NICOTIANA-PLUMBAGINIFOLIA HLG MUTANTS HAVE A MUTATION IN A PHYB-TYPE PHYTOCHROME GENE - THEY HAVE ELONGATED HYPOCOTYLS IN RED-LIGHT, BUT ARE NOT ELONGATED AS ADULT PLANTS

Two new allelic mutants of Nicotiana plumbaginifolia have been isolate d which display a hypocotyl which is long (hlg) when seedlings are gro wn in continuous white light (W). This can be accounted for by the dec reased response to red light (R) of the hypocotyl elongation rate in t hese mutants. Responses to other wavelengths are unaffected in the mut ants. When grown in white light, mature hlg mutants are not elongated with respect to the wild-type; they also bolt and flower later. The sh ade-avoidance responses to red/far red ratio (R:FR) are intact in thes e mutants. Both mutants are deficient in a phyB-like polypeptide that is immunodetectable in the wild-type; both have wild-type levels of a phyA-like polypeptide. These alleles are inherited in a partially domi nant manner, and correspond to single-base missense mutations in a gen e highly homologous to N. tabacum PHYB, which codes for a phytochrome B-type photoreceptor. One allele, hlg-1, has an introduced amino acid substitution; this may define a residue essential for phytochrome prot ein stability. The other allele, hlg-2, has a stop codon introduced C- terminal to the chromophore binding domain. As these phyB mutants are unaffected in shade-avoidance responses, but deficient in perception o f R, it is concluded that the phyB absent in these mutants is responsi ble for R perception in the N. plumbaginifolia seedling, but is not a R:FR sensor in light-grown plants.