Cytokinins are plant hormones that may play essential and crucial roles in various aspects of plant growth and development. Although the functional significance of exogenous cytokinins as to the proliferation and differentiation of cells has been well documented, the biological roles of endogenous cytokinins have remained largely unknown. The recent discovery of the Arabidopsis Histidine Kinase 4 (AHK4)/CRE1/WOL cytokinin receptor in Arabidopsis thaliana strongly suggested that the cellular response to cytokinins involves a two-component signal transduction system. However, the lack of an apparent phenotype in the mutant, presumably because of genetic redundancy, prevented us from determining the in planta roles of the cytokinin receptor. To gain insight into the molecular functions of the three AHK genes AHK2, AHK3, and AHK4 in this study, we identified mutational alleles of the AHK2 and AHK3 genes, both of which encode sensor histidine kinases closely related to AHK4, and constructed a set of multiple ahk mutants. Application of exogenous cytokinins to the resultant strains revealed that both AHK2 and AHK3 function as positive regulators for cytokinin signaling similar to AHK4. The ahk2 ahk4 and ahk3 ahk4 double mutants and the ahk single mutants grew normally, whereas the ahk2 ahk3 double mutants exhibited a semidwarf phenotype as to shoots, such as a reduced leaf size and a reduced influorescence stem length. The growth and development of the ahk2 ahk3 ahk4 triple mutant were markedly inhibited in various tissues and organs, including the roots and leaves in the vegetative growth phase and the influorescence meristem in the reproductive phase. We showed that the inhibition of growth is associated with reduced meristematic activity of cells. Expression analysis involving AHK:b-glucuronidase fusion genes suggested that the AHK genes are expressed ubiquitously in various tissues during postembryonic growth and development. Our results thus strongly suggest that the primary functions of AHK genes, and those of endogenous cytokinins, are triggering of the cell division and maintenance of the meristematic competence of cells to prevent subsequent differentiation until a sufficient number of cells has accumulated during organogenesis.

SummaryAlthough the shoot apical meristem (SAM) is ultimately responsible for post-embryonic development in higher plants, lateral meristems also play an important role in determining the final morphology of the above-ground part. Axillary buds developing at the axils of leaves produce additional shoot systems, lateral branches. The rice TB1 gene (OsTB1) was first identified based on its sequence similarity with maize TEOSINTE BRANCHED 1 (TB1), which is involved in lateral branching in maize. Both genes encode putative transcription factors carrying a basic helix-loop-helix type of DNA-binding motif, named the TCP domain. The genetic locus of OsTB1 suggested that OsTB1 is a real counterpart of maize TB1. Transgenic rice plants overexpressing OsTB1 exhibited markedly reduced lateral branching without the propagation of axillary buds being affected. We also demonstrated that a rice strain carrying a classical morphological marker mutation, fine culm 1 (fc1), contain the loss-of-function mutation of OsTB1 and exhibits enhanced lateral branching. Expression of OsTB1, as examined with a putative promoter-glucuronidase (GUS) gene fusion, was observed throughout the axillary bud, as well as the basal part of the shoot apical meristem, vascular tissues in the pith and the lamina joint. Taking these data together, we concluded that OsTB1 functions as a negative regulator for lateral branching in rice, presumably through expression in axillary buds.

No abstract

In Escherichia coli, recent intensive studies revealed that expression of a certain subset of genes is under the control of the stationary phase‐specific sigma factor, sigma S, which is encoded by the rpoS gene. Since sigma S functions predominantly under certain growth conditions, its activity and/or cellular content has accordingly to be tightly controlled, however, the underlying molecular mechanism is at present unclear. We previously demonstrated that expression of the cbpA gene, encoding an analogue of the DnaJ molecular chaperone, is largely dependent on sigma S function. Here we have found that a mutational lesion of the hns gene, which encodes one of the well‐characterized nucleoid proteins, H‐NS, affects the cellular content of sigma S remarkably and consequently affects the expression of cbpA. Enhanced accumulation of sigma S in hns deletion cells was particularly observed in the logarithmic growth phase and was demonstrated to result from an elevated translational efficiency of rpoS mRNA and also from an increased stability of newly synthesized sigma S. Although H‐NS is known to influence the transcription of a number of apparently unlinked genes on the chromosome, in this study we provide a novel instance in which H‐NS is deeply implicated in post‐transcriptional regulation(s) of the expression of rpoS. As to physiological relevance, it was also demonstrated that hns deletion cells exhibit an extreme thermotolerance even in the logarithmic growth phase, presumably because of the enhanced accumulation of sigma S.

