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Authors: Kei Tsuzuki, Taiki Suzuki, Michio Kuruma, Kotaro Nishiyama, Ken-ichiro Hayashi, Shinya Hagihara and Yoshiya Seto.
bioRxiv (2024)
Abstract: "Root parasitic plants in the Orobancheceae, such as Striga and Orobanche, cause significant damage to crop production. The germination step of these root parasitic plants is induced by host-root-derived strigolactones (SLs). After germination, the radicles elongate toward the host and invade the host root. We have previously discovered that a simple amino acid, tryptophan (Trp), as well as its metabolite, the plant hormone indole-3-acetic acid (IAA), can inhibit radicle elongation of Orobanche minor. These results suggest that auxin plays a crucial role in the radicle elongation step in root parasitic plants. In this report, we used various auxin chemical probes to dissect the auxin function in the radicle growth of O. minor and Striga hermonthica. We found that synthetic auxins inhibited radicle elongation. In addition, auxin receptor antagonist, auxinole, rescued the inhibition of radicle growth by exogenous IAA. Moreover, a polar transport inhibitor of auxin, N-1-naphthylphthalamic acid (NPA), affected radicle tropism. We also proved that exogenously applied Trp is converted into IAA in O. minor seeds, and auxinole partly rescued this radicle elongation. Our data demonstrate a pivotal role of auxin in radicle growth. Thus, manipulation of auxin function in root parasitic plants should offer a useful approach to combat these parasites."
Authors: Yoon Jeong Jang, Taehoon Kim, Makou Lin, Jeongim Kim, Kevin Begcy, Zhongchi Liu and Seonghee Lee.
bioRxiv (2024)
Abstract: "The plant hormone auxin plays a crucial role in regulating important functions in strawberry fruit development. Although a few studies have described the complex auxin biosynthetic and signaling pathway in wild diploid strawberry (Fragaria vesca), the molecular mechanisms underlying auxin biosynthesis and crosstalk in octoploid strawberry fruit development are not fully characterized. To address this knowledge gap, comprehensive transcriptomic analyses were conducted at different stages of fruit development and compared between the achene and receptacle to identify developmentally regulated auxin biosynthetic genes and transcription factors during the fruit ripening process. Similar to wild diploid strawberry, octoploid strawberry accumulates high levels of auxin in achene compared to receptacle. Consistently, genes functioning in auxin biosynthesis and conjugation, such as TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAAs), YUCCA (YUCs), and GRETCHEN HAGEN 3 (GH3s) were found to be primarily expressed in the achene, with low expression in the receptacle. Interestingly, several genes involved in auxin transport and signaling like PIN-FORMED (PINs), AUXIN/INDOLE-3-ACETIC ACID proteins (Aux/IAAs), TRANSPORT INHIBITOR RESPONSE 1 / AUXIN-SIGNALING F-BOX (TIR/AFBs) and AUXIN RESPONSE FACTOR (ARFs) were more abundantly expressed in the receptacle. Moreover, by examining DEGs and their transcriptional profiles across all six developmental stages, we identified key auxin-related genes co-clustered with transcription factors from the NAM-ATAF1,2-CUC2/ WRKYGQK motif (NAC/WYKY), BASIC REGION/ LEUCINE ZIPPER motif (bZIP), and APETALA2/Ethylene Responsive Factor (AP2/ERF) groups. These results elucidate the complex regulatory network of auxin biosynthesis and its intricate crosstalk within the achene and receptacle, enriching our understanding of fruit development in octoploid strawberries."
Authors: Juan He, Xiaoyi Li, Qin Yu, Lu Peng, Li Chen, Jiajia Liu, Jianmei Wang, Xufeng Li and Yi Yang.
Plant, Cell & Environment (2024)
Summary statement: The AGC VIII kinase D6 PROTEIN KINASE (D6PK) phosphorylates PIN-FORMED (PIN) auxin efflux carriers, which is essential for auxin transport in plant cells. This study uncovers a new mechanism for D6PK stability through Cytosolic ABA Receptor Kinases-mediated phosphorylation to regulate auxin-controlled Arabidopsis growth and development.
Abstract: "The polar auxin transport is required for proper plant growth and development. D6 PROTEIN KINASE (D6PK) is required for the phosphorylation of PIN-FORMED (PIN) auxin efflux carriers to regulate auxin transport, while the regulation of D6PK stabilization is still poorly understood. Here, we found that Cytosolic ABA Receptor Kinases (CARKs) redundantly interact with D6PK, and the interactions are dependent on CARKs' kinase activities. Similarly, CARK3 also could interact with paralogs of D6PK, including D6PKL1, D6PKL2, and D6PKL3. The genetic analysis shows that D6PK acts the downstream of CARKs to regulate Arabidopsis growth, including hypocotyl, leaf area, vein formation, and the length of silique. Loss-of-function of CARK3 in overexpressing GFP-D6PK plants leads to reduce the level of D6PK protein, thereby rescues plant growth. In addition, the cell-free degradation assays indicate that D6PK is degraded through 26 S proteasome pathway, while the phosphorylation by CARK3 represses this process in cells. In summary, D6PK stabilization by the CARK family is required for auxin-mediated plant growth and development."
