phytologist (2024)
Abstract: "Arabidopsis thaliana (Arabidopsis) shoot architecture is largely determined by the pattern of axillary buds that grow into lateral branches, the regulation of which requires integrating both local and systemic signals. Nodal explants – stem explants each bearing one leaf and its associated axillary bud – are a simplified system to understand the regulation of bud activation. To explore signal integration in bud activation, we characterised the growth dynamics of buds in nodal explants in key mutants and under different treatments. We observed that isolated axillary buds activate in two genetically and physiologically separable phases: a slow-growing lag phase, followed by a switch to rapid outgrowth. Modifying BRANCHED1 expression or the properties of the auxin transport network, including via strigolactone application, changed the length of the lag phase. While most interventions affected only the length of the lag phase, strigolactone treatment and a second bud also affected the rapid growth phase. Our results are consistent with the hypothesis that the slow-growing lag phase corresponds to the time during which buds establish canalised auxin transport out of the bud, after which they enter a rapid growth phase. Our work also hints at a role for auxin transport in influencing the maximum growth rate of branches."
Authors: Shuang Li, Yanling Yin, Jianmin Chen, Xinyu Cui and Jinmin Fu.
PeerJ (2024)
Abstract: "Tillering/branching pattern plays a significant role in determining the structure and diversity of grass, and trimming has been found to induce tillering in turfgrass. Recently, it has been reported that hydrogen peroxide (H2O2) regulates axillary bud development. However, the role of H2O2 in trimming-induced tillering in bermudagrass, a kind of turfgrass, remains unclear. Our study unveils the significant impact of trimming on promoting the sprouting and growth of tiller buds in stolon nodes, along with an increase in the number of tillers in the main stem. This effect is accompanied by spatial-temporal changes in cytokinin and sucrose content, as well as relevant gene expression in axillary buds. In addition, the partial trimming of new-born tillers results in an increase in sucrose and starch reserves in their leaves, which can be attributed to the enhanced photosynthesis capacity. Importantly, trimming promotes a rapid H2O2 burst in the leaves of new-born tillers and axillary stolon buds. Furthermore, exogenous application of H2O2 significantly increases the number of tillers after trimming by affecting the expression of cytokinin-related genes, bolstering photosynthesis potential, energy reserves and antioxidant enzyme activity. Taken together, these results indicate that both endogenous production and exogenous addition of H2O2 enhance the inductive effects of trimming on the tillering process in bermudagrass, thus helping boost energy supply and maintain the redox state in newly formed tillers."
Authors: Jian You Wang, Guan-Ting Erica Chen, Aparna Balakrishna, Muhammad Jamil, Lamis Berqdar and Salim Al-Babili.
FEBS Letters (2024)
Summary: Previous study elucidated the alternative pathway of strigolactone (SL) biosynthesis, indicating the conversion of 9-cis-3-OH-β-apo-10′-carotenal into a 3-hydroxycarlactone in vitro. Herein, we fed wild-type (WT) and the SL-deficient d17 mutant rice seedlings with 13C-labeled 9-cis-3-OH-β-apo-10′-carotenal and analyzed their SLs by LC–MS/MS. Our results reveal that 9-cis-3-OH-β-apo-10′-carotenal is the SL precursor in planta, filling a knowledge gap in SL biosynthesis.
Abstract: "Strigolactones (SLs) play a crucial role in regulating plant architecture and mediating rhizosphere interactions. They are synthesized from all-trans-β-carotene converted into the intermediate carlactone (CL) via the intermediate 9-cis-β-apo-10′-carotenal. Recent studies indicate that plants can also synthesize 3-OH-CL from all-trans-β-zeaxanthin via the intermediate 9-cis-3-OH-β-apo-10′-carotenal. However, the question of whether plants can form bioactive SLs from 9-cis-3-OH-β-apo-10′-carotenal remains elusive. In this study, we supplied the 13C-labeled 9-cis-3-OH-β-apo-10′-carotenal to rice seedlings and monitored the synthesis of SLs using liquid chromatography-mass spectrometry (LC–MS) and Striga bioassay. We further validated the biological activity of 9-cis-3-OH-β-apo-10′-carotenal-derived SLs using the ccd7/d17 SL-deficient mutant, which demonstrated increased Striga seed-germinating activity and partial rescue of tiller numbers and plant height. Our results establish 9-cis-3-OH-β-apo-10′-carotenal as a significant SL biosynthetic intermediate with implications for understanding plant hormonal functions and potential applications in agriculture."
Authors: Uri Karniel, Amit Koch, Nurit Bar Nun, Dani Zamir and Joseph Hirschberg.
