生长素调控植物重力反应的分子机理研究

重力反应是植物对环境的一种适应现象。生长素参与植物环境适应与发育调控的过程,重力反应过程的核心之一是在重力反应器官形成生长素的浓度梯度,诱导下游基因的差异表达。生长素的合成、代谢、极性运输及信号转导在此过程中发挥了关键作用。该文以拟南芥和水稻的研究为基础,综述了近几年对生长素调控植株重力反应的分子机理的研究进展,并对该领域未来的研究进行展望。     

作物分枝的分子调控研究进展

分枝的数量及角度是决定作物株型的重要农艺性状.有效分枝数决定着作物的穗数或荚果数,进而决定着作物的产量;而分枝角度与光合效率、种植密度和抗病性密切相关,不仅影响作物的产量,也会影响作物的品质.由于分枝在作物生产中具有十分重要的作用,吸引了越来越多的研究者的注意,多个与分枝性状相关的关键基因被鉴定,分枝数目调控的分子机制...     

植物向重力反应中PIN-FORMED介导的生长素极性运输调控

植物的向性,即植物对光或重力等环境刺激信号产生的定向生长反应。在向重力性反应中,植物器官将重力感知为定向环境信号,来控制其器官的生长方向以促进生存。植物激素生长素及其极性运输在植物向重力反应中起着决定性的调控作用。质膜定位的生长素输出蛋白PIN-FORMED(PIN)通过动态的亚细胞极性定位,改变生长素运输的方向以响应环境刺激,由此植物器官间建立的生长素浓度梯度是细胞差异化伸长和器官弯曲的基础,来调控植物的形态建成和生长发育过程。本文主要讨论发生在植物重力感受细胞内早期重力感知和信号转导机制的最新研究进展、PIN介导的生长素极性运输、PIN的极性定位以及质膜蛋白丰度的调控机制等。     

植物干细胞调控的分子机制

植物干细胞位于茎尖分生组织区和根尖分生组织区,是植物胚后发育中新的器官产生的源泉.近几年,在干细胞及其周围组织区发现了一些与干细胞稳态维持有关的基因,这些基因产物与外源性信号（如生长素）一起组成复杂的调控网络控制植物的生长和发育.表观遗传修饰作为控制基因表达的一种方式也对植物干细胞有重要的影响.该文介绍近几年植物干细胞分化调控的最新进展.     

植物重力生物学的Cholodny—Went假说研究进展

本文着重介绍生长素在植物向重性生长反应中作用的研究进展、存在问题和研究方向。     

植物侧根发育的研究进展

侧根是植物根系的重要组成部分,其发生和发育受到内源植物激素和外界环境因素的共同影响。生长素在侧根发生起始、侧根原基的发育和侧根突破母体表皮等阶段均发挥关键作用。研究侧根的发育和形态解剖结构以及信号调控途径等,都具有重要的理论和实践意义。本文结合近年来的研究进展,综述了拟南芥和水稻侧根发育的详细过程和影响因素,重点关注生长素在侧根原基发生和发育过程中的作用。     

生长素信号转导途径与植物胁迫反应相互作用的证据

生长素影响植物多种生理过程,有报道显示生长素可能影响植物对逆境胁迫的反应。我们利用cDNA阵列技术鉴定拟南芥（Arabiopsis thaliana (L.)Heynh.）的生长素应答基因,发现多个胁迫应答基因受生长素抑制,包括Arabidopsis homolog of MEK kinasel（ATMEKK1）,RelA/SpoT homolog 3(At-RSH3),Catalase 1(Cat1)和Ferriitn 1(Fer1)。说明生长素可调节胁迫应答基因的表达,此外,我们还证明吲哚乙酸（LAA）合成途径中的腈水解酶基因nitrilase 1(NIT1)和nitrilae 2（NIT2）受盐胁迫诱导,提示在逆境条件下1AA的合成可能随之增加,我们利用生长素不敏感突变体研究生长素与逆境反应相互作用的信号转导,发现胁迫应答基因在野生型和生长素不敏感突变体auxin resistant 2（axr2）中可被盐胁迫诱导,而在auxin resitant1-3（axl-3)中则不被诱导,说明生长素与逆境胁迫反应的相互作用可能发生在泛素途径。     

