An auxin-auxotrophic mutant of Nicotiana plumbaginifolia

Summary Auxin (indole-3-acetic acid) is considered to be an important signalling molecule in the regulation of plant growth and development but neither auxin synthesis nor its mode of action is clearly understood. To identify genes involved in these processes, mutations were sought that altered the auxin requirement of plant tissues for growth. For the first time mutant plants were obtained that carry a recessive mutation at a single nuclear locus (auxl) which results in an absolute requirement for exogenous auxin for normal growth. In the absence of auxin treatment, mutant plants undergo premature senescence and die.Abbreviations BAP 6-benzylaminopurine - BUdR 5-bromodeoxyuridine - 2,4-D 2,4-dichlorophenoxyacetic acid - FUdR 5-fluorodeoxyuridine - IAA-EE indole-3-acetic acid ethyl ester - IMS indole-3-methanesulfonic acid     

Ethylene‐triggered abscisic acid: A principle in plant growth regulation?

The application of auxins to sensitive plant species or their overproduction in transgenic plants stimulates ethylene biosynthesis via induction of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase. Recent studies with auxin herbicides and indole-3-acetic acid (IAA) have revealed that auxin-stimulated ethylene triggers an increase in the biosynthesis of abscisic acid (ABA), which then functions as a second messenger, leading to growth inhibition and senescence. This raises the question of whether ethylene-triggered ABA is restricted to the action of auxin herbicides or whether it is a widespread phenomenon in the normal plant growth regulation. Our own results and a reappraisal of the literature indicate that ethylene-induced ABA may, indeed, play a role in natural physiological phenomena, such as root gravireaction and suppression of lateral bud growth in apical dominance. In addition, it would be worthwhile to investigate whether ethylene-triggered ABA is involved in other processes which coincide with a strong stimulation of ethylene biosynthesis, such as growth inhibition induced by cytokinins and senescence elicited under stress conditions.     

大豆microRNA基因GmMIR160A负调控植物叶片衰老进程

:叶片衰老是受内外多种因子影响的遗传发育进程.生长素、细胞分裂素和乙烯等多种植物激素是调 控叶片衰老的重要内部因子,它们通过长或短距离运输形成叶片组织内特定的区域分布和浓度梯度,从而直 接或间接参与植物叶片衰老过程.分子遗传学表明,细胞分裂素和乙烯分别是叶片衰老的抑制子和正调节 子,而生长素如何参与叶片衰老的分子机制目前还不清晰.植物体内成熟小分子RNA 由小RNA 基因转录 并通过特定酶加工形成的２１~２３bp的双链RNA分子.这些小分子通过不完全配对方式抑制其靶基因转录 和/或表达,参与植物生长发育多个过程,然而这类小RNA 分子如何调控植物叶片衰老发育过程目前则还鲜 有报告.大豆是重要的油料作物,具有典型的单次结实性衰老特征.研究大豆叶片衰老具有重要的科学意义 和深远的应用价值.该文采用实时荧光定量PCR(qPCR)技术分析大豆(Glycinemax)microRNA基因Gm- MIR１６０A 的表达模式,发现大豆第一复叶中GmMIR１６０A 表达受外源生长素和黑暗处理的诱导,暗示该基 因是生长素快速响应的叶片衰老相关基因.为进一步探究GmMIR１６０A 在大豆叶片发育中的功能,构建了 肾上腺皮质激素(Glucocorticoid,GR)类似物地塞米松(Dexamethasone,DEX)诱导表达GmMIR１６０A 双元表 达载体并通过农杆菌介导的子叶节方法转化野生型大豆.通过抗性筛选和基因组PCR 鉴定并结合表型分 析,共获得了４株诱导表达的稳定遗传转基因植株(株系OXＧ３、OXＧ５、OXＧ７和OXＧ８).GmMIR１６０A 过表达 植株根、茎、叶、花和果实在形态学上与野生型相比无显著差异,但叶片的叶绿素含量增加、最大光量子效率 (Fv/Fm)增强.进一步分子分析发现,转基因大豆叶片中GmARFs 和衰老标记基因(GmCYSP１)表达明显下 降,表明大豆Gma-miR１６０通过抑制靶基因GmARFs 的表达来负调控植物叶片的衰老进程.该文揭示了生 长素通过小分子RNA调控叶片发育一条新途径,为研究植物激素调控植物叶片衰老提供了新的思路.     

