B1-type cyclin-dependent kinases are essential for the formation of stomatal complexes in Arabidopsis thaliana

Cyclin-dependent kinases (CDKs) are key regulators of the cell cycle. In yeasts, only one CDK is sufficient to drive cells through the cell cycle, whereas higher eukaryotes developed a family of related CDKs. Curiously, plants contain a unique class of CDKs (B-type CDKs), whose function is still unclear. We show that the CDKB1;1 gene of Arabidopsis (Arabidopsis thaliana) is highly expressed in guard cells and stomatal precursor cells of cotyledons, suggesting a prominent role for B-type CDKs in stomatal development. In accordance, transgenic Arabidopsis plants with reduced B-type CDK activity had a decreased stomatal index because of an early block of meristemoid division and inhibition of satellite meristemoid formation. Many aberrant stomatal cells were observed, all of them blocked in the G2 phase of the cell cycle. Although division of stomatal precursors was inhibited, cells still acquired stomatal identity, illustrating that stomatal cell differentiation is independent of cellular and nuclear division.     

Regulation of cyclin-dependent kinases in Arabidopsis thaliana

In plants, different families of cyclin-dependent kinases (CDKs) and cyclins have been identified, indicating that also in plants the progression through the cell cycle is regulated by CDKs. In all eukaryotes, CDKs exert their activity through well-controlled phosphorylations of specific substrates on serine/threonine residues. Such post-translational modifications are universal mechanisms in signal transduction pathways. They allow the organism to differentiate, regulate growth and/or adapt to environmental changes, the latter being crucial for plants because of their sedentary life-style. This adaptation might explain the occurrence of a special CDK type with plant-specific features. This review focuses on the involvement of plant CDKs in different phases of the cell cycle in Arabidopsis thaliana and outlines their regulation by binding to other proteins, and by phosphorylation and dephosphorylation.     

Mutational analysis of two Arabidopsis thaliana cyclin-dependent kinases in fission yeast

We have analyzed five mutant alleles of two cyclin-dependent kinases from Arabidopsis thaliana, CDC2aAt and CDC2bAt, in Schizosaccharomyces pombe. Two of the five mutant alleles produced similar phenotypes for both cyclin-dependent kinases. The other three mutants caused phenotypes dependent on the particular cyclin-dependent kinase. Of all the mutant alleles, only two were found to possess a detectable kinase activity. Our mutational analysis lends further support for CDC2aAt being the true orthologue of the yeast cdc2. CDC2bAt, even though quite divergent from S. pombe cdc2, still retains the ability to interact with at least some essential cell cycle regulators, suggesting some functional homology with the yeast protein. Additionally, we demonstrated that the three amino acid deletion in the DL50 mutants results in the loss of the ability to interact with the suc1/CKS1 proteins.     

Photoperiod regulates flower meristem development in Arabidopsis thaliana

Photoperiod has been known to regulate flowering time in many plant species. In Arabidopsis, genes in the long day (LD) pathway detect photoperiod and promote flowering under LD. It was previously reported that clavata2 (clv2) mutants grown under short day (SD) conditions showed suppression of the flower meristem defects, namely the accumulation of stem cells and the resulting production of extra floral organs. Detailed analysis of this phenomenon presented here demonstrates that the suppression is a true photoperiodic response mediated by the inactivation of the LD pathway under SD. Inactivation of the LD pathway was sufficient to suppress the clv2 defects under LD, and activation of the LD pathway under SD conditions restored clv2 phenotypes. These results reveal a novel role of photoperiod in flower meristem development in Arabidopsis. Flower meristem defects of clv1 and clv3 mutants are also suppressed under SD, and 35S:CO enhanced the defects of clv3, indicating that the LD pathway works independently from the CLV genes. A model is proposed to explain the interactions between photoperiod and the CLV genes.     

Regulators of cyclin-dependent kinases are crucial for maintaining genome integrity in S phase

Maintenance of genome integrity is of critical importance to cells. To identify key regulators of genomic integrity, we screened a human cell line with a kinome small interfering RNA library. WEE1, a major regulator of mitotic entry, and CHK1 were among the genes identified. Both kinases are important negative regulators of CDK1 and -2. Strikingly, WEE1 depletion rapidly induced DNA damage in S phase in newly replicated DNA, which was accompanied by a marked increase in single-stranded DNA. This DNA damage is dependent on CDK1 and -2 as well as the replication proteins MCM2 and CDT1 but not CDC25A. Conversely, DNA damage after CHK1 inhibition is highly dependent on CDC25A. Furthermore, the inferior proliferation of CHK1-depleted cells is improved substantially by codepletion of CDC25A. We conclude that the mitotic kinase WEE1 and CHK1 jointly maintain balanced cellular control of Cdk activity during normal DNA replication, which is crucial to prevent the generation of harmful DNA lesions during replication.     