Authors: Xiuzhen Kong, Suhang Yu, Yali Xiong, Xiaoyun Song, Lucia Nevescanin-Moreno, Xiaoqing Wei, Jinliang Rao, Hu Zhou, Malcolm J. Bennett, Bipin K. Pandey and Guoqiang Huang.
Current Biology (2024)
Editor's view: Kong et al. provide evidence for the anchorage functions of root hairs in root penetration, a process that is regulated by OsYUC8-mediated auxin biosynthesis and OsAUX1-mediated auxin transport when roots encounter a compacted barrier.
Highlights • Mechanical impedance induced the upregulation of auxin biosynthesis in root apex • Auxin transport from root tip to hair zone promoted root penetration via root hair • Mutants of OsYUC8 and OsAUX1 had trouble penetrating into the compacted layer • Increased root hair length provided greater anchorage force for root penetration
Abstract: "Compacted soil layers adversely affect rooting depth and access to deeper nutrient and water resources, thereby impacting climate resilience of crop production and global food security. Root hair plays well-known roles in facilitating water and nutrient acquisition. Here, we report that root hair also contributes to root penetration into compacted layers. We demonstrate that longer root hair, induced by elevated auxin response during a root compaction response, improves the ability of rice roots to penetrate harder layers. This compaction-induced auxin response in the root hair zone is dependent on the root apex-expressed auxin synthesis gene OsYUCCA8 (OsYUC8), which is induced by compaction stress. This auxin source for root hair elongation relies on the auxin influx carrier AUXIN RESISTANT 1 (OsAUX1), mobilizing this signal from the root apex to the root hair zone. Mutants disrupting OsYUC8 and OsAUX1 genes exhibit shorter root hairs and weaker penetration ability into harder layers compared with wild type (WT). Root-hair-specific mutants phenocopy these auxin-signaling mutants, as they also exhibit an attenuated root penetration ability. We conclude that compaction stress upregulates OsYUC8-mediated auxin biosynthesis in the root apex, which is subsequently mobilized to the root hair zone by OsAUX1, where auxin promotes root hair elongation, improving anchorage of root tips to their surrounding soil environment and aiding their penetration ability into harder layers."
Authors: Zachary Aldiss, Hannah Robinson, Yasmine Lam, Richard Dixon, Peter A. Crisp, Ian D. Godwin, Andrew Borrell, Lee Hickey and Karen Massel.
bioRxiv (2024)
Abstract: "Roots provide the critical interface where plants acquire nutrients and water, but our limited understanding of the genetic controls modulating root system architecture (RSA) in crop species constrains opportunities to develop future cultivars with improved root systems. However, there is vast knowledge of root developmental genes in model plant species, which has the potential to accelerate progress in crops with more complex genomes, particularly given that genome editing protocols are now available for most species. PIN-FORMED2 (PIN2) encodes a root specific polar auxin transporter, where its absence resulted in roots being unable to orient themselves using gravity, producing a significantly wider root system. To explore the role of PIN2 in a cereal crop, we used CRISPR/Cas9 editing to knockout of PIN2 in barley (Hordeum vulgare). Like Arabidopsis, the roots of barley pin2 loss-of-function mutants displayed an agravitropic response at seedling growth stages, resulting in a significantly shallower and wider root system at later growth stages. Notably, despite the significant change in RSA, there was no change in shoot architecture or total shoot biomass. We discuss the future challenges and opportunities to harness the PIN2 pathway to optimise RSA in crops for a range of production scenarios without a shoot trade-off."
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: Simon Moore, George Jervis, Jennifer F. Topping, Chunli Chen, Junli Liu and Keith Lindsey.
