Complex and shifting interactions of phytochromes regulate fruit development in tomato

Tomato fruit ripening is a complex metabolic process regulated by a genetical hierarchy. A subset of this process is also modulated by light-signaling, as mutants encoding negative regulators of phytochrome signal transduction, show higher accumulation of carotenoids. In tomato phytochromes are encoded by a multi-gene family, namely PhyA, PhyB1, PhyB2, PhyE and PhyF, however, their contribution to fruit development and ripening has not been examined. Using single phytochrome mutants- phyA, phyB1 and phyB2 and multiple mutants- phyAB1, phyB1B2 and phyAB1B2, we compared the on-vine transitory phases of ripening till fruit abscission. The phyAB1B2 mutant showed accelerated transitions during ripening with shortest time to fruit abscission. Comparison of transition intervals in mutants indicated a phase-specific influence of different phytochrome species either singly or in combination on the ripening process. Examination of off-vine ripened fruits indicated that ripening specific carotenoid accumulation was not obligatorily dependent on light and even dark incubated fruits accumulated carotenoids. The accumulation of transcripts and carotenoids in off-vine and on-vine ripened mutant fruits indicated a complex and shifting phase-dependent modulation by phytochromes(s). Our results indicate that in addition to regulating carotenoid levels in tomato fruits, phytochrome(s) also regulate the time required for phase transitions during ripening