Physiological interactions of phytochromes A, B1 and B2 in the control of development in tomato

The role of phytochrome 82 (phyB2) in the control of photomorphogenesis in tomato (Solanum lycopersicum L.) has been investigated using recently isola ted mutants carrying lesions in the PHYB2 gene. The physiological interacti ons of phytochrome A (phyA), phytochrome B1 (phyB1) and phyB2 have also bee n explored, using an isogenic series of all possible mutant combinations an d several different phenotypic characteristics. The loss of phyB2 had a neg ligible effect on the development of white-light-grown wild-type or phyA-de ficient plants, but substantially enhanced the elongated pale phenotype of the phyB1 mutant. This redundancy was also seen in the control of de-etiola tion under continuous red light (R), where the loss of phyB2 had no detecta ble effect in the presence of phyB1. Under continuous R, phyA action was la rgely independent of phyB1 and phyB2 in terms of the control of hypocotyl e longation, but antagonized the effects of phyB1 in the control of anthocyan in synthesis, indicating that photoreceptors may interact differently to co ntrol different traits. Irradiance response curves for anthocyanin synthesi s revealed that phyB1 and phyB2 together mediate all the detectable respons e to high-irradiance R, and, surprisingly, that the phyA-dependent low-irra diance component is also strongly reduced in the phyB1 phyB2 double mutant. This is not associated with a reduction in phyA protein content or respons iveness to continuous far-red light (FR), suggesting that phyB1 and phyB2 s pecifically influence phyA activity under low-irradiance R. Finally, the ph yA phyB1 phyB2 triple mutant showed strong residual responsiveness to suppl ementary daytime FR, indicating that at least one of the two remaining phyt ochromes plays a significant role in tomato photomorphogenesis.