New Insights into the Functions of Cytokinins in Plant Development

Recent breakthroughs in cytokinin research have shed new light on the role of cytokinin in plant development. Loss-of-function mutants of a cytokinin receptor reveal a role for the hormone in establishment of the vasculature during embryonic development. Cytokinin controls the number of early cell divisions via a two-component signaling system. Genetically engineered plants that have a reduced cytokinin content demonstrate the regulatory role of the hormone in control of meristem activity and organ growth during postembryonic development, with opposite roles in roots and shoots. There is increasing evidence from work with transgenic plants and mutant analysis that cytokinins do not perform the previously proposed function as a root-derived signal for the regulation of shoot branching. Root-borne cytokinins might serve as a long-range signal controling other processes at distant sites, such as responding to nutritional status, particularly nitrogen availability.     

Nitrate-specific and cytokinin-mediated nitrogen signaling pathways in plants

Inorganic nitrogen is a limiting factor in plant growth and development. Plants constantly sense changes in nitrogen availability and respond appropriately by modulating gene expression. Plants employ multiple routes for the long-distance signaling and communication of nitrogen status. One of these depends on nitrate itself (nitrate-specific signaling), while another uses cytokinin as a messenger. Recent studies suggest that nitrate-specific signaling functions predominantly in the context of the synthesis of amino acids and nucleic acids. This pathway includes the control of the expression of a wide variety of genes. On the other hand, cytokinin-mediated signaling is related mainly to the control of nitrogen partitioning and development. Nitrogen-dependent cytokinin accumulation and the involvement of His-Asp phosphorelay systems are characteristic of this pathway. The coordination of both regulatory pathways seems to be crucially important for the integration of nitrogen signals at the whole plant level.     

Interaction between phosphate-starvation, sugar, and cytokinin signaling in Arabidopsis and the roles of cytokinin receptors CRE1/AHK4 and AHK3

Cytokinins control key processes during plant growth and development, and cytokinin receptors CYTOKININ RESPONSE 1/WOODEN LEG/ARABIDOPSIS HISTIDINE KINASE 4 (CRE1/WOL/AHK4), AHK2, and AHK3 have been shown to play a crucial role in this control. The involvement of cytokinins in signaling the status of several nutrients, such as sugar, nitrogen, sulfur, and phosphate (Pi), has also been highlighted, although the full physiological relevance of this role remains unclear. To gain further insights into this aspect of cytokinin action, we characterized a mutant with reduced sensitivity to cytokinin repression of a Pi starvation-responsive reporter gene and show it corresponds to AHK3. As expected, ahk3 displayed reduced responsiveness to cytokinin in callus proliferation and plant growth assays. In addition, ahk3 showed reduced cytokinin repression of several Pi starvation-responsive genes and increased sucrose sensitivity. These effects of the ahk3 mutation were especially evident in combination with the cre1 mutation, indicating partial functional redundancy between these receptors. We examined the effect of these mutations on Pi-starvation responses and found that the double mutant is not significantly affected in long-distance systemic repression of these responses. Remarkably, we found that expression of many Pi-responsive genes is stimulated by sucrose in shoots and to a lesser extent in roots, and the sugar effect in shoots of Pi-starved plants was particularly enhanced in the cre1 ahk3 double mutant. Altogether, these results indicate the existence of multidirectional cross regulation between cytokinin, sugar, and Pi-starvation signaling, thus underlining the role of cytokinin signaling in nutrient sensing and the relative importance of Pi-starvation signaling in the control of plant metabolism and development.     

Cytokinin biosynthesis and transport for systemic nitrogen signaling

The plasticity of growth and development in response to environmental changes is one of the essential aspects of plant behavior. Cytokinins play an important role as signaling molecules in the long-distance communication between organs in systemic growth regulation in response to nitrogen. The spatial distribution of the expression sites of cytokinin biosynthesis genes leads to structural differences in the molecular species transported through the xylem and phloem, giving root-borne trans-hydroxylated cytokinins, namely trans-zeatin (tZ) type, a specialized efficacy in regulating shoot growth. Furthermore, root-to-shoot translocation via the xylem, tZ, and its precursor, the tZ riboside, controls different sets of shoot growth traits to fine-tune shoot growth in response to nitrogen availability. In addition to nitrogen, photosynthetically generated sugars positively regulate de novo cytokinin biosynthesis in the roots, and contribute to plant growth under elevated CO₂ conditions. In shoot-to-root signaling, cytokinins also play a role in the regulation of nutrient acquisition and root system growth in cooperation with other types of signaling molecules, such as C-TERMINALLY ENCODED PEPTIDE DOWNSTREAMs. As cytokinin is a key regulator for the maintenance of shoot apical meristem, deepening our understanding of the regulatory mechanisms of cytokinin biosynthesis and transport in response to nitrogen is important not only for basic comprehension of plant growth, but also to ensure the stability of agricultural production.     