生长素调控根向重力性在植物激素实验教学中的应用

根是植物重要的器官,其向重力生长是由生长素的浓度与分布所调控的,是植物对环境信号作出的生理反应。本文对模式植物拟南芥的根进行改变重力方向的刺激,借助含有响应生长素的分子标记的DR5rev:GFP、DR5:3xVenus和DR5:GUS转基因株系,可视化在重力方向改变刺激下根尖两侧的生长素不对称分布,同时观察生长素极性运输载体的相关突变体pin2-T、aux1-T在重力方向改变刺激下的表型,帮助学生深入理解生长素调控拟南芥根向重力生长的生理机制。     

生长素通过调节赤霉素应答反应促进拟南芥根伸长

前人已证实 ,赤霉素通过减少DELLA蛋白的细胞核内浓度从而降低DELLA蛋白的生长抑制效应 ,拟南芥植物细胞核编码至少 5种DELLA蛋白 (GAI、RGA、RGL1、RGL2和RGL3 )。本实验证实缺少RGA(rga_2 4gal_3 )或GAI(gai_t6gal_3 )的根比突变体gal_3的长。同时缺少RGA(rga_2 4gal_3 )或GAI(gai_t6gal_3 )极大地抑制了gal_3的表型 (短根 )。RGA和GAI共同调节根和茎的伸长。它们的抑制作用可被赤霉素所抵消。当生长素运输或信号传导减弱时使得赤霉素所诱导的根细胞核RGA蛋白降解速度变慢。这说明 :通过生长素参与赤霉素介导的DELL…     

生长素信号转导途径与植物胁迫反应相互作用的证据

生长素影响植物多种生理过程,有报道显示生长素可能影响植物对逆境胁迫的反应.我们利用cDNA阵列技术鉴定拟南芥(Arabidopsis thaliana (L.) Heynh.)的生长素应答基因,发现多个胁迫应答基因受生长素抑制,包括Arabidopsis homolog of MEK kinase1 (ATMEKK1),RelA/SpoT homolog 3 (At-RSH3),Catalase 1 (Cat1) 和Ferritin 1 (Fer1),说明生长素可调节胁迫应答基因的表达.此外,我们还证明吲哚乙酸(IAA)合成途径中的腈水解酶基因nitrilase 1 (NIT1) 和nitrilase 2 (NIT2) 受盐胁迫诱导,提示在逆境条件下IAA的合成可能随之增加.我们利用生长素不敏感突变体研究生长素与逆境反应相互作用的信号转导,发现胁迫应答基因在野生型和生长素不敏感突变体auxin resistant 2 (axr2) 中可被盐胁迫诱导,而在auxin resistant 1-3 (axr1-3)中则不被诱导,说明生长素与逆境胁迫反应的相互作用可能发生在泛素途径.     

LAZY1 controls rice shoot gravitropism through regulating polar auxin transport

Tiller angle of rice （Oryza sativa L.） is an important agronomic trait that contributes to grain production, and has long attracted attentions of breeders for achieving ideal plant architecture to improve grain yield. Although enormous efforts have been made over the past decades to study mutants with extremely spreading or compact tillers, the molecular mechanism underlying the control of tiller angle of cereal crops remains unknown. Here we report the cloning of the LAZY1 （LA1） gene that regulates shoot gravitropism by which the rice tiller angle is controlled. We show that LA1, a novel grass-specific gene, is temporally and spatially expressed, and plays a negative role in polar auxin transport （PAT）. Loss-of-function of LA1 enhances PAT greatly and thus alters the endogenous IAA distribution in shoots, leading to the reduced gravitropism, and therefore the tiller-spreading phenotype of rice plants.     