Plant Communications (2024)
Abstract: "The interaction between auxin and cytokinin is important in many aspects of plant development. Experimental measurements of both auxin and cytokinin concentration and reporter gene expression clearly show the coexistence of auxin and cytokinin concentration patterning in Arabidopsis root development. However, in the context of crosstalk between auxin, cytokinin and ethylene, little is known about how auxin and cytokinin concentration patterns simultaneously emerge and how they regulate each other in the Arabidopsis root. This work utilizes a wide range of experimental observations to propose a mechanism for simultaneous patterning of auxin and cytokinin concentration. In addition to the regulatory relationships between auxin and cytokinin, the mechanism reveals that ethylene signalling is an important factor in achieving simultaneous auxin and cytokinin patterning, while also predicting other experimental observations. Combining the mechanism with a realistic in silico root model reproduces experimental observations of both auxin and cytokinin patterning. Predictions made by the mechanism can be compared with a variety of experimental observations, including those conducted by our group and other independent experiments reported by other groups. Examples of these predictions include patterning of auxin biosynthesis rate, PIN1 and PIN2 pattern changes in pin3, 4, 7 mutants, cytokinin patterning change in the pls mutant, PLS patterning, as well as various trends in different mutants. This research unravels a plausible mechanism for simultaneous patterning of auxin and cytokinin concentrations in Arabidopsis root development and suggests a key role for ethylene pattern integration."
Author: Xiufen Dong, Xianfeng Liu, Lina Cheng, Ruizhen Li, Siqi Ge, Sai Wang, Yue Cai, Yang Liu, Sida Meng, Cai-Zhong Jiang, Chun-Lin Shi, Tianlai Li, Daqi Fu, Mingfang Qi and Tao Xu.
Journal of Integrative Plant Biology (2024)
Abstract: "Auxin regulates flower and fruit abscission, but how developmental signals mediate auxin transport in abscission remains unclear. Here, we reveal the role of the transcription factor BEL1-LIKE HOMEODOMAIN11 (SlBEL11) in regulating auxin transport during abscission in tomato (Solanum lycopersicum). SlBEL11 is highly expressed in the fruit abscission zone, and its expression increases during fruit development. Knockdown of SlBEL11 expression by RNA interference (RNAi) caused premature fruit drop at the breaker (Br) and 3 d post-breaker (Br+3) stages of fruit development. Transcriptome and metabolome analysis of SlBEL11-RNAi lines revealed impaired flavonoid biosynthesis and decreased levels of most flavonoids, especially quercetin, which functions as an auxin transport inhibitor. This suggested that SlBEL11 prevents premature fruit abscission by modulating auxin efflux from fruits, which is crucial for the formation of an auxin response gradient. Indeed, quercetin treatment suppressed premature fruit drop in SlBEL11-RNAi plants. DNA affinity purification sequencing (DAP-seq) analysis indicated that SlBEL11 induced expression of the transcription factor gene SlMYB111 by directly binding to its promoter. Chromatin immunoprecipitation-quantitative polymerase chain reaction and electrophoretic mobility shift assay showed that S. lycopersicum MYELOBLASTOSIS VIRAL ONCOGENE HOMOLOG111 (SlMYB111) induces the expression of the core flavonoid biosynthesis genes SlCHS1, SlCHI, SlF3H, and SlFLS by directly binding to their promoters. Our findings suggest that the SlBEL11–SlMYB111 module modulates flavonoid biosynthesis to fine-tune auxin efflux from fruits and thus maintain an auxin response gradient in the pedicel, thereby preventing premature fruit drop."
Author: Enrico Scarpella.
Annual Review of Plant Biology (2024)
Abstract: "Leaves form veins whose patterns vary from a single vein running the length of the leaf to networks of staggering complexity where huge numbers of veins connect to other veins at both ends. For the longest time, vein formation was thought to be controlled only by the polar, cell-to-cell transport of the plant hormone auxin; recent evidence suggests that is not so. Instead, it turns out that vein patterning features are best accounted for by a combination of polar auxin transport, facilitated auxin diffusion through plasmodesmata intercellular channels, and auxin signal transduction—though the latter's precise contribution remains unclear. Equally unclear remain the sites of auxin production during leaf development, on which that vein patterning mechanism ought to depend. Finally, whether that vein patterning mechanism can account for the variety of vein arrangements found in nature remains unknown. Addressing those questions will be the exciting challenge of future research."
Authors: Omid Karami, Azadeh Khadem, Arezoo Rahimi, Nicola Zagari, Simon Aigner and Remko Offringa.
The Plant Journal (2024)
Significance Statement: It has been a longstanding question why the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) is so effective in inducing plant regeneration. Here we show that the secret lies in its inefficient export from plant cells and that using natural auxins can be as effective, even leading to improved regeneration, when their efflux is transiently inhibited.
