UV-B-INDUCED PHOTOMORPHOGENESIS IN ARABIDOPSIS-THALIANA

Relatively little is known about the types of photomorphogenic respons es and signal transduction pathways that plants employ in response to ultraviolet-B (UV-B, 290-320 nm) radiation. In wild-type Arabidopsis s eedlings, hypocotyl growth inhibition and cotyledon expansion were bot h reproducibly promoted by continuous UV-B. The fluence rate response of hypocotyl elongation was examined and showed a biphasic response. W hereas photomorphogenic responses were observed at low doses, higher f luences resulted in damage symptoms. In support of our theory that pho tomorphogenesis, but not damage, occurs at low doses of UV-B, photomor phogenic responses of UV-B sensitive mutants were indistinguishable fr om wild type plants at the low dose. This allowed us to examine UV-B-i nduced photomorphogenesis in photoreceptor deficient plants and consti tutive photomorphogenic mutants. The cry1 cryptochrome structural gene mutant, and phytochrome deficient hy1, phyA and phyB mutant seedlings resembled wild-type seedlings, while phyA/phyB double mutants were le ss sensitive to the photomorphogenic effects of UV-B. These results su ggest that either phyA or phyB is required for UV-B-induced photomorph ogenesis. The constitutive photomorphogenic mutants cop1 and def2 did not show significant inhibition of hypocotyl growth in response to UV- B, while det2 was strongly affected by UV-B irradiation. This suggests that COP1 and DET1 work downstream of the UV-B signaling pathway.