PHOTORESPONSES OF TRANSGENIC ARABIDOPSIS SEEDLINGS EXPRESSING INTRODUCED PHYTOCHROME-B-ENCODING CDNAS - EVIDENCE THAT PHYTOCHROME-A AND PHYTOCHROME-B HAVE DISTINCT PHOTOREGULATORY FUNCTIONS

The photocontrol of hypocotyl elongation has been studied in two trans genic lines of Arabidopsis thaliana which contain elevated levels of p hytochrome B encoded by either an introduced rice- or Arabidopsis-deri ved cDNA driven by the 35S CaMV promoter. Inhibition of hypocotyl grow th in etiolated seedlings of the phyB-transformed lines was saturated at photon fluence rates of continuous red light (R) which were markedl y lower than those required for inhibition of growth in seedlings of t he isogenic wild-type (WT). Inhibition of hypocotyl growth in etiolate d seedlings of the phyB-transgenic lines under continuous far-red irra diation (FR), however, showed the same relationship with fluence rate as WT. Light-grown seedlings of the phyB-transgenic lines responded to end-of-day FR by an acceleration of growth, in a manner comparable wi th WT. This response was unaltered when the end-of-day FR was extended from a 15 min pulse to 14 h of continuous irradiation. The response o f light-grown, phyB-transformed seedlings to decreasing R:FR ratio was also qualitatively similar to WT, i.e. increased elongation growth of the hypocotyl and petioles occurred under low R:FR quantum ratio. How ever, absolute elongation growth was markedly less in the transgenic s eedlings at all R:FR ratios tested than in WT. Together, these data in dicate that seedlings over-expressing phytochrome B are more responsiv e to R than are WT, but are unaltered in their responsiveness to FR. B y contrast, seedlings overexpressing phytochrome A are more responsive than WT to both R and FR; whereas the phytochrome B-deficient mutant hy3 is unresponsive to R while retaining WT-like responsiveness to FR. These data indicate that in WT etiolated seedlings phytochrome A medi ates the effects of continuous FR, and phytochrome B the effects of co ntinuous R. The evidence thus supports the conclusion that these two m olecular species of the photoreceptor have differential regulatory rol es in the plant.