Two alleles of the yellow-green-2 (yg-2) and eight different alleles o
f 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. Hypocot
yl 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 phytochro
me A protein (PHYA), whereas that of phytochrome B protein (PHYB) is n
ormal. in this it resembles the av mutant. The au,yg-2 double mutant h
as 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 t
hat the mutants control two steps in the chromophore biosynthesis path
way. Combination, by crossing, of the yg-2 and ao mutants with a trans
genic tomato line that overexpresses oat phytochrome A3 (PhyA-3) essen
tially failed to restore the WT phenotype under white fluorescent ligh
t conditions, although under greenhouse conditions some evidence for i
ncreased 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 holop
hytochrome was undetectable in the yg-2PhyA-3 and au,PhyA-3 'double' m
utants. These results are compatible with both mutants being disturbed
in phytochrome chromophore biosynthesis.