Abstract: "Plant genome editing and propagation are important tools in crop breeding and production. Both rely heavily on the development of efficient in vitro plant regeneration systems. Two prominent regeneration systems that are widely employed in crop production are somatic embryogenesis (SE) and de novo shoot regeneration. In many of the protocols for SE or shoot regeneration, explants are treated with the synthetic auxin analog 2,4-dichlorophenoxyacetic acid (2,4-D), since natural auxins, such as indole-3-acetic acid (IAA) or 4-chloroindole-3-acetic acid (4-Cl-IAA), are less effective or even fail to induce regeneration. Based on previous reports that 2,4-D, compared to endogenous auxins, is not effectively exported from plant cells, we investigated whether efflux inhibition of endogenous auxins could convert these auxins into efficient inducers of SE in Arabidopsis immature zygotic embryos (IZEs). We show that natural auxins and synthetic analogs thereof become efficient inducers of SE when their efflux is transiently inhibited by co-application of the auxin transport inhibitor naphthylphthalamic acid (NPA). Moreover, IZEs of auxin efflux mutants pin2 or abcb1 abcb19 show enhanced SE efficiency when treated with IAA or efflux-inhibited IAA, confirming that auxin efflux reduces the efficiency of Arabidopsis SE. Importantly, in contrast to the 2,4-D system, where only 50–60% of the embryos converted to seedlings, all SEs induced by transport-inhibited natural auxins converted to seedlings. Efflux-inhibited IAA, like 2,4-D, also efficiently induced SE from carrot suspension cells, whereas IAA alone could not, and efflux-inhibited 4-Cl-IAA significantly improved de novo shoot regeneration in Brassica napus. Our data provides new insights into the action of 2,4-D as an efficient inducer of plant regeneration but also shows that replacing this synthetic auxin for efflux-inhibited natural auxin significantly improves different types of plant regeneration, leading to a more synchronized and homogenous development of the regenerated plants."
Authors: Shalini Yadav, Harish Kumar, Monika Mahajan, Sangram Keshari Sahu, Sharad Kumar Singh and Ram Kishor Yadav.
Development (2024)
Summary: Locally produced auxin and transport control the patterning of the shoot apical meristem into the central zone, peripheral zone and rib meristem in Arabidopsis.
Abstract: "The shoot apical meristem (SAM) of higher plants comprises distinct functional zones. The central zone (CZ) is located at the meristem summit and harbors pluripotent stem cells. Stem cells undergo cell division within the CZ and give rise to descendants, which enter the peripheral zone (PZ) and become recruited into lateral organs. Stem cell daughters that are pushed underneath the CZ form rib meristem (RM). To unravel the mechanism of meristem development, it is essential to know how stem cells adopt distinct cell fates in the SAM. Here, we show that meristem patterning and floral organ primordia formation, besides auxin transport, are regulated by auxin biosynthesis mediated by two closely related genes of the TRYPTOPHAN AMINOTRANSFERASE family. In Arabidopsis SAM, TAA1 and TAR2 played a role in maintaining auxin responses and the identity of PZ cell types. In the absence of auxin biosynthesis and transport, the expression pattern of the marker genes linked to the patterning of the SAM is perturbed. Our results prove that local auxin biosynthesis, in concert with transport, controls the patterning of the SAM into the CZ, PZ and RM."
Authors: Chao Tan, Suxin Li, Jia Song, Xianfu Zheng, Hao Zheng, Weichang Xu, Cui Wan, Tan Zhang, Qiang Bian and Shuzhen Men.
Communications Biology (2024)
Editor's view: Physiological and molecular studies have shown that 3,4-dichlorophenylacetic acid (Dcaa) has auxin-like activity and acts through the auxin signaling pathway in plants. This provides a basis for the application of Dcaa in agricultural practice.
Abstract: "Auxins and their analogs are widely used to promote root growth, flower and fruit development, and yield in crops. The action characteristics and application scope of various auxins are different. To overcome the limitations of existing auxins, expand the scope of applications, and reduce side effects, it is necessary to screen new auxin analogs. Here, we identified 3,4-dichlorophenylacetic acid (Dcaa) as having auxin-like activity and acting through the auxin signaling pathway in plants. At the physiological level, Dcaa promotes the elongation of oat coleoptile segments, the generation of adventitious roots, and the growth of crop roots. At the molecular level, Dcaa induces the expression of auxin-responsive genes and acts through auxin receptors. Molecular docking results showed that Dcaa can bind to auxin receptors, among which TIR1 has the highest binding activity. Application of Dcaa at the root tip of the DR5:GUS auxin-responsive reporter induces GUS expression in the root hair zone, which requires the PIN2 auxin efflux carrier. Dcaa also inhibits the endocytosis of PIN proteins like other auxins. These results provide a basis for the application of Dcaa in agricultural practices."
Authors: Z. Tang, Y. Zhang, Y. Ma, D. Zhao, J. Dong and H. Zhang.
