Uncoupling Auxin and Ethylene Effects in Transgenic Tobacco and Arabidopsis Plants

Overproduction of auxin in transgenic plants also results in the overproduction of ethylene. Plants overproducing both auxin and ethylene display inhibition of stem elongation and growth, increased apical dominance, and leaf epinasty. To determine the relative roles of auxin and ethylene in these processes, transgenic tobacco and Arabidopsis plants expressing the auxin-overproducing tryptophan monooxygenase transgene were crossed to plants expressing an ethylene synthesis-inhibiting 1-aminocyclopropane-1-carboxylate deaminase transgene. Tobacco and Arabidopsis plants with elevated auxin and normal levels of ethylene were obtained by this strategy. Transgenic auxin-overproducing Arabidopsis plants were also crossed with the ethylene-insensitive ein1 and ein2 mutants. Analysis of these plants indicates that apical dominance and leaf epinasty are primarily controlled by auxin rather than ethylene. However, ethylene is partially responsible for the inhibition of stem elongation observed in auxin-overproducing tobacco. Finally, these data show that auxin overproduction can be effectively uncoupled from ethylene overproduction in transgenic plants to enable direct manipulation of plant morphology for agronomic and horticultural purposes.     

生长素与乙烯信号途径及其相互关系研究进展

长期的研究表明, 生长素在调节植物生长发育的各种生理活动中起关键作用, 但对它如何调控这些生理活动却缺乏系统和深入的了解。最近, 细胞核内生长素信号途径的发现为揭示其作用机制带来了曙光。乙烯参与果实成熟及植物对逆境的反应等生理活动, 其信号途径也已得到部分阐明。越来越多的证据表明, 乙烯的作用与生长素对植物生长发育的调控之间有密切的联系。该文概述了生长素与乙烯信号途径的研究进展及其相互关系, 讨论了生长素在植物三重反应中的作用; 并对生长素与乙烯相互关系研究中存在的问题及研究前景进行了探讨。     

Ethylene in vegetative development: a tale with a riddle

The vegetative development of plants is strongly dependent on the action of phytohormones. For over a century, the effects of ethylene on plants have been studied, illustrating the profound impact of this gaseous hormone on plant growth, development and stress responses. Ethylene signaling is under tight self-control at various levels. Feedback regulation occurs on both biosynthesis and signaling. For its role in developmental processes, ethylene has a close and reciprocal relation with auxin, another major determinant of plant architecture. Here, we discuss, in view of novel findings mainly in the reference plant Arabidopsis, how ethylene is distributed and perceived throughout the plant at the organ, tissue and cellular levels, and reflect on how plants benefit from the complex interaction of ethylene and auxin, determining their shape. Furthermore, we elaborate on the implications of recent discoveries on the control of ethylene signaling.     

The Roles of Phytohormones in Development as Studied in Transgenic Plants

The study of transgenic plants has greatly advanced our understanding of the control of development and metabolism. The ability to isolate and modify genes greatly extends the range of what is technically feasible. In the area of hormone biology, transgenic plants have helped to elucidate the pathways of synthesis, the metabolic control points, and the biological functions of the various phytohormones. This review covers the available genes that modulate the metabolism and perception of the phytohormones. One of the most significant conclusions coming out of transgenic plant work is the complex interaction among the different classes of phytohormones. For example, increasing the level of the auxin indole-3-acetic acid (IAA) in a plant has the secondary effect of inducing ethylene biosynthesis. This complication can be circumvented by combining transgenic plants modulating multiple hormones or through the use of available mutants. In this manner, transgenic plants have been utilized to unambiguously define the roles of auxin, cytokinin, and ethylene in the control of apical dominance. The power of transgenic plants as tools in hormone biology is perhaps best illustrated by work on ethylene. In this case, the modular characterization of genes led to elucidation of the biosynthetic pathway. Availability of the biosynthetic genes has permitted detailed analysis of the regulation of synthesis, definition of the role of ethylene in the control of several developmental processes, and the application of that knowledge for agricultural improvement.     

