Mutants as an aid to the study of higher plant photomorphogenesis

Study of photomorphogenesis is often complicated by the interaction of different photoreceptors regulating a given process or by the induction of multiple effects by a single photoreceptor. Mutants in which particular components of the morphogenetic pathways are eliminated provide the possibility of studying a more simplified form of photomorphogenesis. Three classes of photomorphogenetic mutants are proposed: photoreceptor, transduction chain and response mutants. In this study three mutants have been used: two have an elongated hypocotyl when grown in white light (the aurea ( au ) tomato mutant and the long hypocotyl ( lh ) cucumber mutant) and one with an enhanced pigment synthesis (the high pigment ( hp ) tomato mutant). The au mutant appears to be a photoreceptor mutant, lacking spectrophotometrically and immunochemically detectable labile phytochrome (lP). The lh mutant is proposed to lack stable phytochrome ( s P) or its function. These mutants enabled the role of s P, l P. and blue light (BL)/UV- photoreceptor(s) in several photophysiological processes to be studied. The results of these experiments indicate that l P plays a role in both hypocotyl elongation and anthocyanin synthesis in etiolated seedlings. This provides direct evidence that the 'bulk' l P is functional. In etiolated seedlings the au tomato mutant with its deficiency in l P is 'red-blind' and has a shift of fluence rate response curves for hypocotyl inhibition by BL and UV-A approximately l order of magnitude to higher fluence rates. In light-grown plants it is proposed that s P regulates the end-of-day far-red light (FR) response and the inhibition of hypocotyl elongation due to light perception by the cotyledons. Furthermore, these mutants with reduced phytochrome (P) content provide direct experimental evidence that the FR absorbing form of P (Pfr) is the active form indeed. If removal of the red light (RL) absorbing form of P (Pr) is the active photomorphogenetic process, instead of an increase of Pfr, seedlings with a reduced P content would be expected to be short. However, dark-grown seedlings of lh mutant and au mutant are both elongated. The hp mutant with its enhanced anthocyanin synthesis has enabled induction of anthocyanin synthesis in tomato seedlings in response to a single RL pulse to be observed, whereas in wild type this synthesis it too low to be measured. Study of anthocyanin synthesis with the aid of the hp mutant, the au mutant and the au/hp double mutant supports the conclusion that P is the terminal photoreceptor involved in tomato and that BL (operating through the BL/UV-photoreceptor or P) sensitizes the seedling to P action at a later stage. Using a computer-controlled apparatus for continuous growth measurement, designed and constructed for this study, it has been possible to show the differences in kinetics of hypocotyl inhibition by BL or RL in both the lh mutant and its wild type. In BL inhibition started almost immediately after the onset of irradiation, whereas in RL a lag period of several hours was observed.