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Authors: Tian Su, Ziwei Li, Yinghua Zhang, Junqiang Xu and Bin Xu.
Journal of the American Society for Horticultural Science (2024)
Abstract: "Cucumber (Cucumis sativus L.) belongs to the cucumber genus of the Cucurbitaceae family, and the selection of cultivars with minimal or no lateral branches can enhance the cultivation management efficiency. The growth of lateral branches is inhibited by strigolactone. To investigate the regulatory mechanism of strigolactone on the lateral branch development in cucumber, the cultivar LZ1 exhibiting multiple lateral branches was selected as the experimental material. The axillae of the plants were infiltrated with 1, 5, and 10 μmol·L−1 germination releaser 24 (GR24) at the four- to five-leaf stage. It was identified that 1 μmol·L−1 GR24 exhibited the most potent inhibitory effect on cucumber lateral branches. Additionally, exogenous strigolactone decreased the auxin content in the apical bud and axillae and increased the auxin content in the stem. This inhibited polar auxin transport in the axillary bud and promoted polar auxin transport in the apical bud. The content of strigolactone in the axilla region of cucumbers was elevated, whereas the synthesis and expression of cytokinin in the same area were reduced. A low concentration of GR24 induced the expression of cucumber branched 1 (csbrc1), whereas a high concentration of GR24 downregulated the expression of cucumber lateral suppressor (cscls) and blind (csblind), which inhibited the growth of cucumber lateral branches."
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: 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: Shangyu Chen, Xuewei Song, Qixiang Zheng, Yuqi Liu, Jingquan Yu, Yanhong Zhou and Xiaojian Xia.
Journal of Experimental Botany (2023)
Abstract: "Plant architecture imposes a large impact on crop yield. IDEAL PLANT ARCHITECTURE 1 (IPA1), which encodes a SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factor, is a target of molecular design for improving grain yield. However, the roles of SPL transcription factors in regulating tomato (Solanum lycopersicum) plant architecture are unclear. Here, we show that the expression of SPL13 is downregulated in the lateral buds of strigolactones (SLs)-deficient ccd mutants and is induced by GR24 (a synthetic analogue of SL). Knockout of SPL13 by CRISPR/Cas9 resulted in higher levels of cytokinins (CKs) and transcripts of CK synthesis gene ISOPENTENYL TRANSFERASES 1 (IPT1) in the stem nodes and more growth of lateral buds. GR24 suppresses CKs synthesis and lateral bud growth in ccd mutants but is not effective in spl13 mutants. Meanwhile, silencing of IPT1 gene inhibited bud growth of spl13 mutants. Interestingly, SLs levels in root extracts and exudates are significantly increased in spl13 mutants. Molecular studies indicated that SPL13 directly represses the transcription of IPT1 and the SL synthesis genes CAROTENOID CLEAVAGE DIOXYGENASE 7 (CCD7) and MORE AXILLARY GROWTH 1 (MAX1). The results demonstrate that SPL13 acts downstream of SL to suppress lateral bud growth by inhibiting CKs synthesis in tomato. Tuning the expression of SPL13 is a potential approach for decreasing the number of lateral shoots in tomato."
Authors: Christine A Beveridge, Catherine Rameau, Akila Wijerathna-Yapa.
Jornal of Experimental Botany (2023)
Abstract: "The process of apical dominance by which the apical bud/shoot tip of the plant inhibits the outgrowth of axillary buds located below has been studied for more than a century. Different approaches were used over time with first the physiology era, the genetic era, and then the multidisciplinary era. During the physiology era, auxin was thought of as the master regulator of apical dominance acting indirectly to inhibit bud outgrowth via unknown secondary messenger(s). Potential candidates were cytokinin (CK) and abscisic acid (ABA). The genetic era with the screening of shoot branching mutants in different species revealed the existence of a novel carotenoid-derived branching inhibitor and led to the significant discovery of strigolactones (SLs) as a novel class of plant hormones. The re-discovery of the major role of sugars in apical dominance emerged from modern physiology experiments and involves ongoing work with genetic material affected in sugar-signalling. As crops and natural selection rely on the emergent properties of networks such as this branching network, future work should explore the whole network, the details of which are critical but not individually sufficient to solve the wicked problems of sustainable food supply and climate change."