The phosphoinositide-3-OH-kinase-related kinases of Arabidopsis thaliana

The phosphoinositide-3-OH-kinase-related kinases (PIKKs) are atypical protein kinases exclusive to eukaryotes. They mediate the cellular response to a range of stresses, including genome and RNA surveillance and availability of nutrients for growth. Orthologues of five out of the six PIKK family members are present in plant genomes. Recent studies in plant PIKKs have revealed features unique to, and in common with, other PIKKs. This review summarizes the basic knowledge of these proteins in mammals and yeast in comparison with what is known for Arabidopsis and other plants.     

2-Aminoquinazoline inhibitors of cyclin-dependent kinases

The inhibition of cyclin-dependent kinase 4 (Cdk4) causes cell cycle arrest and restores a checkpoint that is absent in the majority of tumor cells. Compounds that inhibit Cdk4 selectively are targeted for treating cancer. Appropriate substitution of 2-aminoquinazolines is demonstrated to produce high levels of selectivity for Cdk4 versus closely related serine-threonine kinases.     

Cell fate in the inflorescence meristem and floral buttress of Arabidopsis thaliana

The flowers and leaves of Arabidopsis are arranged in a spiral with successive organs positioned at intervals of approximately 140°. This simple phyllotaxy determines the organization of the flowers of the inflorescence. Here we describe the analysis of X-ray-induced albino sectors in the L2 layer of the Arabidopsis inflorescence. No evidence was found of lineage restrictions within the inflorescence meristem. Comparison of the spatial relationships between albino and green tissue in the sepals of 43 chimeric inflorescences allowed the generation of a three-dimensional fate map. The map relates the initiation of flowers in the plant apex to their final arrangement. The map was found to be a shallow dome with phyllotaxy superimposed on its surface. A similar map was prepared of the floral buttress and this was found to be a ridge with sepal primordia at its edges. Unlike other fate maps of plants these maps relate the relative positions and sizes of organ primordia in terms of the frequency with which they are of the same somatic phenotype, and not the number of cells giving rise to them.     

Axillary meristem development in the branchless Zu-0 ecotype of Arabidopsis thaliana

Axillary meristems form in the leaf axils during post-embryonic development. In order to initiate the genetic dissection of axillary meristem development, we have characterized the late-flowering branchless ecotype of Arabidopsis thaliana (L.) Heynh., Zu-0. The first-formed rosette leaves of Zu-0 plants all initiate axillary meristems, but later-formed leaves of the rosette remain branchless. Alteration in the meristem development is axillary meristem-specific because the shoot apical and floral meristems develop normally. Scanning electron microscopy, histology and RNA in situ analysis with SHOOTMERISTEMLESS ( STM), a marker for meristematic tissues, show that a mound of cells form and STM mRNA accumulates in barren leaf axils, indicating that axillary meristems initiate but arrest in their development prior to organizing a meristem proper. Expression and retention of the STM RNA in barren leaf axils further suggests that STM expression is not sufficient for the establishment of the axillary meristem proper.     

Requirement of homeobox gene STIMPY/WOX9 for Arabidopsis meristem growth and maintenance

Most organs of flowering plants develop postembryonically from groups of pluripotent cells called meristems [1]. The shoot apical meristem (SAM) is specified by two complementary pathways [2-4]. SHOOT MERISTEMLESS (STM; [5]) defines the entire SAM region [6]. WUSCHEL (WUS), on the other hand, functions in a more restricted set of cells to promote stem-cell fate and is regulated by the CLAVATA genes in a negative feedback loop [7-10]. In contrast, little is known about how the growth of the SAM, which increases in size during vegetative development [11], is regulated. We have characterized STIMPY (STIP; also called WOX9 [12]), a homeobox gene required for the growth of the vegetative SAM, in part by positively regulating WUS expression. In addition, STIP is required in several other aerial organs and the root. What sets STIP apart from STM and WUS is that stip mutants can be fully rescued by stimulating the entry into the cell cycle with sucrose. Therefore, STIP is likely to act in all these tissues by maintaining cell division and preventing premature differentiation. Taken together, our findings suggest that STIP identifies a new genetic pathway integrating developmental signals with cell-cycle control.     

