The gibberellin (GA) biosynthesis mutants Ih-1 and Ih-2 have been used
to examine the physiological role of GAs in pea seed development. The
LH protein is required for the three-step oxidation of ent-kaurene to
ent-kaurenoic acid early in the GA biosynthesis pathway. The allele-s
pecific interaction of Ih-1 and Ih-2 with chemical inhibitors of these
three steps suggests that LH encodes the multifunctional GA biosynthe
sis enzyme ent-kaurene oxidase. Unlike the Ih-2 mutation which reduces
seed weight and decreases seed survival by similar to 50% compared wi
th wild-type plants, the Ih-1 allele has a transient effect on embryo
and seed growth and only slightly increases seed abortion. These seed
phenotypes parallel the effects of the two mutant alleles on GA levels
in young seeds. Detailed examination of the growth of Ih-1 seeds reve
als homeostatic regulation of GA-promoted embryo and seed growth. Alth
ough GA-deficient seeds grow more slowly than WT seeds, decreased assi
milate availability to the developing seeds is not the primary reason
for the altered seed development. Instead, GAs act to promote some pro
cess(es) required for embryo and seed growth and only indirectly influ
ence the distribution of assimilates. How GA deficiency causes seed ab
ortion is not known but it may simply be a consequence of reduced seed
or embryo growth rate. These results demonstrate that even relatively
small changes in the levels of GAs in young seeds can alter seed deve
lopment and suggest that the available GA-related mutants may represen
t only a subset of all possible mutants with reduced GA levels or GA s
ignalling.