Analysis of phytochrome-deficient yellow-green-2 and aurea mutants of tomato

Two alleles of the yellow-green-2 ( yg-2) and eight different alleles of the aurea ( au ) locus of tomato ( Lycopersicon esculentum Mill.) were compared. All are characterized by a paler green colour compared with wild-type (WT), an elongated hypocotyl in red light, and low or below detection limits of spectrophotometrically active phytochrome. Hypocotyl length was variable in white light, ranging from that of WT to more elongated. Immunochemical analysis revealed that etiolated seedlings of the yg-2 mutant have approximately 25% of the WT level of phytochrome A protein (PHYA), whereas that of phytochrome B protein (PHYB) is normal. In this it resembles the au mutant. The au,yg-2 double mutant has a more extreme chlorophyll deficiency than either parent. Since the yg-2 and au mutants have a less severe phenotype at the adult stage, that is, are leaky, the additive effect can be explained by assuming that the mutants control two steps in the chromophore biosynthesis pathway. Combination, by crossing, of the yg-2 and au mutants with a transgenic tomato line that overexpresses oat phytochrome A3 (PhyA-3) essentially failed to restore the WT phenotype under white fluorescent light conditions, although under greenhouse conditions some evidence for increased sensitivity to light was observed. Immunochemically, oat PHYA-3 protein is detectable in both the yg-2,PhyA-3 and au,PhyA-3 'double' mutants. Spectrophotometrical analysis, however, revealed that holophytochrome was undetectable in the yg-2,PhyA-3 and au,PhyA-3 'double' mutants. These results are compatible with both mutants being disturbed in phytochrome chromophore biosynthesis.