New roles of phytochromes during seed germination

Among the most important environmental sensors in plants are the phytochromes, members of a duplicated-gene family of photoreceptors. We characterized the functional diversification of this gene family with respect to a single ecologically important and highly environmentally sensitive process-germination-and thereby addressed the consequence of gene duplication in the evolution of environmental sensitivity. We exposed single and multiple mutants of different phytochromes of Arabidopsis thaliana to dormancy-breaking or dormancy-inducing treatments that resemble different seasonal environmental conditions, and we recorded their germination responses. Much redundancy of function occurred, but an impressive degree of functional diversification was also apparent. Specifically, PHYB was an important contributor to germination when seeds did not receive low-temperature imbibition. It interacted nonadditively with PHYA, whose contribution to germination was redundant to that of PHYB and most apparent when seeds experienced high followed by low temperatures. We also determined that PHYD was necessary for full germination when seeds were exposed to a high temperature during imbibition. PHYE appears to contribute redundantly to germination, except when seeds experienced high followed by low temperatures. Finally, we found new evidence that PHYA and possibly PHYD redundantly contribute to the suppression of germination. The diversification of activities of all these phytochromes during this one process of germination suggests that they all have a combined function in the regulation of germination responses to complex seasonal conditions and that gene duplication can contribute to the evolution of precise responses to variable environments.