Cytokinin signaling

Although cytokinin plays a central role in plant development, our knowledge of the biosynthesis, distribution, perception and signal transduction of cytokinin is limited. Recent molecular-genetic studies have, however, implicated involvement of a two-component system in cytokinin signal transduction. Furthermore, new mutants with altered cytokinin responses and genes involved in cytokinin signaling have been identified.     

细胞分裂素: 代谢、信号转导、交叉反应与农艺性状改良

在高等植物中, 细胞分裂素通过对细胞分裂与分化的调节而广泛参与了对植物生长发育的调控。在过去的10余年, 利用模式植物拟南芥的研究, 在阐明细胞分裂素的代谢、转运与信号转导等方面取得了重要的进展。同时, 关于细胞分裂素与其它信号途径之间存在的广泛交叉反应也受到了人们的注意。根据我们现有的知识, 细胞分裂素信号转导是通过磷酸基团在一个双元组分系统之间的系列传递而完成的, 该过程被称之为“磷酸接力传递”(phosphorelay)。细胞分裂素与其它信号途径的互作可能也主要是通过双元组分系统链接的。双元组分系统中目前已知的主要信号元件不仅表现出功能冗余性, 同时在调控特定的植物生长发育过程时也具有特异性。本文在对细胞分裂素的代谢与转运过程简要评述的基础上,对其信号转导以及与其它信号途径间交叉反应的研究进展进行重点讨论, 并展望细胞分裂素研究对重要农业性状改良的意义。     

细胞分裂素:代谢、信号转导、交叉反应与农艺性状改良

在高等植物中,细胞分裂素通过对细胞分裂与分化的调节而广泛参与了对植物生长发育的调控。在过去的10余年,利用模式植物拟南芥的研究,在阐明细胞分裂素的代谢、转运与信号转导等方面取得了重要的进展。同时,关于细胞分裂素与其它信号途径之间存在的广泛交叉反应也受到了人们的注意。根据我们现有的知识,细胞分裂素信号转导是通过磷酸基团在一个双元组分系统之间的系列传递而完成的,该过程被称之为“磷酸接力传递”（phosphorelay）。细胞分裂素与其它信号途径的互作可能也主要是通过双元组分系统链接的。双元组分系统中目前已知的主要信号元件不仅表现出功能冗余性,同时在调控特定的植物生长发育过程时也具有特异性。本文在对细胞分裂素的代谢与转运过程简要评述的基础上,对其信号转导以及与其它信号途径间交叉反应的研究进展进行重点讨论,并展望细胞分裂素研究对重要农业性状改良的意义。     

A proteomic approach to analyze nitrogen- and cytokinin-responsive proteins in rice roots

Nitrogen plays a central role in rice growth and development because it modulates a wide variety of processes, including cytokinin (CK) metabolism. CK-mediated signaling is also related to nitrogen metabolism. The functional relation between nitrogen and CK are extremely complex and unclear. In this study, a comparative proteomic analysis was carried out to analyze proteins regulated by nitrogen and CK in rice roots. Proteins extracted from rice roots are separated by two-dimensional polyacrylamide gel electrophoresis. Thirty-two protein spots that expressed similarly by nitrogen and CK treatments are selected for identification by mass spectrometry. Of these spots, 28 are successfully identified. These proteins were categorized into classes related to energy, metabolism, disease/defense, protein degradation, signal transduction, transposons, and unclear classification. Energy gives the largest functional category, suggesting that the glycolysis (two enzymes detected) and tricarboxylic acid cycle (six enzymes detected) are accurately regulated by nitrogen and CK, thus promoting the synthesis of amino acid. The identification of novel proteins provides new insights into the coordination of nitrogen and CK in rice. The possible role of these proteins is discussed.     

Two-component elements mediate interactions between cytokinin and salicylic acid in plant immunity

Recent studies have revealed an important role for hormones in plant immunity. We are now beginning to understand the contribution of crosstalk among different hormone signaling networks to the outcome of plant-pathogen interactions. Cytokinins are plant hormones that regulate development and responses to the environment. Cytokinin signaling involves a phosphorelay circuitry similar to two-component systems used by bacteria and fungi to perceive and react to various environmental stimuli. In this study, we asked whether cytokinin and components of cytokinin signaling contribute to plant immunity. We demonstrate that cytokinin levels in Arabidopsis are important in determining the amplitude of immune responses, ultimately influencing the outcome of plant-pathogen interactions. We show that high concentrations of cytokinin lead to increased defense responses to a virulent oomycete pathogen, through a process that is dependent on salicylic acid (SA) accumulation and activation of defense gene expression. Surprisingly, treatment with lower concentrations of cytokinin results in increased susceptibility. These functions for cytokinin in plant immunity require a host phosphorelay system and are mediated in part by type-A response regulators, which act as negative regulators of basal and pathogen-induced SA-dependent gene expression. Our results support a model in which cytokinin up-regulates plant immunity via an elevation of SA-dependent defense responses and in which SA in turn feedback-inhibits cytokinin signaling. The crosstalk between cytokinin and SA signaling networks may help plants fine-tune defense responses against pathogens.     