Author: Sebastian R. Moreno
Plant Physiology (2023)
Excerpts: "In this issue of Plant Physiology, Cao et al. (2023) provide evidence supporting the role of sucrose as the signal that triggers changes in CK, auxin, and gibberellin (GA) concentrations to modulate bud outgrowth and to sustain bud growth. The researchers observed that in pea (Pisum sativum) buds outside the auxin-depletion region after decapitation, bud release is triggered by sugar-elicited changes in CK levels with the subsequent downregulation of the inhibitory role of SL (Figure 1).
"Unlike CK, application of SL on axillary buds represses bud outgrowth (Gomez-Roldan et al., 2018). However, the crosstalk between SL and CK behind bud outgrowth is not completely understood. In this paper, through quantifying gene expression of several CK-related genes, the authors nicely demonstrated that SL inhibits CK levels by increasing CK degradation and decreasing CK biosynthesis. In addition, the authors quantified bud growth after treating with synthetic CK BAP and GR24 (synthetic SL) treatments. They found that sucrose and CK treatments could overcome SL-inhibited bud release. Thus, the authors established a very clear connection between SL, CK, and sucrose during early stage of bud growth and provided insights into the mechanism responsible for releasing axillary buds."
Authors: Da Cao, François Barbier, Elizabeth A. Dun, Franziska Fichtner, Lili Dong, Stephanie C. Kerr and Christine A Beveridge.
bioRxiv (2022)
Abstract: "The inhibition of shoot branching by the growing shoot tip of plants, termed apical dominance, was originally thought to be mediated by auxin. Recently the importance of the shoot tip sink strength during apical dominance has re-emerged with recent studies highlighting roles for sugars in promoting branching. This raises many unanswered questions on the relative roles of auxin and sugars in apical dominance. Here we show that auxin regulation of cytokinins, which promote branching, is significant only after an initial stage of branching we call bud release. During this early bud release stage, rapid cytokinin increases are associated with enhanced sugars. Auxin may also act through strigolactones which have been shown to suppress branching after decapitation, but here we show that strigolactones do not have a significant effect on initial bud outgrowth after decapitation. We report here that when sucrose or cytokinin is abundant, strigolactones are less inhibitory during the bud release stage compared to later stages and that strigolactone treatment rapidly inhibits cytokinin accumulation in pea axillary buds of intact plants. After initial bud release, we find an important role of gibberellin in promoting sustained bud growth downstream of auxin. We are therefore able to suggest a model of apical dominance that integrates auxin, sucrose, strigolactones, cytokinins and gibberellins and describes differences in signalling across stages of bud release to sustained growth."
Authors: Guofang Li, Ming Tan, Juanjuan Ma, Fang Cheng, Ke Li, Xiaojie Liu, Caiping Zhao, Dong Zhang, Libo Xing, Xiaolin Ren, Mingyu Han and Na An.
Journal of Experimental Botany (2021)
Abstract: "Shoot branching is an important factor influencing apple tree architecture. Apple cultivar 'Fuji', which is grown on approximately 75% of the apple-producing area in China, exhibits poor natural branching. Cytokinin (CK) promotes axillary bud outgrowth. The TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) family genes BRANCHED1/2 (BRC1/2) are involved in integrating diverse factors that function locally to inhibit shoot branching. However, the molecular mechanism underlying the CK-mediated promotion of branching involving the repression of BRC1/2 remains unclear. In this study, we revealed that WUSCHEL2 (MdWUS2), which interacts with the co-repressor TOPLESS-RELATED9 (MdTPR9), is activated by CK and regulates branching by inhibiting MdTCP12 (BRC2 homolog) activity. Overexpressing MdWUS2 in Arabidopsis or Nicotiana benthamiana resulted in enhanced branching. The overexpression of MdTCP12 inhibited axillary bud outgrowth in Arabidopsis, indicating that MdTCP12 contributes to the regulation of branching. Additionally, we observed that MdWUS2 interacts with MdTCP12 in vivo and in vitro and suppresses the ability of MdTCP12 to activate the transcription of its target gene HOMEOBOX PROTEIN 53b (MdHB53b). Therefore, our data suggest that MdWUS2 is involved in the CK-mediated inhibition of MdTCP12 that controls bud outgrowth, providing new insights into the regulation of shoot branching by CK."
