Isolation of a full-length mitotic cyclin cDNA clone CycIIIMs from Medicago sativa: chromosomal mapping and expression

The histone phosphatase inhibitory property of plant nucleosome assembly protein-related proteins (NRPs)

SET/I(2)(PP2A), a member of the family of nucleosome assembly proteins (NAPs), has been previously described as a multifunctional protein inhibiting protein phosphatase 2A (PP2A)-mediated histone H3((pSer10)) dephosphorylation during the heat shock response in animal cells. In the present work we demonstrate that its plant orthologs, designated as NAP-related proteins (NRPs), have a similar in vitro biochemical activity and interact with PP2A and histone H3((pSer10))in vivo. Although heat shock gene promoters were found to be associated with histone H3((pSer10))-marked chromatin following a high temperature treatment, heat shock gene expression was not affected in NRP-deficient mutant Arabidopsis thaliana (L.) plantlets. These observations indicate that NRPs are potential regulators of histone dephosphorylation in plants, but they are dispensable for gene expression reorganization in response to heat shock.

Activation of an alfalfa cyclin-dependent kinase inhibitor by calmodulin-like domain protein kinase

Kip-related proteins (KRPs) play a central role in the regulation of the cell cycle and differentiation through modulation of cyclin-dependent kinase (CDK) functions. We have identified a CDK inhibitor gene from Medicago truncatula (Mt) by a yeast two-hybrid screen. The KRPMt gene was expressed in all plant organs and cultured cells, and its transcripts accumulated after abscisic acid and NaCl treatment. The KRPMt protein exhibits seven conserved sequence domains and a PEST motif that is also detected in various Arabidopsis KRPs. In the yeast two-hybrid test, the KRPMt protein interacted with CDK (Medsa;CDKA;1) and D-type cyclins. However, in the pull-down assays, B-type CDK complexes were also detectable. Recombinant KRPMt differentially inhibited various alfalfa CDK complexes in phosphorylation assays. The immunoprecipitated Medsa;CDKA;1/A;2 complex was strongly inhibited, whereas the mitotic Medsa;CDKB2;1 complex was the most sensitive to inhibition. Function of Medsa;CDKB1;1 complex was not inhibited by the KRPMt protein. The mitotic Medsa;CYCB2 and Medsa;CYCA2;1 complexes responded weakly to this inhibitor protein. Kinase complexes from G2/M cells showed increased sensitivity towards the inhibitor compared with those isolated from G1/S-phase cells. In vitro phosphorylation of Medicago retinoblastoma-related protein was also reduced in the presence of KRPMt. Phosphorylation of this inhibitor protein by the recombinant calmodulin-like domain protein kinase (MsCPK3) resulted in enhanced inhibition of CDK function. The data presented emphasize the selective sensitivity of various cyclin-dependent kinase complexes to this inhibitor protein, and suggest a role for CDK inhibitors and CPKs in cross-talk between Ca2+ signalling and regulation of cell-cycle progression in plants.

Characterization of three Rop GTPase genes of alfalfa (Medicago sativa L.)

Three cDNA clones coding for Medicago sativa Rop GTPases have been isolated. The represented genes could be assigned to various linkage groups by genetic mapping. They were expressed in all investigated plant organs, although at different level. Relative gene expression patterns in response to Sinorhizobium infection of roots as well as during somatic embryogenesis indicated their differential participation in these processes. DNA sequences coding for altogether six different Medicago sp. Rop GTPases could be identified in sequence databases. Based on their homology to each other and to their Arabidopsis counterparts, a unified nomenclature is suggested for Medicago Rop GTPases.

Cellular basis of shoot apical meristem development

Shoot apical meristems are composed of proliferating, embryonic type cells, that generate tissues and organs throughout the life of the plant. This review covers the cell biology of the higher plant shoot apical meristem (SAM). The first section describes the molecular basis of plant cell growth and division. The genetic mechanisms, that operate in meristem function and the identification of several key regulators of meristem behavior are described in the second section, and intercellular communication and coordination of cellular behavior in the third part. Finally, we discuss some recent results that indicate interaction between the cellular regulators, such as the cell cycle control genes and developmental regulators.

Cell cycle function of a Medicago sativa A2-type cyclin interacting with a PSTAIRE-type cyclin-dependent kinase and a retinoblastoma protein

In plants multiple A-type cyclins with distinct expression patterns have been isolated and classified into three subgroups (A1-A3), while in animal somatic cells a single type of cyclin A is required for cell-cycle regulation from the S to M phases. We studied the function of an A2-type cyclin from Medicago sativa (Medsa;cycA2) which, in contrast to animal and most plant A-type cyclins, was expressed in all phases of the cell cycle. Using synchronized alfalfa cell cultures and anti-Medsa;CycA2 polyclonal antibodies, we showed that while the mRNA level increased steadily from the late G1 to the G2-M phase, the protein level after a rapid increase in S-phase reached a plateau during the G2 phase. In the yeast two-hybrid system, the Medsa;CycA2 protein interacted with the PSTAIRE-motif-containing cyclin-dependent kinase Cdc2MsA and with the maize retinoblastoma protein. Unexpectedly, the CycA2-associated kinase activity was biphasic: a first activity peak occurred in the S phase while the major one occurred during the G2/M transition, with no apparent dependence upon the actual levels of the Medsa;CycA2 and Cdc2MsA proteins. Immunohistological localization of the cyclin A2 protein by immunofluorescence and immunogold labelling revealed the presence of Medsa;CycA2 in the nucleus of the interphase and prophase cells, while it was undetectable thereafter during mitosis. Together these data suggest that Medsa;CycA2 plays a role both in the S phase and at the G2/M transition.

