Fig. 5.

Proposed model for the roles and regulation of cytokinins (CKs) in tomato fruit development. Before pollination, high levels of CKs are accumulated in ovaries by the expression of CK biosynthetic genes (SlIPT3, SlIPT4, SlLOG6, and SlLOG8). These CKs are involved in the growth of unpollinated ovaries, and their concentrations are decreased after pollination by the expression of SlCKX genes (SlCKX1 and SlCKX7). Concentration of trans-zeatin (tZ) is increased after pollination through the upregulation of other CK biosynthetic genes (SlIPT1, SlIPT2, SlCYP735A1, SlCYP735A2, and SlLOG2). tZ promotes cell division during early fruit development. Concentrations of auxin and gibberellin (GA) also increase after pollination. These hormones are involved in the cell division and expansion, which determine final fruit size. CKs also promote the growth of pedicel and influence the transport of water, nutrients, and assimilates to fruit. The question mark and dotted arrow indicate that CKs might also modulate auxin biosynthesis and/or polar auxin transport, which prevent abscission of ovaries.