Authors: Alexis Porcher, Vincent Guérin, Nathalie Leduc, Anita Lebrec, Jérémy Lothier and Alain Vian.
Plant Physiology (2021)
Abstract: "Rosebush (Rosa ‘Radrazz’) plants are an excellent model to study light control of bud outgrowth since bud outgrowth only arises in the presence of light and never occurs in darkness. Recently, we demonstrated high levels of H2O2 present in the quiescent axillary buds strongly repress the outgrowth process. In light, the outgrowing process occurred after H2O2 scavenging through the promotion of Ascorbic acid-Glutathione (AsA-GSH) dependent pathways and the continuous decrease in H2O2 production. Here we showed Respiratory Burst Oxidase Homologs (RBOH) expression decreased in buds during the outgrowth process in light. In continuous darkness, the same decrease was observed although H2O2 remained at high levels in axillary buds, as a consequence of the strong inhibition of AsA-GSH cycle and GSH synthesis preventing the outgrowth process. Cytokinin (CK) application can evoke bud outgrowth in light as well as in continuous darkness. Furthermore, CKs are the initial targets of light in the photocontrol process. We showed CK application to cultured buds in darkness decreases bud H2O2 to a level that is similar to that observed in light. Furthermore, this treatment restores GSH levels and engages bud burst. We treated plants with buthionine sulfoximine (BSO), an inhibitor of GSH synthesis, to solve the sequence of events involving H2O2/GSH metabolisms in the photocontrol process. This treatment prevented bud burst, even in the presence of CK, suggesting the sequence of actions starts with the positive CK effect on GSH that in turn stimulates H2O2 scavenging, resulting in initiation of bud outgrowth."
Authors: Yanyan Yan, Qinghua Shi and Biao Gong.
Plant and Cell Physiology (2021)
Abstract: "Auxin and cytokinin are two kinds of important phytohormones that mediate outgrowth of axillary buds in plants. How nitric oxide and its regulator of S-nitrosoglutathione reductase (GSNOR) taking part in auxin and cytokinin signaling for controlling axillary buds outgrowth remains elusive. We explained roles of GSNOR during tomato axillary buds outgrowth by physiological, biochemical and genetic approach. GSNOR negatively regulated NO homeostasis. Suppression of GSNOR promoted axillary buds outgrowth via inhibiting the expression of FZY in both apical and axillary buds. Meanwhile, AUX1 and PIN1 were down-regulated in apical buds but up-regulated in axillary buds in GSNOR-suppressed plants. Thus, reduced IAA accumulation was shown in both apical buds and axillary buds of GSNOR-suppressed plants. GSNOR-mediated changes of NO and auxin affected cytokinin biosynthesis, transport, and signaling. And a decreased ratio of auxin: cytokinin was shown in axillary buds of GSNOR-suppressed plants, leading to buds dormancy breaking. We also found that the original NO signaling was generated by nitrate reductase (NR) catalyzing nitrate as substrate. NR-mediated NO reduced the GSNOR activity through S-nitrosylation of Cys-10, then induced a further NO burst, which played the above roles to promote axillary buds outgrowth. Together, GSNOR-mediated NO played important roles in controlling axillary buds outgrowth via altering the homeostasis and signaling of auxin and cytokinin in tomato plants."
