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 induce sporulation in Dictyostelium

The social amoeba Dictyostelium discoideum diverged from the line leading to animals shortly after the separation of plants and animals but it retained characteristics of both kingdoms. A GABA(B)-like receptor and a peptide, SDF-2, with homologs found only in animals, control sporulation, while cytokinins, which act as hormones in plants, keep spores dormant. When SDF-2 binds its receptor DhkA, it reduces the activity of the cAMP phosphodiesterase RegA such that cAMP levels can increase. It has been proposed that the cytokinin discadenine also produces in an increase in cAMP but acts through a different histidine kinase, DhkB. We have found that discadenine and its precursor, isopentenyl adenine, not only maintain spore dormancy but also initiate rapid encapsulation independently of the SDF-2 signal transduction pathway. DhkB and the adenylyl cyclase of late development, AcrA, are members of two component signal transduction families and both are required to transduce the cytokinin signal. As expected, strains lacking the isopentenyl-transferase enzyme chiefly responsible for cytokinin synthesis are defective in sporulation. It appears that SDF-2 and cytokinins are secreted during late development to trigger signal transduction pathways that lead to an increase in the activity of the camp-dependent protein kinase, PKA, which triggers rapid encapsulation as well as ensuring spore dormancy.     

Characterization of cytokinin and adenine transport in Arabidopsis cell cultures

Cytokinins are distributed through the vascular system and trigger responses of target cells via receptor-mediated signal transduction. Perception and transduction of the signal can occur at the plasma membrane or in the cytosol. The signal is terminated by the action of extra- or intracellular cytokinin oxidases. While radiotracer studies have been used to study transport and metabolism of cytokinins in plants, little is known about the kinetic properties of cytokinin transport. To provide a reference dataset, radiolabeled trans-zeatin (tZ) was used for uptake studies in Arabidopsis (Arabidopsis thaliana) cell culture. Uptake kinetics of tZ are multiphasic, indicating the presence of both low- and high-affinity transport systems. The protonophore carbonyl cyanide m-chlorophenylhydrazone is an effective inhibitor of cytokinin uptake, consistent with H(+)-mediated uptake. Other physiological cytokinins, such as isopentenyl adenine and benzylaminopurine, are effective competitors of tZ uptake, whereas allantoin has no inhibitory effect. Adenine competes for zeatin uptake, indicating that the degradation product of cytokinin oxidases is transported by the same systems. Comparison of adenine and tZ uptake in Arabidopsis seedlings reveals similar uptake kinetics. Kinetic properties, as well as substrate specificity determined in cell cultures, are compatible with the hypothesis that members of the plant-specific purine permease family play a role in adenine transport for scavenging extracellular adenine and may, in addition, be involved in low-affinity cytokinin uptake.     

Overexpression of Arabidopsis response regulators,ARR4/ATRR1/IBC7 and ARR8/ATRR3, alters cytokinin responses differentially in the shoot and in callus formation

Arabidopsis ARR4/ATRR1/IBC7 and ARR8/ATRR3 are homologous genes of prokaryotic response regulators that are involved in the His-Asp phosphorelay signal transduction. We analyzed the function of these genes as response regulators using transgenic plants. Overexpression of ARR4 in cultured stems of the transgenics markedly promoted shoot formation in the presence of cytokinin, while overexpression of ARR8 repressed shoot formation and greening of calli. The expression level of cytokinin-inducible genes, cycD3 and cab increased in the ARR4 overexpressor but decreased in the ARR8 overexpressor. By contrast, two drought stress-inducible genes, rd29A and erd1, were expressed in both overexpressors as that in control plants. These results suggest that ARR4 and ARR8 are involved in cytokinin signal transduction, and that ARR4 functions as a positive-regulator, whereas ARR8 functions as a negative-regulator. Furthermore, microarray analysis showed that several genes were up-regulated in the ARR4 overexpressor. Consistent with these results, ARR4 and ARR8 might play important roles in the sensoring system of cytokinin signal transduction pathway in various developmental and environmental conditions and the regulation of gene expression.     

How do cytokinins affect the cell?

The lecture presents modern knowledge of the mechanisms of cytokinin perception and signal transduction to the genes of primary and secondary responses. It also demonstrates the relations between the rapid cytokinin-induced processes and cytokinin-induced physiological effects. The characteristics of the cytokinin regulatory system and its role in the control of plant growth and development are discussed.     

Type-A Arabidopsis response regulators are partially redundant negative regulators of cytokinin signaling

Type-A Arabidopsis (Arabidopsis thaliana) response regulators (ARRs) are a family of 10 genes that are rapidly induced by cytokinin and are highly similar to bacterial two-component response regulators. We have isolated T-DNA insertions in six of the type-A ARRs and constructed multiple insertional mutants, including the arr3,4,5,6,8,9 hextuple mutant. Single arr mutants were indistinguishable from the wild type in various cytokinin assays; double and higher order arr mutants showed progressively increasing sensitivity to cytokinin, indicating functional overlap among type-A ARRs and that these genes act as negative regulators of cytokinin responses. The induction of cytokinin primary response genes was amplified in arr mutants, indicating that the primary response to cytokinin is affected. Spatial patterns of ARR gene expression were consistent with partially redundant function of these genes in cytokinin signaling. The arr mutants show altered red light sensitivity, suggesting a general involvement of type-A ARRs in light signal transduction. Further, morphological phenotypes of some arr mutants suggest complex regulatory interactions and gene-specific functions among family members.     

