The adaptive evolution of plasticity: phytochrome-mediated shade avoidance responses

Many plants display a characteristic suite of developmental"shade avoidance" responses, such as stem elongation and acceleratedreproduction, to the low ratio of red to far-red wavelengths(R:FR) reflected or transmitted from green vegetation. ThisR:FR cue of crowding and vegetation shade is perceived by thephytochrome family of photoreceptors. Phytochrome-mediated responsesprovide an ideal system for investigating the adaptive evolutionof phenotypic plasticity in natural environments. The molecularand developmental mechanisms underlying shade avoidance responsesare well studied, and testable ecological hypotheses exist fortheir adaptive significance. Experimental manipulation of phenotypesdemonstrates that shade avoidance responses may be adaptive,resulting in phenotypes with high relative fitness in the environmentsthat induce those phenotypes. The adaptive value of shade avoidancedepends upon the competitive environment, resource availability,and the reliability of the R:FR cue for predicting the selectiveenvironment experienced by an induced phenotype. Comparativestudies and a reciprocal transplant experiment with Impatienscapensis provide evidence of adaptive divergence in shade avoidanceresponses between woodland and clearing habitats, which mayresult from population differences in the frequency of selectionon shade avoidance traits, as well as differences in the reliabilityof the R:FR cue. Recent rapid progress in elucidating phytochromesignaling pathways in the genetic model Arabidopsis thalianaand other species now provides the opportunity for studyinghow selection on shade avoidance traits in natural environmentsacts upon the molecular mechanisms underlying natural phenotypicvariation.