New Roles of Phytochromes during Seed Germination

Abstract

Among the most important environmental sensors in plants are the phytochromes, members of a duplicatedgene 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 dormancyinducing 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 lowtemperature 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.

A Role for Barley CRYPTOCHROME1 in Light Regulation of Grain Dormancy and Germination

“…R and FR reversibility of germination has been demonstrated in many plant species and appears to be a widely conserved mechanism acting through the PHYB photoreceptor. Recent genetic analysis has revealed that PHY photoreceptors are also implicated in defining conditions to break or maintain dormancy (Heschel et al, 2008). Our work and that of others have previously demonstrated that BL is also a key wavelength in regulating germination of freshly harvested seeds of the grass family.…”

“…In Arabidopsis thaliana seeds, there is also strong evidence that light signaling mediated through the PHYTOCHROME (PHY) family of photoreceptors has a major effect in determining the level of seed dormancy at maturity and the germination conditions required to break dormancy (Heschel et al, 2008). Genetic dissection of the individual roles for the PHYs has shown that they have partially overlapping and diverse functions in defining germination conditions necessary to break or promote dormancy.…”

Evolutionary Studies Illuminate the Structural-Functional Model of Plant Phytochromes

“…These studies revealed a role for phyD in breaking cool-induced seed dormancy and in mediating the effects of photoperiod during seed maturation (Donohue et al, 2007). They further revealed that phyD is important in seed germination when seeds were matured (Dechaine et al, 2009) or imbibed (Heschel et al, 2008) under high temperatures, whereas phyE was important when seeds were imbibed in high temperatures followed by low temperatures (Heschel et al, 2008) or when matured under high temperatures (Dechaine et al, 2009). These data suggest that phyD and phyE contribute to the precision of responses to variable environments.…”

Genetic and physiological bases for phenological responses to current and predicted climates

“…For instance, our model of Arabidopsis phenology is at present silent about the factors that influence the timing of germination, even though recent studies have shown that certain genes in the flowering time network are also involved in seed dormancy and germination behaviour (Heschel et al 2008;Chiang et al 2009). Both maternal environmental factors such as temperature ) and genetic factors (Alonso-Blanco et al 2003;Holdsworth et al 2008;Chiang et al 2009) affect germination behaviour in this species.…”