Two photobiological pathways of phytochrome a activity, only one of which shows dominant negative suppression by phytochrome B

The plant receptor phytochrome A (phyA) mediates responses like hypocotyl g rowth inhibition and cotyledon unfolding that require continuous far-red (F R) light for maximum expression (high-irradiance responses, HIR), and respo nses like seed germination that can be induced by a single pulse of FR (ver y-low-fluence responses, VLFR). It is not known whether this duality result s from either phyA interaction with different end-point processes or from t he intrinsic properties of phyA activity. Etiolated seedlings of Arabidopsi s thaliana were exposed to pulses of FR (3 min) separated by dark intervals of different duration. Hypocotyl-growth inhibition and cotyledon unfolding showed two phases. The first phase (VLFR) between 0.17 and 0.5 pulses.h(-1 ), a plateau between 0.5 and 2 pulses.h(-1) and a second phase (HIR) at hig her frequencies. Reciprocity between fluence rate and duration of FR was ob served within phases, not between phases. The fluence rate for half the max imum effect was 0.1 and 3 mu mol.m(-2).s(-1) for hourly pulses of FR (VLFR) and continuous FR (HIR), respectively. Overexpression of phytochrome B cau sed dominant negative suppression under continuous but not under hourly FR, We conclude that phyA is intrinsically able to initiate two discrete photo responses even when a single end-point process is considered.