Phytochrome a function in red light sensing.

Light signals perceived by the phytochrome family of red (R) and far-red (FR) light-absorbing photoreceptors direct plant growth and development throughout their lifecycle. In contrast to other family members, phyA displays rapid light-induced proteolytic degradation upon conversion to the biologically active Pfr form and mediates high irradiance responses to continuous FR. These unique properties together with limited examples of phyA function in R have resulted in an over-simplified portrayal of phyA as a FR sensor which acts predominantly in seed germination and early stages of seedling de-etiolation. In a recent work, published in The Plant Journal, we report significant phyA activity in Arabidopsis thaliana at high (>100 micromolm(-2)s(-1)) photon irradiances of R. Under these conditions, we observed retarded degradation of a pool of nuclear-localised phyA, consistent with the phenomenon of photoprotection, and showed phyBphyCphyDphyE quadruple null mutants, containing only functional phyA, to de-etiolate and survive to flowering. The photon irradiances used in this study were greater than those routinely used for photomorphogenic analysis in the laboratory but considerably lower than those commonly observed in daylight. In this addendum we present additional analyses of the phyBphyCphyDphyE mutant and discuss the possibility that phyA may perform a significant role in the growth and development of daylight-grown plants.