Genetic dissection of the relative roles of auxin and gibberellin in the regulation of stem elongation in intact light-grown peas

Exogenous gibberellin (GA) and auxin (indoleacetic acid [IAA]) strongly stimulated stem elongation in dwarf GA(1)-deficient le mutants of light-grown pea (Pisom sativum L.): IAA elicited a sharp increase in growth rate after 20 min followed by a slow decline; the GA response had a longer lag (3 h) and growth increased gradually with time. These responses were additive. The effect of CA was mainly in internodes less than 25% expanded, whereas that of IAA was in the older, elongating internodes. IAA stimulated growth by cell extension; GA stimulated growth by an increase in cell length and cell number. Dwarf lkb GA-response-mutant plants elongated poorly in response to CA (accounted for by an increase in cell number) but were very responsive to IAA. CA produced a substantial elongation in lkb plants only in the presence of IAA. Because lkb plants contain low levels of IAA, growth suppression in dwarf lkb mutants seems to be due to a deficiency in endogenous auxin. CA may enhance the auxin induction of cell elongation but cannot promote elongation in the absence of auxin. The effect of CA may, in part, be mediated by auxin. Auxin and CA control separate processes that together contribute to stem elongation. A deficiency in either leads to a dwarfed phenotype.