Toward synthetic plant development

The ability to engineer plant form will enable the production of novel agricultural products designed to tolerate extreme stresses, boost yield, reduce waste, and improve manufacturing practices. While historically, plants were altered through breeding to change their size or shape, advances in our understanding of plant development and our ability to genetically engineer complex eukaryotes are leading to the direct engineering of plant structure. In this review, I highlight the central role of auxin in plant development and the synthetic biology approaches that could be used to turn auxin-response regulators into powerful tools for modifying plant form. I hypothesize that recoded, gain-of-function auxin response proteins combined with synthetic regulation could be used to override endogenous auxin signaling and control plant structure. I also argue that auxin-response regulators are key to engineering development in nonmodel plants and that single-cell -omics techniques will be essential for characterizing and modifying auxin response in these plants. Collectively, advances in synthetic biology, single-cell -omics, and our understanding of the molecular mechanisms underpinning development have set the stage for a new era in the engineering of plant structure.

Recent advances in our understanding of plant development and ability to control gene expression in plants may enable a new strategy for engineering plant form and function.     

The interaction of cytokinin with other signals

Cytokinins are important signalling molecules in plants, and recent studies have begun to shed light on the molecular mechanisms underlying their biosynthesis and response pathways. However, from the time of their discovery, it has been clear that cytokinins interact with other signals to regulate plant growth and development. Herein the interaction of cytokinin with three other signals: light, ethylene, and auxin is discussed. The interaction between light and cytokinin signalling, highlighted by recent analysis of cytokinin signalling mutants is reviewed. A discussion of another aspect of cytokinin cross-talk, its induction of ethylene biosynthesis in etiolated Arabidopsis seedlings, and recent studies that have begun to elucidate the mechanism underlying this regulation is also presented. Finally, there is a brief review of the interaction of auxin and cytokinin, and present novel expression profiling data of Arabidopsis seedlings treated with combinations of these two hormones, which provide insight into this interaction.     

生长素与乙烯信号途径及其相互关系研究进展

长期的研究表明,生长素在调节植物生长发育的各种生理活动中起关键作用,但对它如何调控这些生理活动却缺乏系统和深入的了解。最近,细胞核内生长素信号途径的发现为揭示其作用机制带来了曙光。乙烯参与果实成熟及植物对逆境的反应等生理活动,其信号途径也已得到部分阐明。越来越多的证据表明,乙烯的作用与生长素对植物生长发育的调控之间有密切的联系。该文概述了生长素与乙烯信号途径的研究进展及其相互关系,讨论了生长素在植物三重反应中的作用;并对生长素与乙烯相互关系研究中存在的问题及研究前景进行了探讨。     

The plant hormone auxin: insight in sight.

Physiological experiments conducted over the last 60 years indicate that the plant hormone auxin regulates a diverse set of developmental processes via changes in cell division, cell elongation and cell differentiation. Recent studies using transgenic plants with altered auxin levels support these conclusions and promise to provide more detailed information on the role of auxin during plant development. Although it is possible that all auxin responses are mediated by the same primary biochemical events, the studies described in this review are more consistent with multiple modes of auxin action. The development of molecular and genetic approaches to the study of hormone action should resolve this issue. The accelerated rate of progress in this field suggests that real insight into the mechanism of auxin action may be forthcoming.     

F-box蛋白质在植物生长发育中的功能

在真核生物中, 泛素介导的蛋白降解途径参与了许多生物学过程。SCF复合体是一种非常重要的E3泛素连接酶, 在植物中研究的最为深入。F-box蛋白包含一个F-box 基序, 是SCF复合体的一个亚基, 它决定了底物识别的特异性。目前, 从各种植物中已鉴定出大量的F-box蛋白质, 它们参与了植物激素(乙烯, 生长素, GA, JA)的信号传导以及自交不亲和、花器官发育等生物学过程, F-box蛋白还参与了植物的胁迫反应。最新研究结果显示, 一个F-box蛋白TIR1是生长素的受体。因此, F-box蛋白质介导的泛素化蛋白质降解途径可能是植物基因表达调控的重要机制。     