A novel root gravitropism mutant of Arabidopsis thaliana exhibiting altered auxin physiology

A root gravitropism mutant was isolated from the DuPont Arabidopsis thaliana T-DNA insertional mutagenesis collection. This mutant has reduced root gravitropism, hence the name rgrl. Roots of rgrl are shorter than those of wild-type, and they have reduced lateral root formation. In addition, roots of rgrl coil clockwise on inclined agar plates, unlike wild-type roots which grow in a wavy pattern. The rgrl mutant has increased resistance, as measured by root elongation, to exogenously applied auxins (6-fold to indole-3-acetic acid, 3-fold to 2,4-dichlorophenoxyacetic acid, and 2-fold to napthyleneacetic acid). It is also resistant to polar auxin transport inhibitors (2-fold to triiodobenzoic acid and 3- to 5-fold lo napthyleneacetic acid). The rgrl mutant does not appear to be resistant to other plant hormone classes. When grown in the presence of 10^?2 M 2.4-dichlorophenoxyacetic acid, rgrl roots have fewer root hairs than wild type. All these rgrl phenotypes are Mendelian recessives. Complementation tests indicate that rgrl is not allelic to previously characterized agravitropic or auxin-resistant mutants. The rgrl locus was mapped using visible markers to 1.4 ± 0.6 map units from the CHI locus at 1–65.4. The rgrl mutation and the T-DNA cosegregate, suggesting that rgrl was caused by insertional gene inactivation.     

Circumnutation of rice coleoptiles: its occurrence, regulation by phytochrome, and relationship with gravitropism

It has been found that coleoptiles of dark-grown rice (Oryza sativa L.) seedlings undergo regular circumnutation in circular orbits with periods of about 180 min. Both clockwise and counter-clockwise movements were observed, but individual coleoptiles continued to rotate only in one direction. Light-grown seedlings did not show circumnutation. In fact, dark-grown seedlings were found to cease circumnutating in response to a pulse of red light (R). This light-induced inhibition of circumnutation was demonstrated to involve both a FR-inducible very-low-fluence response, solely mediated by phytochrome A, and a FR-reversible low-fluence response, mediated by phytochrome B and/or C. The R-induced inhibition of circumnutation showed temporal agreement with the R-induced inhibition of coleoptile growth, suggesting that the former results from the latter. However, about 25% of growth activity remained after R treatment, indicating that circumnutation is more specifically regulated by phytochrome. The R-treated coleoptile showed gravitropism. Investigation of the growth differential for gravitropic curvature revealed that gravitropic responsiveness was rather enhanced by R. The results suggested that gravitropism is not a cause of circumnutation. It remained probable, however, that gravity perception is a part of the mechanism of circumnutation. It is speculated that the circumnutation investigated aids the seedling shoot in growing through the soil.     

AtLAZY1 is a signaling component required for gravitropism of the Arabidopsis thaliana inflorescence

The present study identified a family of six A. thaliana genes that share five limited regions of sequence similarity with LAZY1, a gene in Oryza sativa (rice) shown to participate in the early gravity signaling for shoot gravitropism. A T‐DNA insertion into the Arabidopsis gene (At5g14090) most similar to LAZY1 increased the inflorescence branch angle to 81° from the wild type value of 42°. RNA interference lines and molecular rescue experiments confirmed the linkage between the branch‐angle phenotype and the gene consequently named AtLAZY1. Time‐resolved gravitropism measurements of atlazy1 hypocotyls and primary inflorescence stems showed a significantly reduced bending rate during the first hour of response. The subcellular localization of AtLAZY1 protein was investigated to determine if the nuclear localization predicted from the gene sequence was observable and important to its function in shoot gravity responses. AtLAZY1 fused to green fluorescent protein largely rescued the branch‐angle phenotype of atlazy1, and was observed by confocal microscopy at the cell periphery and within the nucleus. Mutation of the nuclear localization signal prevented detectable levels of AtLAZY1 in the nucleus without affecting the ability of the gene to rescue the atlazy1 branch‐angle phenotype. These results indicate that AtLAZY1 functions in gravity signaling during shoot gravitropism, being a functional ortholog of rice LAZY1. The nuclear pool of the protein appears to be unnecessary for this function, which instead relies on a pool that appears to reside at the cell periphery.     