Biologia Plantarum (2024)
Abstract: "Roots are important for plant anchoring, water and nutrient absorption, and other physiological processes. Gravity is a primary determinant of the spatial distribution of plant roots in the soil. Therefore, in-depth understanding of the molecular mechanisms and biochemical networks of root responses to gravity has both theoretical and practical significance in guiding the genetic improvement of plants. Gravitropism, the process through which plants sense the direction of gravity and respond by making the roots grow downward and the stem grow upward, has been widely studied in roots. The perception of gravity and the gravitational growth of roots, key steps in root growth and development, are regulated by auxins and other factors. Here, we review the latest progress in the regulation of root gravitropism by hormone signals and environmental factors from a molecular perspective, and look forward to the direction of future research on root gravitropism."
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Authors: Hongjiao Jiang, Lijun Xie, Zhiqun Gu, Hongyao Mei, Haohao Wang, Jing Zhang, Minmin Wang, Yiteng Xu, Chuanen Zhou and Lu Han.
The Plant Journal (2024)
Significance Statement: Auxins play important roles in various aspects of plant growth and development. In Medicago truncatula, orthologs of PIN1-mediated auxin transportation are involved in various developmental processes and amino acid biosynthesis and metabolism in seeds, which offers insights into the molecular mechanisms governing seed size regulation in crops.
Abstract: "The regulation of seed development is critical for determining crop yield. Auxins are vital phytohormones that play roles in various aspects of plant growth and development. However, its role in amino acid biosynthesis and metabolism in seeds is not fully understood. In this study, we identified a mutant with small seeds through forward genetic screening in Medicago truncatula. The mutated gene encodes MtPIN4, an ortholog of PIN1. Using molecular approaches and integrative omics analyses, we discovered that auxin and amino acid content significantly decreased in mtpin4 seeds, highlighting the role of MtPIN4-mediated auxin distribution in amino acid biosynthesis and metabolism. Furthermore, genetic analysis revealed that the three orthologs of PIN1 have specific and overlapping functions in various developmental processes in M. truncatula. Our findings emphasize the significance of MtPIN4 in seed development and offer insights into the molecular mechanisms governing the regulation of seed size in crops. This knowledge could be applied to enhance crop quality by targeted manipulation of seed protein regulatory pathways."
Authors: Han Tang, Adrijana Smoljan, Minxia Zou, Yuzhou Zhang, Kuan-Ju Lu and Jiří Friml.
bioRxiv (2024)
Abstract: "The plant hormone auxin and its directional transport are crucial for growth and development. PIN auxin transporters, on account of their polarized distribution, are instrumental in guiding auxin flow across tissues. Based on protein length and subcellular localization, the PIN family is classified into two groups: plasma membrane (PM)-localized long PINs and endoplasmic reticulum (ER)-localized short PINs. The origin of PINs was traced to the alga Klebsormidium, with a single PM-localized long KfPIN. Bryophytes, the earliest land plant clade, represent the initial clade harboring the short PINs. We tracked the evolutionary trajectory of the short PINs and explored their function and localization in the model bryophyte Marchantia polymorpha, which carries four short and one long PIN. Our findings reveal that all short MpPINs can export auxin, and they are all PM-localized with MpPINX and MpPINW exhibiting asymmetric distribution. We identified a unique miniW domain within the MpPINW hydrophilic loop region, which is sufficient for its PM localization. Phosphorylation site mutations within the miniW domain abolish the PM localization. These findings not only identify the essential sequence determinant of PINs' PM localization but also provide a unique insight into the evolution of ER-localized PINs. Short MpPINW, which is evolutionarily positioned between the ancestral long PINs and contemporary short PINs, still preserves the critical region essential for its PM localization. We propose that throughout land plant evolution, the unique miniW domain has been gradually lost thus converting the PM-localized short PINs in bryophytes to ER-localized short PINs in angiosperms."
Authors: Riccardo Fusi, Sara Giulia Milner, Serena Rosignoli, Riccardo Bovina, Cristovão De Jesus Vieira Teixeira, Haoyu Lou, Brian S. Atkinson, Aditi N. Borkar, Larry M. York, Dylan H. Jones, Craig J. Sturrock, Nils Stein, Martin Mascher, Roberto Tuberosa, Devin O'Connor, Malcolm J. Bennett, Anthony Bishopp, Silvio Salvi and Rahul Bhosale.