Research Square (2024)
Abstract: "The phytohormones strigolactones (SLs) control root and shoot branching and are exuded from roots into the rhizosphere to stimulate interaction with mycorrhizal fungi. The exuded SLs serve as signaling molecules for the germination of parasitic plants. The broomrape Phelipanche aegyptiaca is a widespread noxious weed in several crop plants, including tomato (Solanum lycopersicum). 'In silico' screening of tomato (M82) mutants identified three lines that showed significantly increased branching. Two mutants, SHOOT BRANCHING 1 (sb1) and SHOOT BRANCHING 2 (sb2), lack SLs due to loss of function mutations in the genes for the carotenoid cleavage dioxygenase enzymes CCD7, and CCD8, respectively. Beyond the increased branching, these mutants were completely resistant to infection by P. aegyptiaca. The third branching mutant, SHOOT BRANCHING 3 (sb3), carried a point mutation in the SLs receptor DWARF14 and was found to be highly susceptible to P. aegyptiaca. SL concentration in roots of the sb3 was two-fold higher than in the wild type due to the upregulation of transcription of SL biosynthesis genes. This phenomenon suggests that the steady-state level of SLs is regulated by a feedback mechanism that involves the SL signaling pathway. Grafting experiments showed that sb1 and sb2 rootstocks protected wild-type tomato scions from P. aegyptiaca infection without a significant yield loss, offering a solution to the broomrape crisis. These results also demonstrate that strigolactones synthesized in the shoots are involved in the control of shoot branching."
Authors: María Florencia Mammarella, Leandro Lucero, Nosheen Hussain, Aitor Muñoz-Lopez, Ying Huang, Lucia Ferrero, Guadalupe L Fernandez-Milmanda, Pablo Manavella, Moussa Benhamed, Martin Crespi, Carlos L. Ballare, José Gutiérrez Marcos, Pilar Cubas and Federico Ariel.
EMBO Journal (2023)
Synopsis: The Arabidopsis lncRNA APOLO influences chromatin architecture to regulate transcription of specific genes, including auxin-responsive genes which are involved in the plant's response to shade avoidance. Here, APOLO is shown to respond to changes in light by influencing transcription and plant behavior. APOLO modulates the expression of BRANCHED1 (BRC1), a master regulator of shoot branching in Arabidopsis, through chromatin looping around the BRC1 promoter. APOLO-mediated chromatin looping is influenced by changes in light exposure and regulates branching behavior. Low-red/far-red light ratio-mediated leaf hyponasty depends on APOLO regulation of auxin-related genes. In vitro-transcribed APOLO directly sprayed onto plants is sufficient to alter plant auxin homeostasis in response to light conditions.
Abstract: "The long noncoding RNA (lncRNA) AUXIN-REGULATED PROMOTER LOOP (APOLO) recognizes a subset of target loci across the Arabidopsis thaliana genome by forming RNA–DNA hybrids (R-loops) and modulating local three-dimensional chromatin conformation. Here, we show that APOLO regulates shade avoidance syndrome by dynamically modulating expression of key factors. In response to far-red (FR) light, expression of APOLO anti-correlates with that of its target BRANCHED1 (BRC1), a master regulator of shoot branching in Arabidopsis thaliana. APOLO deregulation results in BRC1 transcriptional repression and an increase in the number of branches. Accumulation of APOLO transcription fine-tunes the formation of a repressive chromatin loop encompassing the BRC1 promoter, which normally occurs only in leaves and in a late response to far-red light treatment in axillary buds. In addition, our data reveal that APOLO participates in leaf hyponasty, in agreement with its previously reported role in the control of auxin homeostasis through direct modulation of auxin synthesis gene YUCCA2, and auxin efflux genes PID and WAG2. We show that direct application of APOLO RNA to leaves results in a rapid increase in auxin signaling that is associated with changes in the plant response to far-red light. Collectively, our data support the view that lncRNAs coordinate shade avoidance syndrome in A. thaliana, and reveal their potential as exogenous bioactive molecules. Deploying exogenous RNAs that modulate plant–environment interactions may therefore become a new tool for sustainable agriculture."
Authors: Mingjuan Li, Hongye Li, Qidong Zhu, Dong Liu, Zhen Li, Haifei Chen, Jinsong Luo, Pan Gong, Abdelbagi M. Ismail, Zhenhua Zhang
New Phytologist (2024)
Abstract: "Sugar transporter proteins (STPs) play critical roles in regulating plant stress tolerance, growth, and development. However, the role of STPs in regulating crop yield is poorly understood. This study elucidates the mechanism by which knockout of the sugar transporter OsSTP15 enhances grain yield via increasing the tiller number in rice. We found that OsSTP15 is specifically expressed in the shoot base and vascular bundle sheath of seedlings and encodes a plasma membrane-localized high-affinity glucose efflux transporter. OsSTP15 knockout enhanced sucrose and trehalose-6-phosphate (Tre6P) synthesis in leaves and improved sucrose transport to the shoot base by inducing the expression of sucrose transporters. Higher glucose, sucrose, and Tre6P contents were observed at the shoot base of stp15 plants. Transcriptome and metabolome analyses of the shoot base demonstrated that OsSTP15 knockout upregulated the expression of cytokinin (CK) synthesis- and signaling pathway-related genes and increased CK levels. These findings suggest that OsSTP15 knockout represses glucose export from the cytoplasm and simultaneously enhances sugar transport from source leaves to the shoot base by promoting the synthesis of sucrose and Tre6P in leaves. Subsequent accumulation of glucose, sucrose, and Tre6P in the shoot base promotes tillering by stimulating the CK signaling pathway."
