植物激素生长素调控植物生长发育及环境适应的多个过程,包括胚胎发育、器官发生和向性生长等.生长素发挥生物学功能主要依赖于经典的TIR1/AFB-auxin-Aux/IAA-ARF信号转导途径.其中,由4个保守结构域组成的典型Aux/IAA蛋白作为TIR1/AFB的共受体在生长素信号转导过程中发挥关键作用.然而,近年来发现缺乏保守结构域的非典型Aux/IAA蛋白也参与生长素的应答与调控作用.该文从蛋白结构、生物学功能及参与生长素信号转导等方面综述了非典型Aux/IAA蛋白的研究进展,探讨和展望了非典型Aux/IAA蛋白的研究方向.

活性氧与生长素在植物生长发育过程中均发挥着重要作用,它们的信号转导与调控机制一直是植物科学研究的重要领域.细胞核内经典通路TIR1/AFB-Aux/IAA-ARF通过转录调控来介导生长素信号,然而生长素信号在细胞质和细胞核之间进行传递的过程尚不清楚.近期,华东师范大学李超团队发现氧化还原信号参与了受生长素调控的根毛发育过程.生长素受体蛋白TIR1/AFB2 发生氧化后促进其向细胞核迁移,启动根毛发育的生长素转录信号,而这一过程同时受上游FER/LLG1-RAC/ROP-RBOHC分子模块的调控.该研究深入解析了根毛发育过程中生长素与氧化还原信号的交谈模式,是蛋白质氧化翻译后修饰调控生物学过程的经典范式.

The discovery and characterization of the nuclear AUX-indole-3-acetic acid(IAA)-TIR1/AFB auxin co-receptor complex in the model plant Arabidopsis at the outset of the 21st century was a hallmark event in biological science.Two decades of subsequent discovery demonstrated the central role of the nuclear auxin perception mechanism in virtually every stage of Viridiplantae development.Nuclear auxin receptors were also shown to be essential for later stages of differential hypocotyl,coleoptile,and root tropisms,but the most rapid responses were found to persist in Arabidopsis mutants lacking important components of the receptor mechanism(Napier,2021;Peer and Murphy,2022).In particular,rapid cellular elongation observed in tropic growth or after experimental application of auxin is mediated by H+extrusion across the plasma membrane(PM).The resulting apoplastic acidification activates extensin proteins that loosen cell walls to allow for turgor-driven cellular elongation.These initial events occur in a time frame that precludes nuclear signaling and now can be clearly attributed to a cell surface auxin co-receptor complex and downstream signaling intermediaries.

Dear Editor,Auxin is the major plant hormone regulating growth and devel-opment(Friml,2022).Forward genetic approaches have identified major components of auxin signaling and established the canonical mechanism mediating transcriptional and thus developmental reprogramming in Arabidopsis thaliana.In this textbook view,TRANSPORT INHIBITOR RESPONSE 1(TIR1)/AUXIN-SIGNALING F-BOX(AFB)proteins are auxin receptors,which act as F-box subunits determining the substrate specificity of the Skp1-Cullin1-F box protein(SCF)type E3 ubiquitin ligase complex.

The phytohormone auxin triggers root growth inhibition within seconds via a non-transcriptional pathway.Among members of the TIR1/AFB auxin receptor family,AFB1 has a primary role in this rapid response.However,the unique features that confer this specific function have not been identified.Here we show that the N-terminal region of AFB1,including the F-box domain and residues that contribute to auxin binding,is essential and sufficient for its specific role in the rapid response.Substitu-tion of the N-terminal region of AFB1 with that of TIR1 disrupts its distinct cytoplasm-enriched localiza-tion and activity in rapid root growth inhibition by auxin.Importantly,the N-terminal region of AFB1 is indispensable for auxin-triggered calcium influx,which is a prerequisite for rapid root growth inhibition.Furthermore,AFB1 negatively regulates lateral root formation and transcription of auxin-induced genes,suggesting that it plays an inhibitory role in canonical auxin signaling.These results suggest that AFB1 may buffer the transcriptional auxin response,whereas it regulates rapid changes in cell growth that contribute to root gravitropism.