Distinct and Dynamic Auxin Activities During Reproductive Development

Flowering plants have evolved sophisticated and complicated reproductive structures to ensure optimal conditions for the next generation. Successful reproduction relies on careful timing and coordination of tissue development, which requires constant communication between these tissues. Work on flower and fruit development over the last decade places the phytohormone auxin in a key role as a master of patterning and tissue specification of reproductive organs. Although many questions still remain, it is now clear that auxin mediates its function in flowers and fruits through an integrated process of biosynthesis, transport, and signaling, as well as interaction with other hormonal pathways. In addition, the knowledge obtained so far about auxin function already allows researchers to develop tools for crop improvement and precision agriculture.Flower and fruit development requires a precise patterning of organs and tissues, which also have to be coordinated for the successful fulfillment of the tasks inherited. The plant hormone auxin has received a lot of attention for its prominent role in organ positioning as well as in organ polarity formation and differentiation, and flowers and fruits make no exception to the dependency on auxin in these aspects. In addition, auxin appears to participate in the coordination of processes within, as well as between, floral organs, aiding for example in successful fertilization. In this article, we focus on the role of auxin in the establishment of the flower, and specifically in the development and dehiscence of the reproductive floral organs.     

Genetic approaches to auxin action

Answers to long-standing questions concerning the molecular mechanism of auxin action and auxin's exact functions in plant growth and development are beginning to be uncovered through studies using mutant and transgenic plants. We review recent work in this area in vascular plants. A number of conclusions can be drawn from these studies. First, auxin appears essential for cell division and viability, as auxin auxotrophs isolated in tissue culture are dependent on auxin for growth and cannot be regenerated into plants even when auxin is supplied exogenously. Secondly, plants with transgenes that alter auxin levels are able to regulate cellular auxin concentrations by synthesis and conjugation; wild-type plants are probably also capable of such regulation. Thirdly, the phenotypes of transgenic plants with altered auxin levels and of mutant plants with altered sensitivity to auxin confirm earlier physiological studies which indicated a role for auxin in regulation of apical dominance, in development of roots and vascular tissue, and in the gravitropic response. Finally, the cloning of a mutationally identified gene important for auxin action, along with accumulating biochemical evidence, hints at a major role for protein degradation in the auxin response pathway.     

Genetic approaches to auxin action

Answers to long-standing questions concerning the molecular mechanism of auxin action and auxin's exact functions in plant growth and development are beginning to be uncovered through studies using mutant and transgenic plants. We review recent work in this area in vascular plants. A number of conclusions can be drawn from these studies. First, auxin appears essential for cell division and viability, as auxin auxotrophs isolated in tissue culture are dependent on auxin for growth and cannot be regenerated into plants even when auxin is supplied exogenously. Secondly, plants with transgenes that alter auxin levels are able to regulate cellular auxin concentrations by synthesis and conjugation; wild-type plants are probably also capable of such regulation. Thirdly, the phenotypes of transgenic plants with altered auxin levels and of mutant plants with altered sensitivity to auxin confirm earlier physiological studies which indicated a role for auxin in regulation of apical dominance, in development of roots and vascular tissue, and in the gravitropic response. Finally, the cloning of a mutationally identified gene important for auxin action, along with accumulating biochemical evidence, hints at a major role for protein degradation in the auxin response pathway.     

Multilevel interactions between ethylene and auxin in Arabidopsis roots

Hormones play a central role in the coordination of internal developmental processes with environmental signals. Herein, a combination of physiological, genetic, cellular, and whole-genome expression profiling approaches has been employed to investigate the mechanisms of interaction between two key plant hormones: ethylene and auxin. Quantification of the morphological effects of ethylene and auxin in a variety of mutant backgrounds indicates that auxin biosynthesis, transport, signaling, and response are required for the ethylene-induced growth inhibition in roots but not in hypocotyls of dark-grown seedlings. Analysis of the activation of early auxin and ethylene responses at the cellular level, as well as of global changes in gene expression in the wild type versus auxin and ethylene mutants, suggests a simple mechanistic model for the interaction between these two hormones in roots, according to which ethylene and auxin can reciprocally regulate each other's biosyntheses, influence each other's response pathways, and/or act independently on the same target genes. This model not only implies existence of several levels of interaction but also provides a likely explanation for the strong ethylene response defects observed in auxin mutants.     

