The Physiology of Photomorphogenetic Tomato Mutants

Photomorphogenesis of higher plants is a complex process resulting from the co-action of at least 3 different photoreceptors: phytochrome, a blue light (B)/UV-A photoreceptor (cryptochrome) and a UV-B photoreceptor (Mohr, 1986). The existence of multiple photoreceptor types, eg. type I (PI) or light-labile phytochrome and type II (PH) or light-stable phytochrome, adds to the complexity (Furuya, 1989; Tomizawa et al., 1990). The assignment of specific functions to the distinct molecular species of the photoreceptor is therefore being studied with the aid of photomorphogenetic mutants in which certain parts of the morphogenetic pathway are eliminated or altered. The relevance of the changed part in the mutant is directly indicated by its difference in response compared to its isogenic wild type (Kooraneef and Kendrick, 1986). Mutants can be found (isolated) from natural populations or varieties (cultivars) or more efficiently after mutagenic treatment: using e.g. chemicals, irrradiation; somaclonal variation; transposon insertion; transformation; introduction of antisense RNA.