Genes affecting phenotypic plasticity in Arabidopsis: pleiotropic effects and reproductive fitness of photomorphogenic mutants

Many plants exhibit characteristic photomorphogenic shade 'avoidance' respo nses to crowding and vegetation shade; this plasticity is often hypothesize d to be adaptive. We examined the contribution of specific photomorphogenic loci to plastic shade avoidance responses in the annual crucifer Arabidops is thaliana by comparing single-gene mutants defective at those loci with w ild type plants exhibiting normal photomorphogenesis. The hy1 and hy2 mutan ts, deficient in all functional phytochromes, were less plastic than the wi ld type in response to a nearby grass canopy or to a low-red/far-red light ratio characteristic of vegetation shade. These mutants displayed constitut ively shade-avoiding phenotypes throughout the life cycle regardless of the treatment: they bolted at an earlier developmental stage and were characte rized by reduced branching. In contrast, the hy4 mutant, deficient in blue light reception, exhibited greater plasticity than the wild type in respons e to vegetation shade after the seedling stage. This mutant produced more l eaves before bolting and more basal branches under normal light conditions when compared to the wild type. These results indicate that specific photom orphogenic loci have different and sometimes antagonistic pleiotropic effec ts on the plastic response to vegetation shade throughout the life cycle of the plant. The fitness of the constitutively shade-avoiding phytochrome-de ficient mutants was lower than that of the plastic wild type under normal l ight, but was not different in the vegetation shade treatments, where all g enotypes converged toward similar shade avoidance phenotypes. This outcome supports one key prediction of the adaptive plasticity hypothesis: that ina ppropriate expression of shade avoidance traits is maladaptive.