Shedding light on auxin movement: Light-regulation of polar auxin transport in the photocontrol of plant development

By being sessile, plants have evolved a remarkable capacity to perceive and respond to changes in environmental conditions throughout their life cycle. Light represents probably the most important environmental factor that impinge on plant development because, other than supplying the energy source for photosynthesis, it also provides seasonal and positional information that are essential for the plant survival and fitness. Changes in the light environment can dramatically alter plant morphogenesis, especially during the early phases of plant life, and a compelling amount of evidence indicates that light-mediated changes in auxin homeostasis are central in these processes. Auxin exerts its morphogenetic action through instructive hormone gradients that drive developmental programs of plants. Such gradients are formed and maintained via an accurate control on directional auxin transport. This review summarizes the recent advances in understanding the influence of the light environment on polar auxin transport.     

A Link between ethylene and auxin uncovered by the characterization of two root-specific ethylene-insensitive mutants in Arabidopsis

The plant hormone ethylene participates in the regulation of a variety of developmental processes and serves as a key mediator of plant responses to biotic and abiotic stress factors. The diversity of ethylene functions is achieved, at least in part, by combinatorial interactions with other hormonal signals. Here, we show that ethylene-triggered inhibition of root growth, one of the classical effects of ethylene in Arabidopsis thaliana seedlings, is mediated by the action of the WEAK ETHYLENE INSENSITIVE2/ANTHRANILATE SYNTHASE alpha1 (WEI2/ASA1) and WEI7/ANTHRANILATE SYNTHASE beta1 (ASB1) genes that encode alpha- and beta-subunits of a rate-limiting enzyme of Trp biosynthesis, anthranilate synthase. Upregulation of WEI2/ASA1 and WEI7/ASB1 by ethylene results in the accumulation of auxin in the tip of primary root, whereas loss-of-function mutations in these genes prevent the ethylene-mediated auxin increase. Furthermore, wei2 and wei7 suppress the high-auxin phenotypes of superroot1 (sur1) and sur2, two auxin-overproducing mutants, suggesting that the roles of WEI2 and WEI7 in the regulation of auxin biosynthesis are not restricted to the ethylene response. Together, these findings reveal that ASA1 and ASB1 are key elements in the regulation of auxin production and an unexpected node of interaction between ethylene responses and auxin biosynthesis in Arabidopsis. This study provides a mechanistic explanation for the root-specific ethylene insensitivity of wei2 and wei7, illustrating how interactions between hormones can be used to achieve response specificity.     

Ethylene is required in tobacco to successfully compete with proximate neighbours

Plants sense neighbours even before these cause a decrease in photosynthetic light availability. Light reflected by proximate neighbours signals a plant to adjust growth and development, in order to avoid suppression by neighbour plants. These phenotypic changes are known as the shade‐avoidance syndrome and include enhanced shoot elongation and more upright‐positioned leaves. In the present study it was shown that these shade‐avoidance traits in tobacco (Nicotiana tabacum) are also induced by low concentrations of ethylene. Furthermore, it was shown that transgenic plants, insensitive to ethylene, have a delayed appearance of shade‐avoidance traits. The increase in both leaf angles and stem elongation in response to neighbours are delayed in ethylene‐insensitive plants. These data show that ethylene is an important component in the regulation of neighbour‐induced, shade‐avoidance responses. Consequently, ethylene‐insensitive plants lose competition with wild‐type neighbours, demonstrating that sensing of ethylene is required for a plant to successfully compete for light.