A CURLY LEAF homologue controls both vegetative and reproductive development of tomato plants

The Enhancer of Zeste Polycomb group proteins, which are encoded by a small gene family in Arabidopsis thaliana, participate to the control of plant development. In the tomato (Solanum lycopersicum), these proteins are encoded by three genes (SlEZ1, SlEZ2 and SlEZ3) that display specific expression profiles. Using a gene specific RNAi strategy, we demonstrate that repression of SlEZ2 correlates with a general reduction of H3K27me3 levels, indicating that SlEZ2 is part of an active PRC2 complex. Reduction of SlEZ2 gene expression impacts the vegetative development of tomato plants, consistent with SlEZ2 having retained at least some of the functions of the Arabidopsis CURLY LEAF (CLF) protein. Notwithstanding, we observed significant differences between transgenic SlEZ2 RNAi tomato plants and Arabidopsis clf mutants. First, we found that reduced SlEZ2 expression has dramatic effects on tomato fruit development and ripening, functions not described in Arabidopsis for the CLF protein. In addition, repression of SlEZ2 has no significant effect on the flowering time or the control of flower organ identity, in contrast to the Arabidopsis clf mutation. Taken together, our results are consistent with a diversification of the function of CLF orthologues in plants, and indicate that although partly conserved amongst plants, the function of EZ proteins need to be newly investigated for non-model plants because they might have been recruited to specific developmental processes.

A new C-type cyclin-dependent kinase from tomato expressed in dividing tissues does not interact with mitotic and G1 cyclins

Cyclin-dependent kinases (CDKs) form a conserved superfamily of eukaryotic serine-threonine protein kinases whose activity requires the binding of a cyclin protein. CDKs are involved in many aspects of cell biology and notably in the regulation of the cell cycle. Three cDNAs encoding a C-type CDK, and a member of each B-type CDK subfamily, were isolated from tomato (Lycopsersicon esculentum Mill.) and designated Lyces;CDKC;1 (accession no. AJ294903), Lyces; CDKB1;1 (accession no. AJ297916), and Lyces;CDKB2;1 (accession no. AJ297917). The predicted amino acid sequences displayed the characteristic PITAIRE (CDKC), PPTALRE (CDKB1), and PPTTLRE (CDKB2) motives in the cyclin-binding domain, clearly identifying the type of CDK. The accumulation of all transcripts was associated preferentially with dividing tissues in developing tomato fruit and vegetative organs. In contrast to that of CDKA and CDKBs, the transcription pattern of Lyces;CDKC;1 was shown to be independent of hormone and sugar supply in tomato cell suspension cultures and excised roots. This observation, together with the absence of a patchy expression profile in in situ hybridization experiments, suggests a non-cell cycle regulation of Lyces;CDKC;1. Using a two-hybrid assay, we showed that Lyces;CDKC;1 did not interact with mitotic and G1 cyclins. The role of plant CDKCs in the regulation of cell division and differentiation is discussed with regard to the known function of their animal counterparts.

Endoreduplication in higher plants

Cell polyploidisation can be achieved by endoreduplication, which consists of one or several rounds of DNA synthesis in the absence of mitosis. As a consequence, chromosomes with 2n chromatids are produced without change in the chromosome number. Endoreduplication is the most common mode of polyploidisation in plants and can be found in many cell types, especially in those undergoing differentiation and expansion. Although accumulating data reveal that this process is developmentally regulated, it is still poorly understood in plants. At the molecular level, the increasing knowledge on plant cell cycle regulators allows the acquisition of new tools and clues to understand the basis of endoreduplication control and, in particular, the switch between cell proliferation and cell differentiation.

Molecular characterization of the expression of distinct classes of cyclins during the early development of tomato fruit

Early fruit development in tomato (Lycopersicon esculentum Mill.) proceeds from two distinct phases of growth, essentially cell division and cell expansion. In this study, we investigated the expression characteristics of the key cell-cycle regulators, mitotic and G1 cyclins, during tomato fruit development. We isolated six genes designated Lyces;CycA1;1, Lyces;CycA2;1, Lyces; CycA3;1, Lyces;CycB1:1 and Lyces;CycB2;1 encoding tomato mitotic cyclins, and Lyces;CycD3;1 encoding a G1 cyclin. The accumulation of transcripts was predominantly associated with mitotically active organs: developing fruits, young leaves and roots, and with cell-suspension cultures under appropriate sugar feeding conditions. Transcripts for all the isolated cyclin genes could be detected in the epidermis and pericarp of fruit tissues where some slight mitotic activity still remained at the onset of ripening. However, Lyces;CycA3;1 and Lyces;CycD3;1 were expressed in the gel tissue at the late stage of fruit development, suggesting that they are involved in endoreduplication of the differentiated and giant cells of the gel tissue.

CDK-related protein kinases in plants

Cyclin-dependent kinases (CDK) form a conserved superfamily of eukaryotic serine-threonine protein kinases, which require binding to a cyclin protein for activity. CDK are involved in different aspects of cell biology and notably in cell cycle regulation. The comparison of nearly 50 plant CDK-related cDNAs with a selected set of their animal and yeast counterparts reveals five classes of these genes in plants. These are described here with respect to their phylogenetic, structural and functional properties. A plant-wide nomenclature of CDK-related genes is proposed, using a system similar to that of the plant cyclin genes. The most numerous class, CDKA, includes genes coding for CDK with the PSTAIRE canonical motif. CDKB makes up a class of plant-specific CDK divided into two groups: CDKB1 and CDKB2. CDKC, CDKD and CDKE form less numerous classes. The CDKD class includes the plant orthologues of metazoan CDK7, which correspond to the CDK-activating kinase (CAK). At present, no functional information is available in plants for CDKC and CDKE.

Molecular and biochemical characterization of the involvement of cyclin-dependent kinase A during the early development of tomato fruit

Following fruit set, the early development of tomato (Lycopersicon esculentum Mill.) fruit comprises two distinct phases: a cell division phase and a consecutive phase of cell expansion until the onset of ripening. In this study, we analyzed cytological and molecular changes characterizing these early phases of tomato fruit development. First we investigated the spatial and temporal regulation of the mitotic activity during fruit development. The DNA content of isolated nuclei from the different fruit tissues was determined by flow cytometry analysis. The results confirm the data of mitotic activity measurements and show that cell differentiation, leading to expanded cells, is characterized by endoreduplication. Second, we isolated two cDNAs, named Lyces;CDKA1 (accession no. Y17225) and Lyces;CDKA2 (accession no. Y17226), encoding tomato homologs of the cyclin-dependent kinase (CDK) p34(cdc2). Tomato CDKA gene expression was followed at both the transcriptional and translational levels during fruit development. The transcripts for Lyces;CDKA1 and Lyces;CDKA2 and the corresponding CDKA proteins are predominantly accumulated during the phase of cell division between anthesis and 5 d post anthesis (DPA). In whole fruits, the maximum CDK activity was obtained between 5 and 10 DPA. The determination of the kinase activity using protein extracts from the different fruit tissues was in agreement with mitotic activity analysis. It showed the particular disappearance of the activity in the gel tissue as early as 15 DPA. The overall data of CDK activity measurements suggest a strong post-translational regulation of CDK at the temporal and spatial levels during early tomato fruit development.