New Phytologist (2024)
Abstract: "Barley (Hordeum vulgare) is an important global cereal crop and a model in genetic studies. Despite advances in characterising barley genomic resources, few mutant studies have identified genes controlling root architecture and anatomy, which plays a critical role in capturing soil resources. Our phenotypic screening of a TILLING mutant collection identified line TM5992 exhibiting a short-root phenotype compared with wild-type (WT) Morex background. Outcrossing TM5992 with barley variety Proctor and subsequent SNP array-based bulk segregant analysis, fine mapped the mutation to a cM scale. Exome sequencing pinpointed a mutation in the candidate gene HvPIN1a, further confirming this by analysing independent mutant alleles. Detailed analysis of root growth and anatomy in Hvpin1a mutant alleles exhibited a slower growth rate, shorter apical meristem and striking vascular patterning defects compared to WT. Expression and mutant analyses of PIN1 members in the closely related cereal brachypodium (Brachypodium distachyon) revealed that BdPIN1a and BdPIN1b were redundantly expressed in root vascular tissues but only Bdpin1a mutant allele displayed root vascular defects similar to Hvpin1a. We conclude that barley PIN1 genes have sub-functionalised in cereals, compared to Arabidopsis (Arabidopsis thaliana), where PIN1a sequences control root vascular patterning."
Author: Martin Balcerowicz
The Plant Journal (2024)
Excerpts: " The synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) stands out as the most effective inducer of somatic embryogenesis, whereas other synthetic auxins like 1-naphthaleneacetic acid (1-NAA) and natural auxins such as indole-3-acetic acid (IAA) yield inferior results (Gaj, 2004). However, the underlying reasons for 2,4-D's superior efficacy have remained elusive."
"He observed that 2,4-D is perceived by the same canonical auxin signalling pathway that senses IAA, suggesting that the superior effectiveness of 2,4-D is not due to differences in perception or signalling (Karami et al., 2023). However, 2,4-D differs in its cellular availability, accumulating to higher levels within plant cells and exhibiting lower efficiency as a substrate for auxin efflux carriers than IAA (Delbarre et al., 1996; Seifertová et al., 2014). In the highlighted study, Karami and colleagues investigated whether these properties contribute to making 2,4-D a potent inducer of somatic embryogenesis."
"Given that 2,4-D is a poor substrate for auxin efflux, they examined the impact of the auxin efflux inhibitor naphthylphthalamic acid (NPA) on the efficacy of various synthetic and natural auxins to induce embryo formation. Strikingly, combined treatment with IAA and NPA was as effective in generating somatic embryos as treatment with 2,4-D (Figure 1). NPA also enhanced the efficacy of other natural and synthetic auxins, with highest embryo numbers observed for a combination of 4-chloroindole-3-acetic acid (4-Cl-IAA) and NPA. Monitoring the expression of a pDR5::GUS auxin reporter showed that combined treatment of IAA or synthetic auxins with NPA elicited a stronger auxin response than treatment with either compound alone. These observations demonstrate that many auxins can efficiently induce somatic embryogenesis when auxin efflux is inhibited, likely due to increased intracellular auxin levels."
Authors: Yu Chen, Yansong Fu, Yanwei Xia, Youzhi Miao, Jiahui Shao, Wei Xuan, Yunpeng Liu, Weibing Xun, Qiuyan Yan, Qirong Shen and Ruifu Zhang.
Cell Reports (2023)
Editor's view: Chen et al. show that 2-AA, a compound identified from T. guizhouense NJAU4742, promotes plant lateral root development via the canonical auxin pathway as a stimulator and increases ROS deposition in the Casparian strip as an IAA mimic, which finally enhances endodermal cell wall remodeling and facilitates lateral root emergence.
Highlights: • 2-AA identified from T. guizhouense NJAU4742 can promote lateral root development • 2-AA regulates auxin signaling and transport in the canonical auxin pathway • 2-AA enhances endodermal cell wall remodeling via an RBOHF-induced ROS burst
Abstract: "Trichoderma spp. have evolved the capacity to communicate with plants by producing various secondary metabolites (SMs). Nonhormonal SMs play important roles in plant root development, while specific SMs from rhizosphere microbes and their underlying mechanisms to control plant root branching are still largely unknown. In this study, a compound, anthranilic acid (2-AA), is identified from T. guizhouense NJAU4742 to promote lateral root development. Further studies demonstrate that 2-AA positively regulates auxin signaling and transport in the canonical auxin pathway. 2-AA also partly rescues the lateral root numbers of CASP1pro:shy2-2, which regulates endodermal cell wall remodeling via an RBOHF-induced reactive oxygen species burst. In addition, our work reports another role for microbial 2-AA in the regulation of lateral root development, which is different from its better-known role in plant indole-3-acetic acid biosynthesis. In summary, this study identifies 2-AA from T. guizhouense NJAU4742, which plays versatile roles in regulating plant root development."
Authors: Haihai Wang, Yongcai Huang, Yujie Li, Yahui Cui, Xiaoli Xiang, Yidong Zhu, Qiong Wang, Xiaoqing Wang, Guangjin Ma, Qiao Xiao, Xing Huang, Xiaoyan Gao, Jiechen Wang, Xiaoduo Lu, Brian A. Larkins, Wenqin Wang and Yongrui Wu.