Protein phosphatase 1 catalytic subunit isoforms from alfalfa: biochemical characterization and cDNA cloning

The catalytic subunit of protein phosphatase 1 (PP1c) was purified from an alfalfa (Medicago sativa) microcallus cell culture. The preparation was inhibited by rabbit muscle inhibitor-2 and okadaic acid and had a molecular mass of 35 kDa. Five distinct cDNAs termed MsPP1alpha, -beta, -gamma, -delta, and -epsilon were cloned from a M. sativa somatic embryo library. MsPP1alpha was identical to a cDNA reported earlier [A. Páy, M. Pirck, L. Bögre, H. Hirt, and E. Heberle-Bors Mol. Gen. Genet. 244, 176-182, 1994], while the others represented novel isoforms encoded by separate genes. The predicted amino acid sequences of MsPP1alpha, -beta, -gamma, -delta, and -epsilon were highly similar to each other and to other known PP1c sequences. The GST-MsPP1ss fusion protein expressed in Escherichia coli was catalytically active and was inhibited by inhibitor-2 and okadaic acid. Affinity-purified polyclonal MsPP1antipeptide antibody detected a protein of 36 kDa in crude cell extracts. These results proved that the cDNA clone encoded an active PP1c which was very similar to the purified enzyme. The mRNA and protein concentrations of PP1c as well as the specific activity of protein phosphatase 1 did not change during the cell cycle in a synchronized alfalfa cell culture. On the other hand, the isoforms exhibited different steady-state mRNA levels in different plant organs suggesting tissue-specific functions.

The plant cell cycle in context

Biological scientists are eagerly confronting the challenge of understanding the regulatory mechanisms that control the cell division cycle in eukaryotes. New information will have major implications for the treatment of growth-related diseases and cancer in animals. In plants, cell division has a key role in root and shoot growth as well as in the development of vegetative storage organs and reproductive tissues such as flowers and seeds. Many of the strategies for crop improvement, especially those aimed at increasing yield, involve the manipulation of cell division. This review describes, in some detail, the current status of our understanding of the regulation of cell division in eukaryotes and especially in plants. It also features an outline of some preliminary attempts to exploit transgenesis for manipulation of plant cell division.

Immunolocalization of a novel annexin-like protein encoded by a stress and abscisic acid responsive gene in alfalfa

We report here on the isolation and characterization of a full-length cDNA clone from alfalfa termed AnnMs2 encoding a 333 amino acid long polypeptide that shows 32-37% sequence identity with both mammalian and plant annexins, and has four tandem repeats. While other plant annexins exhibit a high level of sequence similarity to each other (up to 77% identity at amino acid level), AnnMs2 appears to be a distinct type of plant annexins. All the four endonexin folds contain the conserved eukaryotic motif within this alfalfa protein, but this element is considerably different in the second repeat. The AnnMs2 gene is expressed in various tissues of alfalfa with elevated mRNA accumulation in root and flower. This gene is activated in cells or tissues exposed to osmotic stress, abscisic acid (ABA) or water deficiency. The recombinant AnnMs2 protein is able to bind to phospholipid in the presence of Ca2+. Indirect immunofluorescence studies using affinity purified rabbit anti-AnnMs2 peptide antibody show mainly nucleolar localization, but the protein sequence lacks the usual nuclear localization signal. The potential role of this novel annexin-like protein in the basic and stress-induced cellular functions is discussed.

Plant cyclins: a unified nomenclature for plant A-, B- and D-type cyclins based on sequence organization

The comparative analysis of a large number of plant cyclins of the A/B family has recently revealed that plants possess two distinct B-type groups and three distinct A-type groups of cyclins. Despite earlier uncertainties, this large-scale comparative analysis has allowed an unequivocal definition of plant cyclins into either A or B classes. We present here the most important results obtained in this study, and extend them to the case of plant D-type cyclins, in which three groups are identified. For each of the plant cyclin groups, consensus sequences have been established and a new, rational, plant-wide naming system is proposed in accordance with the guidelines of the Commission on Plant Gene Nomenclature. This nomenclature is based on the animal system indicating cyclin classes by an upper-case roman letter, and distinct groups within these classes by an arabic numeral suffix. The naming of plant cyclin classes is chosen to indicate homology to their closest animal class. The revised nomenclature of all described plant cyclins is presented, with their classification into groups CycA1, CycA2, CycA3, CycB1, CycB2, CycD1, CycD2 and CycD3.

G2-and early-M-specific expression of the NTCYC1 cyclin gene in Nicotiana tabacum cells

We have previously reported the isolation of a cDNA encoding a mitotic cyclin, NTCYC1, from a tobacco cell suspension library. Here we describe the expression patterns of NTCYC1 and of Ntsuc1, a suc 1 plant homologue, in synchronized tobacco cell suspensions. Furthermore, the expression pattern of this cyclin is compared to that of Ntcdc2-1, a Nicotiana tabacum homologue of cdc2. While no NTCYC1 transcript was detected in cells synchronized in the G1 and S phases, NTCYC1 expression was observed in late G2 and early M phases, disappearing in the G1' of a new cell cycle. On the other hand, Ntsuc1 and Ntcdc2-1 exhibited a constitutive expression during the cell cycle. A functional analysis performed by microinjecting NTCYC1 mRNA into immature Xenopus oocytes, indicates that NTCYC1 could participate in the control of the G2/M transition in plant cells. Subsequently NTCYC1 expression was used to assess the status of mesophyll cells in expanded leaves of N. tabacum. Depending on leaf position along the shoot axis, a large population of mesophyll cells appeared with a 4C DNA content, suggesting a G2 arrest. It was found that leaves with such a population also contained high levels of NTCYC1 transcripts. With respect to these results concerning a naturally occurring G2-arrested cell population, the regulation of NTCYC1 expression in planta is discussed.