Accumulation of maize response regulator proteins in mesophyll cells after cytokinin treatment

The maize response regulator genes ZmRR1 and ZmRR2 respond to cytokinin, and the translated products seem to be involved in nitrogen signal transduction mediated by cytokinin through the His-Asp phosphorelay. To elucidate the physiological function of the proteins, we examined the temporal and spatial distribution in maize leaves by immunochemical analysis and use of transgenic plants. ZmRR1 and ZmRR2 polypeptides could be distinctively detected by western blotting. The polypeptides accumulated in leaves within 5 h of the supply of nitrate to nitrogen-depleted maize, and the accumulation was transient. The extent of induction was larger in the leaf tip, which is rich in photosynthetically matured cells, than elsewhere. In leaves, the polypeptides accumulated mostly in mesophyll cells. Histochemical analyses of transgenic maize harboring a ZmRR1 promoter-beta-glucuronidase fusion gene also showed most of the expression to be in these cells. These results suggest that ZmRR1 and ZmRR2 are induced in mesophyll cells and function in nitrogen signal transduction mediated by cytokinin.     

Biosensors for the Detection of Organophosphorous Pesticides

Cytokinins are master regulators of plant growth and development. They are involved in the regulation of many important physiological and metabolic processes. Recent progress in cytokinin research at the molecular level, including identification of related genes and cytokinin receptors, plus elucidation of signal transduction, has greatly increased our understanding of cytokinin actions. Although still in its infant stage, molecular breeding of crops with altered cytokinin metabolism, when combined with the transgenic approach, has shown very promising potential for application to agriculture. In this review we briefly introduce recent progress in cytokinin molecular biology, discuss applications of cytokinin genetic engineering to agriculture, and present implications and future research directions.     

Genetic engineering of cytokinins and their application to agriculture

Cytokinins are master regulators of plant growth and development. They are involved in the regulation of many important physiological and metabolic processes. Recent progress in cytokinin research at the molecular level, including identification of related genes and cytokinin receptors, plus elucidation of signal transduction, has greatly increased our understanding of cytokinin actions. Although still in its infant stage, molecular breeding of crops with altered cytokinin metabolism, when combined with the transgenic approach, has shown very promising potential for application to agriculture. In this review we briefly introduce recent progress in cytokinin molecular biology, discuss applications of cytokinin genetic engineering to agriculture, and present implications and future research directions.     

Gravity signal transduction in primary roots

AIMS: The molecular mechanisms that correlate with gravity perception and signal transduction in the tip of angiosperm primary roots are discussed. SCOPE: Gravity provides a cue for downward orientation of plant roots, allowing anchorage of the plant and uptake of the water and nutrients needed for growth and development. Root gravitropism involves a succession of physiological steps: gravity perception and signal transduction (mainly mediated by the columella cells of the root cap); signal transmission to the elongation zone; and curvature response. Interesting new insights into gravity perception and signal transduction within the root tip have accumulated recently by use of a wide range of experimental approaches in physiology, biochemistry, genetics, genomics, proteomics and cell biology. The data suggest a network of signal transduction pathways leading to a lateral redistribution of auxin across the root cap and a possible involvement of cytokinin in initial phases of gravicurvature. CONCLUSION: These new discoveries illustrate the complexity of a highly redundant gravity-signalling process in roots, and help to elucidate the global mechanisms that govern auxin transport and morphogenetic regulation in roots.     

Structural basis for cytokinin receptor signaling: an evolutionary approach

Cytokinins are ubiquitous plant hormones; their signal is perceived by sensor histidine kinases—cytokinin receptors. This review focuses on recent advances on cytokinin receptor structure, in particular sensing module and adjacent domains which play an important role in hormone recognition, signal transduction and receptor subcellular localization. Principles of cytokinin binding site organization and point mutations affecting signaling are discussed. To date, more than 100 putative cytokinin receptor genes from different plant species were revealed due to the total genome sequencing. This allowed us to employ an evolutionary and bioinformatics approaches to clarify some new aspects of receptor structure and function. Non-transmembrane areas adjacent to the ligand-binding CHASE domain were characterized in detail and new conserved protein motifs were recovered. Putative mechanisms for cytokinin-triggered receptor activation were suggested.     

Environmental perception avenues: the interaction of cytokinin and environmental response pathways

Cytokinins were discovered in the 1950s by their ability to promote cell division in cultured plant cells. Recently, there have been significant breakthroughs in our understanding of the biosynthesis, metabolism, perception and signal transduction of this phytohormone. These advances, coupled with physiological and other approaches, have enabled remarkable progress to be made in our understanding of the interactions between cytokinin function and environmental inputs. In this review, we first highlight the most recent advances in our understanding of cytokinin biosynthesis, metabolism and signalling. We then discuss how various environmental signals interact with these pathways to modulate plant growth, development and physiology.     

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.     

Plant response regulators implicated in signal transduction and circadian rhythm

The so-called 'response regulators' were originally discovered as common components of the widespread histidine (His)-->aspartate (Asp) phosphorelay signal transduction system in prokaryotes. Through the course of evolution, higher plants have also come to employ such prokaryotic response regulators (RRs) for their own signal transduction, such as the elicitation of plant hormone (e.g. cytokinin) responses. Furthermore, plants have evolved their own atypical variants of response regulators, pseudo response regulators (PRRs), which are used to modulate sophisticated biological processes, including circadian rhythms and other light-signal responses. Recent studies using the model plant Arabidopsis thaliana have begun to shed light on the interesting functions of these plant response regulators.