EXPRESSION OF FUNCTIONAL OAT PHYTOCHROME A IN TRANSGENIC RICE

To investigate the biological functions of phytochromes in monocots, w e generated, by electric discharge particle bombardment, transgenic ri ce (Oryza sativa cv Gulfmont) that constitutively expresses the oat ph ytochrome A apoprotein. The introduced 124-kD polypeptide bound chromo phore and assembled into a red- and far-red-light-photoreversible chro moprotein with absorbance spectra indistinguishable from those of phyt ochrome purified from etiolated oats. Transgenic lines expressed up to 3 and 4 times more spectrophotometrically detectable phytochrome than wild-type plants in etiolated and green seedlings, respectively. Upon photoconversion to the far-red-absorbing form of phytochrome, oat phy tochrome A was degraded in etiolated seedlings with kinetics similar t o those of endogenous rice phytochromes (half-life approximately 20 mi n). Although plants overexpressing phytochrome A were phenotypically i ndistinguishable from wild-type plants when grown under high-fluence w hite light, they were more sensitive as etiolated seedlings to light p ulses that established very low phytochrome equilibria. This indicates that the introduced oat phytochrome A was biologically active. Thus, rice ectopically expressing PHY genes may offer a useful model to help understand the physiological functions of the various phytochrome iso forms in monocotyledonous plants.