Gibberellins and the Growth of Excised Tomato Roots: Comparison ofgib-1Mutant and Wild Type and Responses to Applied GA3and 2S, 3SPaclobutrazol

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Abstract
Clones of excised roots of wild type tomato (Lycopersicon esculentum, Mill., cv. Moneymaker) and a near-isogenic GA-deficient mutant (gib-1/gib-1) were cultured in modified White's medium containing 1.5% w/v sucrose. The linear elongation rate of the main axis of the gib-1 mutant was 40% less than that of the wild type. In addition, the main axis of the gib-1 mutant was thicker than that of the wild type but main axis volume growth was the same in both genotypes, indicating that the gib-1 allele was affecting the orientation of root expansion. There was no evidence to suggest that the gib-1 allele affected either the pattern of emergence or the density of lateral roots. Elongation rate and thickness of gib-1 mutant roots were restored to those of the wild type by the addition of low concentrations (0.1–1.0 μM) of gibberellic acid (GA3). These concentrations of GA3 caused a slight reduction in extension growth of wild type roots, indicating that endogenous GAs were not limiting elongation of normal roots in culture. The GA biosynthesis inhibitor, 2S, 3S paclobutrazol, at 0.1 μM, significantly reduced elongation of wild type roots and this inhibition was counteracted by 0.1 μM GA3. It is concluded that the difference in growth between the gib-1 mutant and the wild type represented GA-dependent growth. Low concentrations of 2S, 3S paclobutrazol caused only a small (5%) reduction in growth of the gib-1 mutant and this growth inhibition was not reversed by GA3. This observation, and the fact that gib-1 mutant roots grow in the absence of added GA3, suggested that part of root growth was GA-independent. However, the possibilities that the gib-1 mutant is ‘leaky’ and that paclobutrazol does not inhibit GA biosynthesis completely cannot be excluded.