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Authors: Yuqi Liu, Shangyu Chen, Sikander Pal, Jingquan Yu, Yanhong Zhou, Lam-Son Phan Tran and Xiaojian Xia.
New Crops (2024)
Abstract: "Plants have evolved varied structures for environmental adaptation. Shoot branching, as a part of plant architecture, influences the allocation of sugars produced by photosynthesis and thus greatly impacts crop yields. The activity of axillary meristem, and apical dominance governs the shoot branching patterns. In this review, we summarize the key factors involved in the formation of lateral branches, and the mechanisms of how these factors are interconnected. In particular, we focus on recent advances in understanding how sugar and environmental signals affect the hormonal signaling network to regulate apical dominance. Ultimately, we propose that epigenetic modifications are critical mechanisms underlying the plasticity of shoot branching, and that precise targeted gene editing is promising for shaping the ideal plant architecture."
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: 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."
Authors: Qian Zhao, Hao Chen, Dong Zhang and Juanjuan Ma.
Scientia Horticulturae (2023)
Highlights • MdRR9 has an ORF of 648 bp, which showed the highest expression level in the stem and was up-regulated earlier in response to 6-BA than decapitation treatment. • The subcellular localization of MdRR9 was in the nucleus. Ectopic expression of MdRR9 in tomato indicated that MdRR9 plays a positive role in branch control. • Yeast double-hybrid assay showed that MdRR9 may interact with MADS-box transcription factor 22-like (AGL9).
Abstract: "Response regulator 9 (RR9) is a typical member of type-A RRs whose transcription is rapidly increased by exogenous cytokinin (CTK). CTK stimulates the outgrowth of axillary buds in apples (Malus domestica). Previous studies showed that the decreased bud outgrowth activity of type-A rr mutants is paradoxical given that type-A RRs act as negative feedback regulators of CTK signaling in Arabidopsis. However, no systematic study of RR9 has been performed in apples. In this study, the genomic sequences of MdRR9, located on chromosome 14, were obtained. Sequencing identified MdRR9 as having an open reading frame (ORF) of 648 bp. Expression pattern showed MdRR9 has the highest expression level in the stem. MdRR9 was up-regulated earlier in response to 6-benzylaminopurine (6-BA, a synthetic type of CTK) than decapitation treatment. The subcellular localization of MdRR9 was in the nucleus. Ectopic expression of MdRR9 in tomatoes indicates that MdRR9 plays a positive role in branch control. A yeast double-hybrid assay showed that MdRR9 may interact with MADS-box transcription factor 22-like (MdAGL9), which gives us a new perspective to study the regulation of the bud outgrowth mechanism of RR9 in the future."
Authors: Han Dong, Jiachun Wang, Xuewei Song, Chaoyi Hu, Changan Zhu, Ting Sun, Zhiwen Zhou, Zhangjian Hu, Xiaojian Xia , Jie Zhou , Kai Shi, Yanhong Zhou, Christine H. Foyer and Jingquan Yu.
PNAS (2023)
Significance: The orchestration of shoot architecture, which is a major determinant of crop productivity, requires the regulated suppression or activation of -bud outgrowth. We show that light quality regulated tomato bud outgrowth is dependent on the mobile signaling protein HY5 and that HY5 dependent light signaling alone is sufficient to regulate bud outgrowth. HY5 promotes bud growth by direct and brassinosteroid mediated suppression of BRC1. BRC1 blocks the accumulation of cytokinin and gibberellin that regulate bud growth in tomato. In this way, HY5 represses gibberellin mediated stem elongation during photomorphogenesis while activating branching through BRC1 dependent phytohormone regulation. The HY5–BRC1 module thus plays predominant roles in the shoot architecture by orchestrating light quality dependent changes in stem elongation and bud outgrowth.