Authors: Chang Su, Andrzej Kokosza, Xiaonan Xie, Aleš Pěnčík, Youjun Zhang, Pasi Raumonen, Xueping Shi, Sampo Muranen, Melis Kucukoglu Topcu, Juha Immanen, Risto Hagqvist, Omid Safronov, Juan Alonso-Serra, Gugan Eswaran, Mirko Pavicic Venegas, Karin Ljung, Sally Ward, Ari Pekka Mähönen, Kristiina Himanen, Jarkko Salojärvi, Alisdair R. Fernie, Ondřej Novák, Ottoline Leyser, Wojtek Pałubicki, Ykä Helariutta and Kaisa Nieminen.
PNAS (2023)
Significance What makes a tree a tree instead of a bush? Through a candidate gene approach, we identified a natural bush-like (short and highly branching) SL-deficient birch mutant, kanttarelli, with an early STOP codon in an essential SL biosynthesis gene, BpMAX1. The number of higher-order branches was increased in the mutant and in phenocopying transgenic RNAi -lines. Intriguingly, the auxin concentration formed a gradient along the main stem in the WT, with more auxin in the uppermost internodes and less toward the base, whereas in the transgenic line this gradient was absent. Mathematical modeling showed that this difference in auxin distribution may result from the differing architectures. Our results could be applied in the breeding of trees with an optimized architecture.
Abstract: "Due to their long lifespan, trees and bushes develop higher order of branches in a perennial manner. In contrast to a tall tree, with a clearly defined main stem and branching order, a bush is shorter and has a less apparent main stem and branching pattern. To address the developmental basis of these two forms, we studied several naturally occurring architectural variants in silver birch (Betula pendula). Using a candidate gene approach, we identified a bushy kanttarelli variant with a loss-of-function mutation in the BpMAX1 gene required for strigolactone (SL) biosynthesis. While kanttarelli is shorter than the wild type (WT), it has the same number of primary branches, whereas the number of secondary branches is increased, contributing to its bush-like phenotype. To confirm that the identified mutation was responsible for the phenotype, we phenocopied kanttarelli in transgenic BpMAX1::RNAi birch lines. SL profiling confirmed that both kanttarelli and the transgenic lines produced very limited amounts of SL. Interestingly, the auxin (IAA) distribution along the main stem differed between WT and BpMAX1::RNAi. In the WT, the auxin concentration formed a gradient, being higher in the uppermost internodes and decreasing toward the basal part of the stem, whereas in the transgenic line, this gradient was not observed. Through modeling, we showed that the different IAA distribution patterns may result from the difference in the number of higher-order branches and plant height. Future studies will determine whether the IAA gradient itself regulates aspects of plant architecture."
Authors: Ying Liu and Guohua Xu.
Molecular Plant (2023)
Excerpts: "Song et al. (2023) discovered that N-induced tillering relies on a balance between N and Fe status in rice. More importantly, a central transcriptional regulator of rice N signaling, NIN-LIKE PROTEIN 4 (OsNLP4) (Wu et al., 2021), plays a vital role in this process. The authors found that OsNLP4 directly regulates the expression of a large set of genes involved in N and Fe uptake, utilization, and signaling pathways, thus coordinating N–Fe balance in rice. Further investigation revealed that OsNLP4 can bind to the promoter of the strigolactone (SL) signaling gene OsD3, repressing its expression to shut down SL signaling, stimulate tillering, and improve yield (Song et al., 2023). These results highlight OsNLP4 as a vital integrator that coordinates rice tillering and productivity with external N and Fe availability (Figure 1)."
"Under balanced N–Fe conditions, the nuclear retention of OsNLP4 is promoted in a nitrate-dependent manner, which allows OsNLP4 to activate the expression of a large set of genes involved in N and Fe uptake and utilization and suppress D3 expression and SL signaling, thus stimulating tillering and improving grain yield in rice (Song et al., 2023). An appropriate N–Fe balance also enables OsNLP4 to up-regulate the expression of several ROS-scavenging genes and reduce ROS (H2O2) levels in rice."
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