Effects of gibberellin mutations on tolerance to apical meristem damage in Arabidopsis thaliana

To examine the role of gibberellin hormones (GAs) in tolerance to apical meristem damage (AMD), we characterized the reaction norms of several GA-deficient and insensitive mutants of Arabidopsis thaliana in response to AMD and compared them to those of the wild type, Landsberg, from which they were derived. We included 'natural' genotypes of A. thaliana--accessions with shorter lab histories--in order to evaluate how representative Landsberg is of other genotypes. The GA mutations did not alter the level of tolerance to AMD, which was consistent with equal compensation for all genotypes. Generally, the reaction norms to AMD did not differ among the GA mutants themselves, or between the GA mutants and Landsberg. The GA mutations did affect the overall phenotypes of the plants, but these effects were not simply related to whether the mutation was early or late in the biochemical pathways. The GA-insensitive mutant was phenotypically different from the GA-deficient mutants and from Landsberg. The natural populations differed significantly from Landsberg, particularly in attributes related to size and inflorescence production, one more example of the need for researchers to be careful when generalizing the results of studies based upon laboratory strains. Our results indicate that early-flowering genotypes of A. thaliana can be remarkably tolerant to AMD, and that GA deficiency/insensitivity does not hinder tolerance to AMD, at least in this genetic background. Moreover, we confirm that mutations at regulatory loci can have noncatastrophic effects on fitness, as recently found by other investigators.     

Characterization of microstructure and cell wall components of Arabidopsis thaliana overexpressing cyclin-dependent kinase inhibitor 2

Overexpression of a cyclin-dependent kinase inhibitor (KRP2) caused changes in the general morphology in the leaves of Arabidopsis thaliana. The wild type plant had obovate leaves with entire margins whereas the transgenic line had leaves with denticulate margins. The epidermal cells and stomata of the adult transgenic leaves were significantly larger than those of the wild-type plants and the number of stomata was in proportion to the number of epidermal cells. No apparent differences in thickness and structure of cell walls of the mesophyll cells between the two samples were observed. The smaller amount of cell wall material in the transgenic leaves caused by the larger cell size was also apparent in the lower dry weight of the transgenic leaves. The chemical analysis revealed the main differences to be in pectin and neutral sugar contents, and especially in the amounts of glucose, all being higher in the leaves of the KRP2 transgenic plants. p-Coumaric acid content varied more in the transgenic leaf material than in the control one reflecting possibly fewer cross-links in the cell walls of transgenic plants.     

Shoot apical meristem structure and STM expression in has mutant of Arabidopsis thaliana

Wild-type and the handshake (has) mutants in Arabidopsis thaliana were analyzed. Compared to the wild-type, has mutants display a number of morphological alterations, which can largely be traced back to altered meristem function. Analyses of apical meristem of mutant plants showed that mutation affected meristem structure and patterns of STM expression.     

The SEP4 gene of Arabidopsis thaliana functions in floral organ and meristem identity

The ABC model of flower organ identity is widely recognized as providing a framework for understanding the specification of flower organs in diverse plant species. Recent studies in Arabidopsis thaliana have shown that three closely related MADS-box genes, SEPALLATA1 (SEP1), SEP2 and SEP3, are required to specify petals, stamens, and carpels because these organs are converted into sepals in sep1 sep2 sep3 triple mutants. Additional studies indicate that the SEP proteins form multimeric complexes with the products of the B and C organ identity genes. Here, we characterize the SEP4 gene, which shares extensive sequence similarity to and an overlapping expression pattern with the other SEP genes. Although sep4 single mutants display a phenotype similar to that of wild-type plants, we find that floral organs are converted into leaf-like organs in sep1 sep2 sep3 sep4 quadruple mutants, indicating the involvement of all four SEP genes in the development of sepals. We also find that SEP4 contributes to the development of petals, stamens, and carpels in addition to sepals and that it plays an important role in meristem identity. These and other data demonstrate that the SEP genes play central roles in flower meristem identity and organ identity.     

A SCARECROW-based regulatory circuit controls Arabidopsis thaliana meristem size from the root endodermis

Planta - SCARECROW controls Arabidopsis root meristem size from the root endodermis tissue by regulating the DELLA protein RGA that in turn mediates the regulation of ARR1 levels at the transition...