Abstract: "Light plays an important role in determining plant architecture, which greatly influences crop yield. However, the precise mechanisms by which light signaling regulates bud outgrowth remain to be identified. Here, we show that light regulates bud outgrowth via both HY5 and brassinosteroid (BR)-dependent pathways in tomato. Inactivation of the red-light photoreceptor PHYB, or deficiencies in PHYB or the blue-light photoreceptor CRY1a, inhibits bud outgrowth and leads to decreased accumulation of HY5 protein and increased transcript level of BRANCHED1 (BRC1), a central integrator of branching signals. HY5, functioning as a mobile systemic signal from leaves, promotes bud outgrowth by directly suppressing BRC1 transcript and activating the transcript of BR biosynthesis genes within the lateral buds in tomato. Furthermore, BRC1 prevents the accumulation of cytokinin (CK) and gibberellin (GA) by directly inhibiting the transcript of CK synthesis gene LOG4, while increasing the transcript levels of CK and GA degradation genes (CKX7, GA2ox4, and GA2ox5), leading to an arrest of bud outgrowth. Moreover, bud outgrowth occurs predominantly in the day due to the suppression of BRC1 transcript by HY5. These findings demonstrate that light-inducible HY5 acts as a systemic signaling factor in fine-tuning the bud outgrowth of tomato."
Authors: Da Cao, Tinashe Chabikwa, Francois Barbier, Elizabeth A. Dun, Franziska Fichtner, Lili Dong, Stephanie C. Kerr and Christine A. Beveridge.
Plant Physiology (2023)
Short summary: Sugars and cytokinin initiate bud release independently of auxin and suppress inhibition by SL. Afterward, auxin in buds regulates gibberellin to promote sustained bud growth.
Abstract: "The inhibition of shoot branching by the growing shoot tip of plants, termed apical dominance, was originally thought to be mediated by auxin. Recently the importance of the shoot tip sink strength during apical dominance has re-emerged with recent studies highlighting roles for sugars in promoting branching. This raises many unanswered questions on the relative roles of auxin and sugars in apical dominance. Here we show that auxin depletion after decapitation is not always the initial trigger of rapid cytokinin increases in buds that are instead correlated with enhanced sugars. Auxin may also act through strigolactones which have been shown to suppress branching after decapitation, but here we show that strigolactones do not have a significant effect on initial bud outgrowth after decapitation. We report here that when sucrose or cytokinin is abundant, strigolactones are less inhibitory during the bud release stage compared to during later stages and that strigolactone treatment rapidly inhibits cytokinin accumulation in pea (Pisum sativum) axillary buds of intact plants. After initial bud release, we find an important role of gibberellin in promoting sustained bud growth downstream of auxin. We are therefore able to suggest a model of apical dominance that integrates auxin, sucrose, strigolactones, cytokinins and gibberellins and describes differences in signalling across stages of bud release to sustained growth."
Authors: Minglei Yi, Heyu Yang, Shaohui Yang and Jiehua Wang.
bioRxiv (2021)
Abstract: "Plant branching is usually prevented by an actively proliferating apex. In poplars, one GRAS family member, SHORT-ROOT2 (PtSHR2), was preferentially expressed in axillary buds (AXBs) and was inducible during bud maturation and activation. Overexpression of PtSHR2 (PtSHR2OE) in hybrid poplar impaired the apical dominance and simultaneously promoted the outgrowth of axillary branches below the maturation point (BMP), accompanied by regulated expression of genes critical for axillary meristem initiation and bud formation. Following a detained phenotypic characterization, we compared the IAA and trans-zeatin levels in apical shoots and AXBs of wild-type and PtSHR2OE trees, together with gene expression analyses and defoliation, decapitation, and hormone reapplication assays. PtSHR2OE AXBs contained a significantly lower ratio of auxin to cytokinin than wild-type AXBs, particularly in those below the BMP. Decapitation induced a faster bud burst in PtSHR2OE trees than in wild-type plants, and it could be strongly inhibited by exogenously applied auxin and cytokinin biosynthesis inhibitor, but only partially inhibited by N-1-naphthylphthalamic acid (NPA). An impaired basipetal auxin transport, rather than an insufficient auxin biosynthesis or auxin insensitivity, disturbed the local hormonal homeostasis in PtSHR2OE AXBs, which in turn enhanced the axillary bud initiation and promoted the bud release."
