THE CUCUMBER LONG HYPOCOTYL MUTANT LACKS A LIGHT-STABLE PHYB-LIKE PHYTOCHROME

A novel cDNA sequence homologous to a phytochrome B (phyB) gene that w as isolated in a library from tobacco tissue has been used in an Esche richia coli expression system to raise anti-phytochrome B (anti-PHYB) polypeptide-specific monoclonal antibodies. The specificity of these a ntibodies has been tested by cross-reactivity against purified pea lig ht-labile type 1 and light-stable type 2 phytochromes, with some antib odies reacting with the type 2 and none with the type 1 phytochromes. One such antibody, monoclonal mAT1, has been employed to analyze the p hytochrome molecular species present in a photomorphogenic long hypoco tyl (lh) mutant of cucumber. The results indicated that the mutant con tains wild-type levels of the light-labile type 1 phytochrome polypept ide (PHYA), which has an apparent molecular mass of approximately 120 kD, but shows < 1% (detection limit) of a light-stable polypeptide rec ognized by mAT1 in wild-type seedlings. This protein, not detectable i n the lh mutant, has the properties of light-stable type 2 phytochrome , has an apparent molecular mass of 116 to 117 kD, and remains at cons tant levels under continuous low-fluence-rate red light. Therefore, we conclude that the lh mutant lacks at least one type 2 phytochrome-lik e polypeptide, most probably a phyB gene product. The correlation betw een the lack of this protein and the deficiency or absence of physiolo gical responses to a light-stable phytochrome species in this mutant h elps to identify the physiological roles played by the products of dif ferent subfamilies within the phytochrome gene family.