Characterization of genes involved in cytokinin signaling and metabolism from rice

Two-component signaling elements play important roles in plants, including a central role in cytokinin signaling. We characterized two-component elements from the monocot rice (Oryza sativa) using several complementary approaches. Phylogenetic analysis reveals relatively simple orthologous relationships among the histidine kinases in rice and Arabidopsis (Arabidopsis thaliana). In contrast, the histidine-containing phosphotransfer proteins (OsHPs) and response regulators (OsRRs) display a higher degree of lineage-specific expansion. The intracellular localizations of several OsHPs and OsRRs were examined in rice and generally found to correspond to the localizations of their dicot counterparts. The functionality of rice type-B OsRRs was tested in Arabidopsis; one from a clade composed of both monocot and dicot type-B OsRRs complemented an Arabidopsis type-B response regulator mutant, but a type-B OsRR from a monocot-specific subfamily generally did not. The expression of genes encoding two-component elements and proteins involved in cytokinin biosynthesis and degradation was analyzed in rice roots and shoots and in response to phytohormones. Nearly all type-A OsRRs and OsHK4 were up-regulated in response to cytokinin, but other cytokinin signaling elements were not appreciably affected. Furthermore, multiple cytokinin oxidase (OsCKX) genes were up-regulated by cytokinin. Abscisic acid treatment decreased the expression of several genes involved in cytokinin biosynthesis and degradation. Auxin affected the expression of a few genes; brassinosteroid and gibberellin had only modest effects. Our results support a shared role for two-component elements in mediating cytokinin signaling in monocots and dicots and reveal how phytohormones can impact cytokinin function through modulating gene expression.     

The role of phytohormones in cotton fiber development

Since 1974, when Beasley and Ting discovered that fertilized ovules of cotton can be cultured in media supplemented with GA along with auxin, the effect of all types of phytohormones on fiber development has been widely studied. Many phytohormones, including GA, IAA, brassinosteroid (Br), ABA, ethylene (Et), and cytokinins (Ck), all have been demonstrated to play important roles during cotton fiber development. In recent years, the rapid development of genomic analysis and the accumulation of high-quality cotton ESTs allowed us to probe phytohormonal gene expression during fiber development. Many phytohormonal genes, including GA-, IAA-, ABA-, Br-, Et-, and Ck-related genes, participating in phytohormone biosynthesis pathways and signal transduction pathway accumulated in the process of cotton fiber development.     

Analysis of protein interactions within the cytokinin-signaling pathway of Arabidopsis thaliana

The signal of the plant hormone cytokinin is perceived by membrane-located sensor histidine kinases and transduced by other members of the plant two-component system. In Arabidopsis thaliana, 28 two-component system proteins (phosphotransmitters and response regulators) act downstream of three receptors, transmitting the signal from the membrane to the nucleus and modulating the cellular response. Although the principal signaling mechanism has been elucidated, redundancy in the system has made it difficult to understand which of the many components interact to control the downstream biological processes. Here, we present a large-scale interaction study comprising most members of the Arabidopsis cytokinin signaling pathway. Using the yeast two-hybrid system, we detected 42 new interactions, of which more than 90% were confirmed by in vitro coaffinity purification. There are distinct patterns of interaction between protein families, but only a few interactions between proteins of the same family. An interaction map of this signaling pathway shows the Arabidopsis histidine phosphotransfer proteins as hubs, which interact with members from all other protein families, mostly in a redundant fashion. Domain-mapping experiments revealed the interaction domains of the proteins of this pathway. Analyses of Arabidopsis histidine phosphotransfer protein 5 mutant proteins showed that the presence of the canonical phospho-accepting histidine residue is not required for the interactions. Interaction of A-type response regulators with Arabidopsis histidine phosphotransfer proteins but not with B-type response regulators suggests that their known activity in feedback regulation may be realized by interfering at the level of Arabidopsis histidine phosphotransfer protein-mediated signaling. This study contributes to our understanding of the protein interactions of the cytokinin-signaling system and provides a framework for further functional studies in planta.     