Cytochalasin D does not inhibit gravitropism in roots

It is generally thought that sedimenting plastids are responsible for gravity sensing in higher plants. We directly tested the model generated by the current statolith hypothesis that the gravity sensing that leads to gravitropism results from an interaction between the plastids and actin microfilaments. We find that the primary roots of rice, corn, and cress undergo normal gravitropism and growth even when exposed to cytochalasin D, a disruptor of actin microfilaments. These results indicate that an interaction between amyloplasts and the actin cytoskeleton is not critical for gravity sensing in higher plants and weaken the current statolith hypothesis.     

TAC4 controls tiller angle by regulating the endogenous auxin content and distribution in rice

Tiller angle, an important component of plant architecture, greatly influences the grain yield of rice (Oryza sativa L.). Here, we identified Tiller Angle Control 4 (TAC4) as a novel regulator of rice tiller angle. TAC4 encodes a plant‐specific, highly conserved nuclear protein. The loss of TAC4 function leads to a significant increase in the tiller angle. TAC4 can regulate rice shoot gravitropism by increasing the indole acetic acid content and affecting the auxin distribution. A sequence analysis revealed that TAC4 has undergone a bottleneck and become fixed in indica cultivars during domestication and improvement. Our findings facilitate an increased understanding of the regulatory mechanisms of tiller angle and also provide a potential gene resource for the improvement of rice plant architecture.     

Auxin metabolism and transport during gravitropism

Auxins are endogenous, growth-regulating compounds in plants: for decades investigators have hypothesized that plants change their growth rates and patterns in response to environmental signals by changing their transport of, metabolism of, or sensitivity to their endogenous auxins. The Cholodny-Went hypothesis, for example, postulates that plants respond to tropic signals by changing the distribution of free indoleacetic acid within their tissues. This hypothesis was based on data from experiments investigating phototropism and gravitropism in oat ( Avena sativa L.) and maize ( Zea mays L.) coleoptiles. The results of recent experiments support the Cholodny-Went hypothesis for maize coleoptile gravitropism. Recent experiments conducted on the gravitropisms of other developmental stages of grasses, and other species of plants, however, indicate that the Cholodny-Went hypothesis may not adequately describe how all plants respond to gravity.     

The promotion of gravitropism in Arabidopsis roots upon actin disruption is coupled with the extended alkalinization of the columella cytoplasm and a persistent lateral auxin gradient

The actin cytoskeleton has been implicated in regulating plant gravitropism. However, its precise role in this process remains uncertain. We have shown previously that disruption of the actin cytoskeleton with Latrunculin B (Lat B) strongly promoted gravitropism in maize roots. These effects were most evident on a clinostat as curvature that would exceed 90 degrees despite short periods of horizontal stimulation. To probe further the cellular mechanisms underlying these enhanced gravity responses, we extended our studies to roots of Arabidopsis. Similar to our observations in other plant species, Lat B enhanced the response of Arabidopsis roots to gravity. Lat B (100 nm) and a stimulation time of 5-10 min were sufficient to induce enhanced bending responses during clinorotation. Lat B (100 nm) disrupted the fine actin filament network in different regions of the root and altered the dynamics of amyloplasts in the columella but did not inhibit the gravity-induced alkalinization of the columella cytoplasm. However, the duration of the alkalinization response during continuous gravistimulation was extended in Lat B-treated roots. Indirect visualization of auxin redistribution using the DR5:beta-glucuronidase (DR5:GUS) auxin-responsive reporter showed that the enhanced curvature of Lat B-treated roots during clinorotation was accompanied by a persistent lateral auxin gradient. Blocking the gravity-induced alkalinization of the columella cytoplasm with caged protons reduced Lat B-induced curvature and the development of the lateral auxin gradient. Our data indicate that the actin cytoskeleton is unnecessary for the initial perception of gravity but likely acts to downregulate gravitropism by continuously resetting the gravitropic-signaling system.     

How do plant shoots bend up? — The initial step to elucidate the molecular mechanisms of shoot gravitropism usingArabidopsis thaliana

In higher plants, shoots show a negative gravitropic response. To elucidate the molecular mechanisms of this phenomenon, mutational analyses usingArabidopsis thaliana are in progress. This minireview aims to present recent developments in the genetic analysis of shoot gravitropism in this organism. We focus mainly on our studies on the novelshootgravitropic (sgr) mutants inArabidopsis thaliana that have dramatic defects in shoot gravitropism.