Mediation of Herbicide Effects by Hormone Interactions

Chemical manipulation of the phytohormone system involves the use of herbicides for weed control in modern crop production. In the latter case, only compounds interacting with the auxin system have gained practical importance. Auxin herbicides mimic the overdose effects of indole-3-acetic acid (IAA), the principal natural auxin in higher plants. With their ability to control, particularly, dicotyledonous weeds in cereal crops, the synthetic auxins have been among the most successful herbicides used in agriculture. A newly discovered sequential hormone interaction plays a decisive role in their mode of action. The induction of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase in ethylene biosynthesis is the primary target process, following auxin herbicide signalling. Although the exact molecular target site has yet to be identified, it appears likely to be at the level of auxin receptor(s) for perception or signalling, leading ultimately to species- and organ-specific de novo enzyme synthesis. In sensitive dicots, ethylene causes epinastic growth and tissue swelling. Ethylene also triggers the biosynthesis of abscisic acid (ABA), mainly through the stimulated cleavage of xanthophylls to xanthoxal, catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED). ABA mediates stomatal closure which limits photosynthetic activity and biomass production, accompanied by an overproduction of reactive oxygen species. Growth inhibition, senescence and tissue decay are the consequences. Recent results suggest that ethylene-triggered ABA is not restricted to the action of auxin herbicides. It may function as a module in the signalling of a variety of stimuli leading to plant growth regulation. An additional phenomenon is caused by the auxin herbicide quinclorac which also controls grass weeds. Here, quinclorac induces the accumulation of phytotoxic levels of cyanide, a co-product of ethylene, which ultimately derives from herbicide-induced ACC synthase activity in the tissue. Phytotropins are a further group of hormone-related compounds which are used as herbicides. They inhibit polar auxin transport by interacting with a regulatory protein, the NPA-binding protein, of the auxin efflux carrier. This causes an abnormal accumulation of IAA and applied synthetic auxins in plant meristems. Growth inhibition, loss of tropic responses and, in combination with auxin herbicides, synergistic effects are the consequences.     

植物的环境信号分子茉莉酸及其生物学功能

茉莉酸信号分子参与植物生长发育众多生理过程的调控,尤其是作为环境信号分子能有效地介导植物对生物及非生物胁迫的防御反应。迄今已知具有信号分子生理功能的至少包括茉莉酸(jasmonic acid,JA)以及茉莉酸甲酯(methyl jasmonate,Me JA)和茉莉酸-异亮氨酸复合物(jasmonoyl-isoleucine,JA-Ile)等茉莉酸衍生物,统称为茉莉酸类化合物(jasmonates,JAs)。从环境信号分子角度介绍了茉莉酸信号的启动(环境信号感知与转导、茉莉酸类化合物合成)、传递(局部传递、维管束传输、空气传播)和生物学功能(茉莉酸信号受体、调控的转录因子、参与的生物学过程)。     

生长素合成途径的研究进展

生长素是一类含有一个不饱和芳香族环和一个乙酸侧链的内源激素, 参与植物生长发育的许多过程。植物和一些侵染植物的病原微生物都可以通过改变生长素的合成来调节植株的生长。吲哚-3-乙酸(IAA)是天然植物生长素的主要活性成分。近年来, 随着IAA生物合成过程中一些关键调控基因的克隆和功能分析, 人们对IAA的生物合成途径有了更加深入的认识。IAA的生物合成有依赖色氨酸和非依赖色氨酸两条途径。依据IAA合成的中间产物不同, 依赖色氨酸的生物合成过程通常又划分成4条支路: 吲哚乙醛肟途径、吲哚丙酮酸途径、色胺途径和吲哚乙酰胺途径。该文综述了近几年在IAA生物合成方面取得的新进展。     

AXR1 acts after lateral bud formation to inhibit lateral bud growth in Arabidopsis

The AXR1 gene of Arabidopsis is required for many auxin responses. The highly branched shoot phenotype of mature axr1 mutant plants has been taken as genetic evidence for a role of auxin in the control of shoot branching. We compared the development of lateral shoots in wild-type Columbia and axr1-12 plants. In the wild type, the pattern of lateral shoot development depends on the developmental stage of the plant. During prolonged vegetative growth, axillary shoots arise and develop in a basal-apical sequence. After floral transition, axillary shoots arise rapidly along the primary shoot axis and grow out to form lateral inflorescences in an apical-basal sequence. For both patterns, the axr1 mutation does not affect the timing of axillary meristem formation; however, subsequent lateral shoot development proceeds more rapidly in axr1 plants. The outgrowth of lateral inflorescences from excised cauline nodes of wild-type plants is inhibited by apical auxin. axr1-12 nodes are resistant to this inhibition. These results provide evidence for common control of axillary growth in both patterns, and suggest a role for auxin during the late stages of axillary shoot development following the formation of the axillary bud and several axillary leaf primordia.     

