fhy3-1 retains inductive responses of phytochrome A

The fhy3 mutation of Arabidopsis impairs phytochrome A (phyA)-mediated inhi bition of hypocotyl growth without affecting the levels of phyA measured sp ectrophotometrically or immunochemically. We investigated whether the fhy3- 1 mutation has similar effects on very low fluence responses (VLFR) and hig h irradiance responses (HIR) of phyA. When exposed to hourly pulses of far- red light, etiolated seedlings of the wild type or of the fhy3-1 mutant sho wed similar inhibition of hypocotyl growth, unfolding of the cotyledons, an thocyanin synthesis, and greening upon transfer to white light. In the wild type, continuous far-red light was significantly more effective than hourl y far-red pulses (at equal total fluence). In the fhy3-1 mutant, hourly pul ses were as effective as continuous far-red light, i.e. the failure of reci procity typical of HIR was not observed. Germination was similarly promoted by continuous or pulsed far-red in wild-type and fhy3-1 seeds. Thus, for h ypocotyl growth, cotyledon unfolding, greening, and seed germination, the f hy3-1 mutant retains VLFR but is severely impaired in HIR. These data are c onsistent with the idea that VLFR and HIR involve divergent signaling pathw ays of phyA.