IDENTIFICATION OF 2 LOCI INVOLVED IN PHYTOCHROME EXPRESSION IN NICOTIANA-PLUMBAGINIFOLIA AND LETHALITY OF THE CORRESPONDING DOUBLE MUTANT

Four Nicotiana plumbaginifolia mutants exhibiting long hypocotyls and chlorotic cotyledons under white light, have been isolated from M2 see ds following mutagenesis with ethyl methane sulphonate. In each of the se mutants, this partly etiolated in white light (pew) phenotype is du e to a recessive nuclear mutation at a single locus. Complementation a nalysis indicates that three mutants, dap5, ems28 and ems3-6-34, belon g to a single complementation group called pew1, while dap1 defines th e pew2 locus. The mutants at pew1 contain normal levels of immunochemi cally detectable apoprotein of the phytochrome that is relatively abun dant in etiolated seedlings, but are deficient in spectrophotometrical ly detectable phytochrome, whether seedlings are grown in darkness or light. Moreover, biliverdin, a precursor of the phytochrome chromophor e, restores light-regulated responses in pew1 mutants and increases th eir level of photoreversible phytochrome when grown in darkness. These results indicate that the pew1 locus may be involved in chromophore b iosynthesis. The mutant at the pew2 locus displays no photoreversible phytochrome in etiolated seedlings, but does contain normal levels of photoreversible phytochrome when grown in the light. Biliverdin had li ttle effect on light-regulated responses in this mutant. In addition, biliverdin did not alter the level of phytochrome in etiolated seedlin gs. These observations lead us to propose that this mutant could be af fected in the phyA gene itself. We have also obtained the homozygous d ouble mutant at the pew1 and pew2 loci. This double mutant is lethal a t an early stage of development, consistent with a critical role for p hytochrome in early development of higher plants.