Temperature-regulation of plant architecture

As sessile organisms, plants have evolved great plasticity to adapt to their surrounding environment. Temperature signals regulate the timing of multiple developmental processes and have dramatic effects on plant architecture and biomass. The modulation of plant architecture by temperature is of increasing relevance with regard to crop productivity and global climate change. Unlike many other organisms, the mechanisms through which plants sense changes in ambient temperature remain elusive. Multiple studies have identified crosstalk between ambient temperature sensing, light signaling, cold acclimation and pathogen response pathways. The regulation of plant architecture by temperature appears to involve the complex integration of multiple hormone signaling networks. Gibberellin (GA), Salicylic Acid (SA) and cytokinin have been implicated in the regulation of plant growth during chilling, whilst a predominant role for auxin is observed at high temperatures. This mini-review summarizes current knowledge of plant growth regulation by temperature and crosstalk with other abiotic and biotic stress signaling pathways.Key words: temperature, architecture, elongation, growth, hormone, auxin, gibberellin, salicylic acid, biomass     

Ethylene and auxin-induced cell growth in relation to auxin transport and metabolism and ethylene production in the semi-aquatic plant,Regnellidium diphyllum

Cell elongation in the rachis of the semiaquatic fern Regnellidium diphyllum is induced by the addition of ethylene or indoleacetic acid (IAA). Experiments with whole plants or rachis segments have shown that ethylene-induced growth requires the presence of auxin. Ethylene does not cause a modification in either endogenous auxin levels or in the extent of auxin metabolism but auxin transport is reduced. Rates of ethylene production in Regnellidium are not altered by either mechanical excitation or by the addition of auxin. A two-hormone control of cell expansion is proposed in which an initial, auxin-dependent growth event pre-conditions the cells to a further subsequent (or synchronous) ethylene-dependent growth event.Abbreviation IAA indole-3yl-acetic acid     

Polar auxin transport: controlling where and how much.

Auxin is transported through plant tissues, moving from cell to cell in a unique polar manner. Polar auxin transport controls important growth and developmental processes in higher plants. Recent studies have identified several proteins that mediate polar auxin transport and have shown that some of these proteins are asymmetrically localized, paving the way for studies of the mechanisms that regulate auxin transport. New data indicate that reversible protein phosphorylation can control the amount of auxin transport, whereas protein secretion through Golgi-derived vesicles and interactions with the actin cytoskeleton might regulate the localization of auxin efflux complexes.     

Auxin inhibition of decapitation-induced branching is dependent on graft-transmissible signals regulated by genes Rms1 and Rms2

Decapitation-induced axillary bud outgrowth is a vital mechanism whereby shoots are able to continue normal growth and development. In many plants, including wild-type garden pea (Pisum sativum L.), this process can be inhibited by exogenous auxin. Using the ramosus (rms) increased branching mutants of pea, we present evidence that this response to auxin is dependent on graft-transmissible substance(s) regulated by the genes Rms1 and Rms2. The response to exogenous auxin is massively diminished in decapitated rms1 and rms2 mutant plants. However, basipetal auxin transport is not reduced in intact or decapitated mutants. Grafting rms1 or rms2 shoots onto wild-type rootstocks restored the auxin response, indicating that Rms1 and Rms2 gene action in the rootstock is sufficient to enable an auxin response in mutant shoots. We conclude that Rms1 and Rms2 act in the rootstock and shoot to control levels of mobile substance(s) that interact with exogenous auxin in the inhibition of bud outgrowth after decapitation. At least for rms1, the reduced auxin response is unlikely to be due to an inability of auxin to decrease xylem sap cytokinin content, as this is already low in intact rms1 plants. Consequently, we have genetic evidence that auxin action in decapitated plants depends on at least one novel long-distance signal.     

Altered life cycle in Arabidopsis plants expressing PsUGT1, a UDP-glucuronosyltransferase-encoding gene from pea

Alfalfa (Medicago sativa) and Arabidopsis were used as model systems to examine molecular mechanisms underlying developmental effects of a microsomal UDP-glucuronosyltransferase-encoding gene from pea (Pisum sativum; PsUGT1). Alfalfa expressing PsUGT1 antisense mRNA under the control of the cauliflower mosaic virus (CaMV) 35S promoter exhibited delayed root emergence, reduced root growth, and increased lateral root development. The timing of root emergence in wild-type and antisense plants was correlated with the transient accumulation of auxin at the site of root emergence. Cell suspension cultures derived from the antisense alfalfa plants exhibited a delay in cell cycle from 24-h in the wild-type plants to 48-h in the antisense plants. PsUGT1::uidA was introduced into Arabidopsis to demonstrate that, as in alfalfa and pea, PsUGT1 expression occurs in regions of active cell division. This includes the root cap and root apical meristems, leaf primordia, tips of older leaves, and the transition zone between the hypocotyl and the root. Expression of PsUGT1::uidA colocalized with the expression of the auxin-responding reporter DR5::uidA. Co-expression of DR5::uidA in transgenic Arabidopsis lines expressing CaMV35S::PsUGT1 revealed that ectopic expression of CaMV35S::PsUGT1 is correlated with a change in endogenous auxin gradients in roots. Roots of ecotype Columbia expressing CaMV35S::PsUGT1 exhibited distinctive responses to exogenous naphthalene acetic acid. Completion of the life cycle occurred in 4 to 6 weeks compared with 6 to 7 weeks for wild-type Columbia. Inhibition of endogenous ethylene did not correct this early senescence phenotype.     