Nature Communications (2024
Editor's view: The mutation of ARFTF17 results in the development of flint kernel architecture in dent maize by reducing excessive pericarp length. This discovery holds significant potential for enhancing grain quality in elite, high-yielding dent maize hybrids.
Abstract: "Dent and flint kernel architectures are important characteristics that affect the physical properties of maize kernels and their grain end uses. The genes controlling these traits are unknown, so it is difficult to combine the advantageous kernel traits of both. We found mutation of ARFTF17 in a dent genetic background reduces IAA content in the seed pericarp, creating a flint-like kernel phenotype. ARFTF17 is highly expressed in the pericarp and encodes a protein that interacts with and inhibits MYB40, a transcription factor with the dual functions of repressing PIN1 expression and transactivating genes for flavonoid biosynthesis. Enhanced flavonoid biosynthesis could reduce the metabolic flux responsible for auxin biosynthesis. The decreased IAA content of the dent pericarp appears to reduce cell division and expansion, creating a shorter, denser kernel. Introgression of the ARFTF17 mutation into dent inbreds and hybrids improved their kernel texture, integrity, and desiccation, without affecting yield."
Authors: Lulu Zheng, Yongfeng Hu, Tianzhao Yang, Zhen Wang, Daoyuan Wang, Letian Jia, Yuanming Xie, Long Luo, Weicong Qi, Yuanda Lv, Tom Beeckman, Wei Xuan and Yi Han.
Nature Communications (2024)
One-sentence summary: This study reports that the SOMBRERO, a root cap-localized transcription factor, determines root halotropic response to salt stress via spatiotemporally modulating AUX1-dependent auxin redistribution in the root tip.
Abstract: "Plants are capable of altering root growth direction to curtail exposure to a saline environment (termed halotropism). The root cap that surrounds root tip meristematic stem cells plays crucial roles in perceiving and responding to environmental stimuli. However, how the root cap mediates root halotropism remains undetermined. Here, we identified a root cap-localized NAC transcription factor, SOMBRERO (SMB), that is required for root halotropism. Its effect on root halotropism is attributable to the establishment of asymmetric auxin distribution in the lateral root cap (LRC) rather than to the alteration of cellular sodium equilibrium or amyloplast statoliths. Furthermore, SMB is essential for basal expression of the auxin influx carrier gene AUX1 in LRC and for auxin redistribution in a spatiotemporally-regulated manner, thereby leading to directional bending of roots away from higher salinity. Our findings uncover an SMB-AUX1-auxin module linking the role of the root cap to the activation of root halotropism."
Authors: Ivan Kulich, Julia Schmid, Anastasia Teplova, Linlin Qi and Jiří Friml.
eLife (2024)
Editor's view: This fundamental study addresses the earliest events that enable plant roots to reorient growth in response to gravity. Compelling molecular and cell biological data establish that plasma membrane localization of the LAZY or NEGATIVE GRAVITROPIC RESPONSE OF ROOTS (NGR) protein family is required for rapid and polar redirection of D6 protein kinase, an activator of the PIN3 auxin transporter. This work complements and extends recent publications on the NGR family in gravity sensing (PMID: 37741279 and PMID: 37561884). Collectively these papers advance our understanding of rapid plant gravity sensing and response.
Abstract: "Root gravitropic bending represents a fundamental aspect of terrestrial plant physiology. Gravity is perceived by sedimentation of starch-rich plastids (statoliths) to the bottom of the central root cap cells. Following gravity perception, intercellular auxin transport is redirected downwards leading to an asymmetric auxin accumulation at the lower root side causing inhibition of cell expansion, ultimately resulting in downwards bending. How gravity-induced statoliths repositioning is translated into asymmetric auxin distribution remains unclear despite PIN auxin efflux carriers and the Negative Gravitropic Response of roots (NGR) proteins polarize along statolith sedimentation, thus providing a plausible mechanism for auxin flow redirection. In this study, using a functional NGR1-GFP construct, we visualized the NGR1 localization on the statolith surface and plasma membrane (PM) domains in close proximity to the statoliths, correlating with their movements. We determined that NGR1 binding to these PM domains is indispensable for NGR1 functionality and relies on cysteine acylation and adjacent polybasic regions as well as on lipid and sterol PM composition. Detailed timing of the early events following graviperception suggested that both NGR1 repolarization and initial auxin asymmetry precede the visible PIN3 polarization. This discrepancy motivated us to unveil a rapid, NGR-dependent translocation of PIN-activating AGCVIII kinase D6PK towards lower PMs of gravity-perceiving cells, thus providing an attractive model for rapid redirection of auxin fluxes following gravistimulation."
Authors: Jerry D. Cohen and Lucia C. Strader.