Characterization of an Arabidopsis gene that mediates cytokinin signaling in shoot apical meristem development

Cytokinins are adenine derivatives that regulate numerous plant growth and developmental processes, including apical and floral meristem development, stem growth, leaf senescence, apical dominance, and stress tolerance. However, not much is known about how cytokinin biosynthesis and metabolism is regulated. We identified a novel Arabidopsis gene, ALL, encoding an aldolase-like enzyme that regulates cytokinin signaling. An Arabidopsis mutant, all-1D, in which ALL is activated by the nearby insertion of the 35S enhancer, exhibited extreme dwarfism with rolled, dark-green leaves and reduced apical dominance, symptomatic of cytokinin-overproducing mutants. Consistent with this, ARR4 and ARR5, two representative primary cytokinin-responsive genes, were significantly induced in all-1D. Whereas SHOOT MERISTEMLESS (STM) and KNAT1, which regulate meristem development, were also greatly induced, expression of REV and PHV that regulate lateral organ polarity was inhibited. ALL encodes an aldolase-like enzyme that belongs to the HpcH/HpaI aldolase family in prokaryotes and is down-regulated by exogenous cytokinin, possibly through a negative feedback pathway. We propose that ALL is involved in cytokinin biosynthesis or metabolism and acts as a positive regulator of cytokinin signaling during shoot apical meristem development and determination of lateral organ polarity.     

Cross talk between gibberellin and cytokinin: the Arabidopsis GA response inhibitor SPINDLY plays a positive role in cytokinin signaling

SPINDLY (SPY) is a negative regulator of gibberellin (GA) responses; however, spy mutants exhibit various phenotypic alterations not found in GA-treated plants. Assaying for additional roles for SPY revealed that spy mutants are resistant to exogenously applied cytokinin. GA also repressed the effects of cytokinin, suggesting that there is cross talk between the two hormone-response pathways, which may involve SPY function. Two spy alleles showing severe (spy-4) and mild (spy-3) GA-associated phenotypes exhibited similar resistance to cytokinin, suggesting that SPY enhances cytokinin responses and inhibits GA signaling through distinct mechanisms. GA and spy repressed numerous cytokinin responses, from seedling development to senescence, indicating that cross talk occurs early in the cytokinin-signaling pathway. Because GA3 and spy-4 inhibited induction of the cytokinin primary-response gene, type-A Arabidopsis response regulator 5, SPY may interact with and modify elements from the phosphorelay cascade of the cytokinin signal transduction pathway. Cytokinin, on the other hand, had no effect on GA biosynthesis or responses. Our results demonstrate that SPY acts as both a repressor of GA responses and a positive regulator of cytokinin signaling. Hence, SPY may play a central role in the regulation of GA/cytokinin cross talk during plant development.     

细胞分裂素结合蛋白的研究进展

迄今为止,已从多种植物中分离到细胞分裂素结合蛋白(CBPs),它们可能在细胞分裂素的信号转导、体内运输及代谢中起作用。根据现有研究结果认为,大多数CTKs受体可能位于膜上,通过与G_蛋白耦联的信号转导系统或双组分信号转导系统完成CTKs信号的跨膜转导。少数CTKs受体可能位于细胞质中,与胞内CTKs结合后进入细胞核,直接调节基因的表达。本文综述了近年来对CBPs的研究进展,分析了CTKs受体的可能类型及CBPs作用的可能机制。     

Cytokinins: Biosynthesis metabolism and perception

Summary Cytokinins are essential hormones for plant growth and development. They are also of vital importance for in vitro manipulations of plant cells and tissues. The biological activities and chemistry of cytokinins are well defined but very little is known about their mode of action and it is only recently that cytokinin genes have been identified in plants. This review summarizes the current status of knowledge on cytokinin biosynthesis, metabolism and signal transduction, with an emphasis on genes encoding metabolic enzymes and putative receptors, and genes rapidly induced by cytokinins.     

Comparison of phytohormone signaling mechanisms

Plant hormones are crucial signaling molecules that coordinate all aspects of plant growth, development and defense. A great deal of attention has been attracted from biologists to study the molecular mechanisms for perception and signal transduction of plant hormones during the last two decades. Tremendous progress has been made in identifying receptors and key signaling components of plant hormones. The holistic picture of hormone signaling pathways is extremely complicated, this review will give a general overview of perception and signal transduction mechanisms of auxin, gibberellin, cytokinin, abscisic acid, ethylene, brassinosteroid, and jasmonate.