Underground gibberellin activity: differential gibberellin response in tomato shoots and roots

Summary Gibberellins (GAs) are growth-promoting hormones that regulate organ growth, mainly via cell elongation. Contradicting reports leave an open question of whether GA is as important for root elongation as it is for stem elongation. Here we have addressed this question focusing on tomato ( Solanum lycopersicum ) primary-root elongation. We used a combination of physiological, molecular and genetic approaches to tackle this question. Tomato has three GA receptors; GID1a, GID1b1 and GID1b2. The loss of all three receptors, strongly suppressed stem elongation and leaf expansion, but had a relatively minor effect on primary root elongation. The effect of GA on cell elongation and gene-expression was much weaker in roots, than in shoots, reaching saturation at lower hormone concentrations. Our results imply that this differential response to GA in shoots and roots is caused by the lower expression of the dominant GA receptor GID1a in roots. We show that the differential activity of GA between shoots and roots affects root-to-shoot ratio, and speculate that this evolved as an adaptive mechanism to changing environments.