The Plant Cell (2024)
Abstract: "The phytohormone auxin is at times called the master regulator of plant processes and has been shown to be a central player in embryo development, the establishment of the polar axis, early aspects of seedling growth, as well as growth and organ formation during later stages of plant development. The Plant Cell has been key, since the inception of the journal, to developing an understanding of auxin biology. Auxin regulated plant growth control is accomplished by both changes in the levels of active hormones and the sensitivity of plant tissues to these concentration changes. In this historical review, we chart auxin research as it has progressed in key areas and highlighting the role The Plant Cell played in these scientific developments. We focus on understanding auxin-responsive genes, transcription factors, reporter constructs, perception and signal transduction processes. Auxin metabolism is discussed from the development of tryptophan auxotrophic mutants, the molecular biology of conjugate formation and hydrolysis, indole-3-butyric acid metabolism and transport, and key steps in indole-3-acetic acid biosynthesis, catabolism and transport. This progress leads to an expectation of a more comprehensive understanding of the systems biology of auxin and the spatial and temporal regulation of cellular growth and development."
Authors: Andrea R. Kohler, Andrew Scheil, Joseph L. Hill, Jr., Jeffrey R. Allen, Jameel M. Al-Haddad, Charity Z. Goeckeritz, Lucia C. Strader, Frank W. Telewski and Courtney A. Hollender.
Plant Physiology (2024)
One-sentence summary: Polar auxin transport associated with gravitropism and lateral shoot and root orientation requires the highly conserved WEEP protein in peach.
Abstract: "Trees with weeping shoot architectures are valued for their beauty and are a resource for understanding how plants regulate posture control. The peach (Prunus persica) weeping phenotype, which has elliptical downward arching branches, is caused by a homozygous mutation in the WEEP gene. Little is known about the function of WEEP despite its high conservation throughout Plantae. Here, we present the results of anatomical, biochemical, biomechanical, physiological, and molecular experiments that provide insight into WEEP function. Our data suggest that weeping peach trees do not have defects in branch structure. Rather, transcriptomes from the adaxial (upper) and abaxial (lower) sides of standard and weeping branch shoot tips revealed flipped expression patterns for genes associated with early auxin response, tissue patterning, cell elongation, and tension wood development. This suggests that WEEP promotes polar auxin transport toward the lower side during shoot gravitropic response, leading to cell elongation and tension wood development. In addition, weeping peach trees exhibited steeper root systems and faster lateral root gravitropic response. This suggests that WEEP moderates root gravitropism and is essential to establishing the set-point angle of lateral roots from the gravity vector. Additionally, size-exclusion chromatography indicated that WEEP proteins self-oligomerize, like other proteins with sterile alpha motif (SAM) domains. Collectively, our results from weeping peach provide insight into polar auxin transport mechanisms associated with gravitropism and lateral shoot and root orientation."
Authors: HuiFang Xu and Xu Chen.
Scientia Sinica Vitae (2024)
Abstract: "Auxin is widely involved in plant growth and development and its adaptation to the environment, and is the most important hormone in plants. In the past two decades, studies based on the model plant Arabidopsis thaliana have confirmed that establishment of auxin gradient through biosynthesis, metabolism, polar transport and signaling pathways determines organogenesis and polarity of plant organs. With the development of gene editing and molecular breeding research, how to apply the theoretical results related to auxin pathway in crop improvement, and coordinate the ideal plant/root type of crops through the selection and combination of dominant genes is a key issue in this field. In this review, we summarize the latest research progress in auxin field in the past five years, refer to the contribution of auxin to the improvement of rice agronomic traits, and discuss and look forward to the possibility of auxin application on soybean breeding."
Authors: Maria Florencia Ercoli, Alexandra Shigenaga, Artur Teixeira de Araujo Junior, Rashmi Jain and Pamela Ronald.
bioRxiv (2024)
Abstract: "In Arabidopsis roots, growth initiation and cessation are organized into distinct zones. How regulatory mechanisms are integrated to coordinate these processes and maintain proper growth progression over time is not well understood. Here, we demonstrate that the peptide hormone PLANT PEPTIDE CONTAINING SULFATED TYROSINE 1 (PSY1) promotes root growth by controlling cell elongation. Higher levels of PSY1 lead to longer differentiated cells with a shootward displacement of characteristics common to mature cells. PSY1 activates genes involved in the biosynthesis of flavonols, a group of plant-specific secondary metabolites. Using genetic and chemical approaches, we show that flavonols are required for PSY1 function. Flavonol accumulation downstream of PSY1 occurs in the differentiation zone, where PSY1 also reduces auxin and reactive oxygen species (ROS) activity. These findings support a model where PSY1 signals the developmental-specific accumulation of secondary metabolites to regulate the extent of cell elongation and the overall progression to maturation."
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