Expression of gibberellin 20-oxidase1 (AtGA20ox1) in Arabidopsis seedlings with altered auxin status is regulated at multiple levels

Bioactive gibberellins (GAs) affect many biological processes including germination, stem growth, transition to flowering, and fruit development. The location, timing, and level of bioactive GA are finely tuned to ensure that optimal growth and development occur. The balance between GA biosynthesis and deactivation is controlled by external factors such as light and by internal factors that include auxin. The role of auxin transport inhibitors (ATIs) and auxins on GA homeostasis in intact light-grown Arabidopsis thaliana (L.) Heynh. seedlings was investigated. Two ATIs, 1-N-naphthylthalamic acid (NPA) and 1-naphthoxyacetic acid (NOA) caused elevated expression of the GA biosynthetic enzyme AtGA20-oxidase1 (AtGA20ox1) in shoot but not in root tissues, and only at certain developmental stages. It was investigated whether enhanced AtGA20ox1 gene expression was a consequence of altered flow through the GA biosynthetic pathway, or was due to impaired GA signalling that can lead to enhanced AtGA20ox1 expression and accumulation of a DELLA protein, Repressor of ga1-3 (RGA). Both ATIs promoted accumulation of GFP-fused RGA in shoots and roots, and this increase was counteracted by the application of GA(4). These results suggest that in ATI-treated seedlings the impediment to DELLA protein degradation may be a deficiency of bioactive GA at sites of GA response. It is proposed that the four different levels of AtGA20ox1 regulation observed here are imposed in a strict hierarchy: spatial (organ-, tissue-, cell-specific) > developmental > metabolic > auxin regulation. Thus results show that, in intact auxin- and auxin transport inhibitor-treated light-grown Arabidopsis seedlings, three other levels of regulation supersede the effects of auxin on AtGA20ox1.     

Organ-specific effects of the auxin herbicide 2,4-D on the oxidative stress and senescence-related parameters of the stems of pea plants

Oxidative stress and senescence have been shown to participate in the toxicity mechanism of auxin herbicides in the leaves and roots of sensitive plants. However, their role in stem toxicity has not been studied yet. In this work, we report the effect of foliar or root applications of the auxin herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) on the parameters of oxidative stress and senescence of stems of pea (Pisum sativum L.) plants. Contrary to their effect on the pea leaves, in the stems 2,4-D applications did not cause oxidative stress, as shown by the parameters of lipid peroxidation, protein carbonyls, and total and protein thiols. Moreover, they inhibited the superoxide radical (O₂^.−)-producing xanthine oxidase (XOD) activity and stimulated the antioxidant activities of catalase (CAT), guaiacol peroxidase (GPOX), ascorbate peroxidase (APX), glutathione reductase (GR), glutathione S-transferase (GST) and Krebs cycle NAD⁺-isocitrate dehydrogenase (IDH). Applications of 2,4-D also did not induce senescence in the pea stems, as shown by the increase of proteins, the lack of stimulation of proteolytic activity, and the inhibition of senescence-related isocitrate lyase (ICL) activity. However, they stimulated the H₂O₂-producing acyl-CoA oxidase (ACOX) activity of fatty acid beta oxidation. Results suggest that oxidative stress and senescence are not involved in the mechanism of toxicity of 2,4-D in the stems of pea plants, and that these phenomena are not whole-plant toxicity mechanisms for auxin herbicides in susceptible plants. Results also suggest that the effect of 2,4-D on the oxidative metabolism of pea plants might be organ-specific.     

The transcription factor FUSCA3 controls developmental timing in Arabidopsis through the hormones gibberellin and abscisic acid

Although plants continually produce different organs throughout their life cycle, little is known about the factors that regulate the timing of a given developmental program. Here we report that the restricted expression of FUS3 to the epidermis is sufficient to control foliar organ identity in Arabidopsis by regulating the synthesis of two hormones, abscisic acid and gibberellin. These hormones in turn regulate the rates of cell cycling during organ formation to determine whether an embryonic or adult leaf will emerge. We also show that FUS3 expression is influenced by the patterning hormone, auxin, and therefore acts as a nexus of hormone action during embryogenesis. The identification of lipophillic hormones downstream of a heterochronic regulator in Arabidopsis has parallels to mechanisms of developmental timing in animals and suggests a common logic for temporal control of developmental programs between these two kingdoms.