His-Asp phosphorelays are evolutionary-conserved powerful biological tactics for intracellular signal transduction. Such a phosphorelay is generally made up of "sensor histidine (His)-kinases", "response regulators", and "histidine-containing (HPt) phosphotransmitters". In the higher plant, Arabidopsis thaliana, results from recent intensive studies suggested that His-Asp phosphorelays may be widely used for propagating environmental stimuli, such as phytohormones (e.g., ethylene and cytokinin). In this study, we first inspected extensively the occurrence of Arabidopsis response regulators in order to compile and characterize them. The results showed that this higher plant has, at least, 14 members of the family of response regulators that can be classified into two distinct subtypes (type-A and type-B), as judged from their structural designs, biochemical properties, and expression profiles. Comparative studies were conducted for each representative (ARR3 and ARR4 for type-A, and ARR10 for type-B). It was suggested that expression of the type-A response regulator is cytokinin-inducible, while that of the type-B response regulator appears to be not. Results from yeast two-hybrid analyses suggested that the type-B response regulator may have an ability to stably interact with a set of HPt phosphotransmitters (AHPs). These and other results will be discussed with special reference to the His-Asp phosphorelay signaling network in Arabidopsis thaliana.

The H-NS protein is a major constituent of the Escherichia coli nucleoid structure and is implicated in the compact organization of the chromosome. Based on recent genetic evidence, this protein appears to influence the transcription of a variety of apparently unlinked genes on the chromosome, although the underlying molecular mechanism is not fully understood. In this study, we carried out a series of in vitro transcription assays including purified H-NS with special reference to the osmotically inducible proV promoter of the proVWX operon (or proU), whose expression is known to be derepressed by lesions of the hns (osmZ) gene. Here, H-NS was revealed to selectively inhibit an early step(s) of proV transcription initiation through its direct binding to the promoter region. It was thus demonstrated that H-NS functions directly as a transcriptional repressor.Under the in vitro conditions used, this in vitro inhibitory effect of H-NS was affected by changes in the superhelical density of template DNAs and more significantly by the concentration of potassium (K+) ions. These results are also discussed with regard to the mechanism underlying regulation of the proV promoter in response to the medium osmolarity.

Escherichia coli heat‐shock proteins GroES and GroEL are essential cytoplasmic proteins, which have been termed ‘chaperonins’ because of their ability to assist protein assembly of bacteriophage capsids and multimeric enzymes of foreign origin. In this report we show that temperature‐sensitive mutations in groES and groEL genes cause defective export of the plasmid‐encoded beta‐lactamase (Bla) in vivo. Since efficient translocation of proteins across biological membranes is thought to be supported by cytoplasmic factors that protect presecretory molecules from being misfolded, these results suggest that both GroES and GroEL proteins possess a chaperone function by which they facilitate export of Bla. The translocation of other secretory proteins, however, appears to depend minimally on GroE, suggesting that GroE interacts only with a specific class of secreted proteins.

We previously identified a set of structurally related genes, AHK2, 3 and 4, each encoding a sensor histidine kinase in Arabidopsis thaliana. To determine the relevant biological functions, we identified a loss-of-function mutation of the AHK4 gene. The mutant exhibited the cytokinin-resistant phenotype not only in inhibition of root growth by cytokinin but also in greening and shoot induction of calli. Moreover, AHK4 expressed in budding yeast showed histidine kinase activity in a manner dependent on the presence of cytokinin. These results strongly suggested that AHK4 is involved in the cytokinin-signaling pathway, as a direct receptor molecule, in Arabidopsis.