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.

Cyclins play important roles in cell division and cell expansion. They also interact with cyclin-dependent kinases to control cell cycle progression in plants. Our genome-wide analysis identified 52 expressed cyclin genes in tomato. Phylogenetic analysis of the deduced amino sequences of tomato and Arabidopsis cyclin genes divided them into 10 types, A-, B-, C-, D-, H-, L-, T-, U-, SDS- and J18. Pfam analysis indicated that most tomato cyclins contain a cyclin-N domain. C-, H- and J18 types only contain a cyclin-C domain, and U-type cyclins contain another potential cyclin domain. All of the cyclin genes are distributed throughout the tomato genome except for chromosome 8, and 30 of them were found to be segmentally duplicated; they are found on the duplicate segments of chromosome 1, 2, 3, 4, 5, 6, 10, 11 and 12, suggesting that tomato cyclin genes experienced a mass of segmental duplication. Quantitative real-time polymerase chain reaction analysis indicates that the expression patterns of tomato cyclin genes were significantly different in vegetative and reproductive stages. Transcription of most cyclin genes can be enhanced or repressed by exogenous application of gibberellin, which implies that gibberellin maybe a direct regulator of cyclin genes. The study presented here may be useful as a guide for further functional research on tomato cyclins.