Authors: Francisco Javier Pérez, Ximena Noriega and Sebastián Rubio.
Antioxidants (2021)
Abstract: "Changes in the level of hydrogen peroxide (H2O2) is a good indicator to monitor fluctuations in cellular metabolism and in the stress responses. In this study, the changes in H2O2 content during bud endodormancy (ED) and budbreak were analysed in grapevine (Vitis vinifera L.). The results showed a gradual increase in the H2O2 content during the development of bud ED, which was mainly due to an increase in the activity of peroxidases (PODs). The maximum H2O2 content reached in the grapevine buds coincided with the maximum depth of bud ED. In contrast, during budbreak, the H2O2 content decreased. As the plant hormones cytokinin (CK) and auxin play an important role in budbreak and growth resumption in grapevine, the effect of exogenous applications of H2O2 on the expression of genes involved in CK and auxin metabolism was analysed. The results showed that H2O2 represses the expression of the CK biosynthesis genes VvIPT3a and VvLOG1 and induces the expression of the CK-inactivating gene VvCKX3, thus reducing potentially the CK content in the grapevine bud. On the other hand, H2O2 induced the expression of the auxin biosynthesis genes VvAMI1 and VvYUC3 and of the auxin transporter gene VvPIN3, thus increasing potentially the auxin content and auxin transport in grapevine buds. In general, the results suggest that H2O2 in grapevine buds is associated with the depth of ED and negatively regulates its budbreak."
Authors: Xiaojian Xia, Han Dong, Yanling Yin, Xuewei Song, Xiaohua Gu, Kangqi Sang, Jie Zhou, Kai Shi, Yanhong Zhou, Christine H. Foyer, and Jingquan Yu.
PNAS (2021)
Significance: For almost a century, auxin had been well-known as the master regulator of apical dominance. Recently, however, sugars were shown to be the initial regulator of apical dominance, while strigolactones (SLs) and cytokinins (CKs) act downstream of auxin to control bud outgrowth. However, the interactions of the different pathways have remained outstanding questions. Here, we report that brassinosteroids (BRs) are essential for the release of apical dominance in tomato. CK signaling relays information from auxin, SL, and sugars to promote the production of BRs, which activate the BZR1 transcription factor to suppress the expression of BRANCHED1, an inhibitor of bud outgrowth. These findings demonstrate that hormonal and metabolic pathways impinge on a common BR signaling for controlling shoot branching.
Abstract: "The control of apical dominance involves auxin, strigolactones (SLs), cytokinins (CKs), and sugars, but the mechanistic controls of this regulatory network are not fully understood. Here, we show that brassinosteroid (BR) promotes bud outgrowth in tomato through the direct transcriptional regulation of BRANCHED1 (BRC1) by the BR signaling component BRASSINAZOLE-RESISTANT1 (BZR1). Attenuated responses to the removal of the apical bud, the inhibition of auxin, SLs or gibberellin synthesis, or treatment with CK and sucrose, were observed in bud outgrowth and the levels of BRC1 transcripts in the BR-deficient or bzr1 mutants. Furthermore, the accumulation of BR and the dephosphorylated form of BZR1 were increased by apical bud removal, inhibition of auxin, and SLs synthesis or treatment with CK and sucrose. These responses were decreased in the DELLA-deficient mutant. In addition, CK accumulation was inhibited by auxin and SLs, and decreased in the DELLA-deficient mutant, but it was increased in response to sucrose treatment. CK promoted BR synthesis in axillary buds through the action of the type-B response regulator, RR10. Our results demonstrate that BR signaling integrates multiple pathways that control shoot branching. Local BR signaling in axillary buds is therefore a potential target for shaping plant architecture."
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