植物激素生长素影响植物生长中几乎所有的方面,包括植物细胞的生长、分裂和分化,以及植物从胚胎发育到生殖发育的各个过程中.生长素的研究主要围绕合成与代谢、运输和信号转导而展开.植物细胞对生长素的响应和信号转导主要通过细胞核内的TIR1/AFB-Aux/IAA-ARF信号通路来完成.另外,细胞表面起始的信号通路主要调控生长素的快速响应,而SKP2A通路介导了生长素对细胞周期的调控.概述了三条生长素信号转导通路目前的研究进展,并介绍了生长素信号转导中具体的研究工具.基于该领域的研究现状,提出了生长素信号转导方面有待进一步研究的方向,包括TIR1/AFB-Aux/IAA-ARF复合体的调控机制研究、生长素调控冠部和根部生长的差异性研究、以及对细胞表面起始信号途径的进一步探索.

In Arabidopsis thaliana,canonical auxin-dependent gene regulation is mediated by 23 transcription factors from the AUXIN RESPONSE FACTOR(ARF)family that interact with auxin/indole acetic acid repressors(Aux/IAAs),which themselves form co-receptor complexes with one of six TRANSPORT INHIBITOR1/AUXIN-SIGNALLlNG F-BOX(TIR1/AFB)proteins.Different combinations of co-receptors drive specific sensing outputs,allowing auxin to control a myriad of processes.ARF6 and ARF8 are positive regulators of adventitious root initiation upstream of jasmonate,but the exact auxin co-receptor complexes control-ling the transcriptional activity of these proteins has remained unknown.Here,using loss-of-function mu-tants we show that three Aux/IAA genes,IAA6,IAA9,and IAA 17,act additively in the control of adventitious root(AR)initiation.These three IAA proteins interact with ARF6 and/or ARF8 and likely repress their activity in AR development.We show that TIR1 and AFB2 are positive regulators of AR formation and TIR1 plays a dual role in the control of jasmonic acid(JA)biosynthesis and conjugation,as several JA biosynthesis genes are up-regulated in the tir1-1 mutant.These results lead us to propose that in the presence of auxin,TIR1 and AFB2 form specific sensing complexes with IAA6,IAA9,and/or IAA 17 to modulate JA homeostasis and control AR initiation.

植物激素生长素参与调控植物生长发育的各个过程,包括胚胎发育、器官发生和向性运动等.植物通过协调生长素的合成代谢、极性运输以及信号转导来实现对不同生长发育过程的精准调控.生长素的功能依赖于其信号被感知后经由信号转导通路转换为下游复杂多样的反应.经典的生长素信号转导通路阐明了细胞核内从SCFTIR1/AFB受体到Aux/IAA蛋白的泛素化降解最终通过ARF转录因子调控基因转录的完整生长素响应过程.该核内信号通路揭示了生长素转录调控生长发育的诸多分子机制,但植物生长发育调控过程中仍有许多生长素响应过程无法通过该经典信号通路解析.重点阐述生长素非经典信号通路的调控机制及其对植物生长发育的重要作用,并讨论和展望生长素非经典信号通路研究目前所面临的挑战以及研究前景.

Auxin,one of the first identified and most widely studied phytohormones,has been and will re-main a hot topic in plant biology.After more than a century of passionate exploration,the mys-teries of its synthesis,transport,signaling,and metabolism have largely been unlocked.Due to the rapid development of new technologies,new methods,and new genetic materials,the study of auxin has entered the fast lane over the past 30 years.Here,we highlight advances in understanding auxin signaling,including auxin perception,rapid auxin responses,TRANSPORT INHIBITOR RESPONSE 1 and AUXIN SIGNALING F-boxes (TIR1/AFBs)-mediated transcriptional and non-transcriptional branches,and the epi-genetic regulation of auxin signaling.We also focus on feedback inhibition mechanisms that prevent the over-amplification of auxin signals.In addition,we cover the TRANSMEMBRANE KINASE-mediated non-canonical signaling,which converges with TIR1/AFBs-mediated transcriptional regulation to coordinate plant growth and development.The identification of additional auxin signaling components and their regulation will continue to open new avenues of research in this field,leading to an increasingly deeper,more comprehensive understanding of how auxin signals are interpreted at the cellular level to regulate plant growth and development.

The phytohormone auxin is essential for plant growth and devel-opment as well as cellular and systemic responses to environ-mental cues.The auxin receptor TRANSPORT INHIBITOR RESPONSE 1/AUXIN-SIGNALING F-BOX(TIR1/AFB)lies within an E3 ubiquitin ligase complex(SCFTIR1/AFB)consisting of Skp,Cullin,and F-box proteins.Auxin brings together TIR1/AFB and the transcriptional repressor Auxin/INDOLE-3-ACETIC ACID(Aux/IAA),which allows the SCFTIR1/AFB complex to transfer acti-vated ubiquitin to Aux/IAA,thereby targeting it for proteasomal degradation.