Signaling via Histidine-Containing Phosphotransfer Proteins in Arabidopsis

The Arabidopsis genome encodes a number of proteins with similarity to two-component phosphorelay signaling elements, including hybrid receptor histidine kinases, two classes of response regulator proteins (type-A and type-B ARRs) and a family of six histidine-containing phosphotransfer proteins (AHPs), five of which contain a conserved His residue that is required for phosphorelay signaling. The current model for cytokinin signaling includes a multistep phosphorelay: three histidine kinases and at least five type-B ARRs have been shown to act as positive regulators of cytokinin signaling, while a number of type-A ARRs, and AHP6, act as negative regulators of the pathway. In our recent Plant Cell paper, we provided genetic evidence that at least four AHPs can act as positive regulators of cytokinin signaling, affecting responses to cytokinin in the root and the shoot. In this addendum, we discuss the role of AHPs in cytokinin signaling and speculate on their potential interactions with other signaling pathways in Arabidopsis.Key Words: Arabidopsis, cytokinin, two-component signaling, phosphorelay, AHP     

ACC synthase and its cognate E3 ligase are inversely regulated by light

1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) is the key enzyme in ethylene biosynthesis, catalyzing the conversion of S-adenosylmethionine (AdoMet) to ACC, which is the immediate precursor of ethylene. The regulation of ACS protein stability plays an important role in controlling ethylene biosynthesis. We have recently shown that 14-3-3 positively regulates ACS protein stability by both a direct effect and via downregulation of the stability of the E3 ligases regulating its turnover, Ethylene Overproducer1 (ETO1)/ETO1-like (EOL). Here, we report that treatment of etiolated Arabidopsis seedlings with light rapidly increases the stability of ACS5 protein. In contrast, light destabilizes the ETO1/EOLs proteins, suggesting that light acts to increase ethylene biosynthesis in part through a decrease in the level of the ETO1/EOL proteins. This demonstrates that the ETO1/EOLs are regulated in response to at least one environmental cue and that their regulated degradation may represent a novel input controlling ethylene biosynthesis.     

Cytokinin and auxin inhibit abscisic acid-induced stomatal closure by enhancing ethylene production in Arabidopsis

Cytokinins and auxins are major phytohormones involved in various aspects of plant growth and development. These phytohormones are also known to antagonize the effects of abscisic acid (ABA) on stomatal movement, and to affect ethylene biosynthesis. As ethylene has an antagonistic effect on ABA-induced stomatal closure, the possibility that the antagonistic effects of these phytohormones on ABA were mediated through ethylene biosynthesis was investigated. Both the cytokinin, 6-benzyladenine (BA), and the auxin, 1-naphthaleneacetic acid (NAA), antagonized ABA-induced stomatal closure in a manner similar to that following application of the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC). However, these effects were negated when ethylene signalling, perception, or biosynthesis were blocked. As stomatal aperture is regulated by changes in guard cell volume, ABA application was found to reduce the volume of the guard cell protoplasts (GCP). It was found that BA, NAA, or ACC application compensated perfectly for the reduction in GCP volume by ABA application in WT plants. The above observations suggest that cytokinins and auxins inhibit ABA-induced stomatal closure through the modulation of ethylene biosynthesis, and that ethylene inhibits the ABA-induced reduction of osmotic pressure in the guard cells.     

乙烯、超长链脂肪酸、活性氧、油菜素内酯和赤霉素相互作用调控棉纤维伸长发育的分子机制研究

棉纤维细胞的快速极性生长与植物激素的合成、细胞膜和细胞壁的合成、细胞壁的松弛和延展密切相关。其中,植物激素作为调控因子一直是纤维发育领域研究的热点。该文对植物激素乙烯、油菜素内酯、赤霉素、活性氧以及超长链脂肪酸的生物合成及信号转导途径进行了综述,并介绍了它们在棉纤维发生发育过程中的作用机理及相互关系的最新研究进展,为深入阐明纤维细胞伸长的调控规律,最终提高棉纤维产量和品质提供理论依据。     

植物荫蔽胁迫的激素信号响应

植物的生长发育与光信号密切相关, 外界光强、光质的变化会改变植物的生长发育状态。在自然或人工生态系统中, 植株个体的光环境往往会被其周围植物所影响, 导致荫蔽胁迫, 其主要表现为光合有效辐射以及红光与远红光比值(R:FR)降低。荫蔽胁迫对植物生长发育的多个时期均有影响, 如抑制种子萌发、促进幼苗下胚轴伸长及促进植物花期提前等, 这对农业生产不利, 会导致作物产量以及品质的降低。植物激素是调控植物生长发育的关键内源因子。大量研究表明, 生长素(IAA)、赤霉素(GA)及油菜素甾醇(BR)等植物激素均参与介导植物的荫蔽胁迫响应。当植物处于荫蔽胁迫时, 光信号的改变会影响植物激素的合成及信号转导。不同植物激素对荫蔽胁迫的响应各不相同, 但其信号通路之间却存在互作关系, 从而形成复杂的网络状调控路径。该文总结了几种主要植物激素(生长素、赤霉素、油菜素甾醇及乙烯)响应荫蔽胁迫的机理, 重点论述了荫蔽胁迫对植物激素合成及信号通路的影响, 以及植物激素调控荫蔽胁迫下植物生长的分子机理, 并对未来潜在的研究热点进行了分析。