Reduced gibberellin biosynthesis and response in fruits of the auxin insensitive diageotropica tomato mutant

Auxin has a central role in determining tomato fruit growth and development, and most of its action is mediated by gibberellins (GAs). The diageotropica (dgt) mutant of tomato exhibits many physiological responses that are related to a defective auxin sensitivity. In this paper we investigated the effects of the dgt mutation on tomato gibberellin biosynthesis regulation during fruit-set and early growth of pollinated fruits. In spite of an initial accumulation of active GAs in dgt ovaries, their content is significantly reduced at later stages. Indeed, at the beginning of rapid fruit growth, dgt fruits display a lower amount of GA(1) and its direct catabolite GA(8). Consistently, transcripts of GA 20-oxidase genes (GA20ox1, GA20ox2, GA20ox3) are low in the mutant. Moreover, low expression of genes encoding GA catabolism enzymes (GA 2 beta-hydroxylases) does not lead to an increase in the amount of active GAs, supporting the hypothesis that GA 20-oxidase genes downregulation might bottleneck the synthesis of active GAs in dgt. Interestingly, exogenous GA(3) application has little effect on dgt ovaries. GA(3)-treated fruits of the mutant are smaller than those of its wild type as a result of fewer and smaller pericarp cells. Consistently, GA(3) treatment in the dgt ovaries produces negligible effects on cell endoreduplication revealed by a lower nuclear DNA content in pericarp and locular tissue cells. The lack of DELLA-mediated constraint on GA signal in the double mutant dgt pro did not cause an increase in size and weight in pollinated fruits, suggesting that GA signalling is unable to overcome the inhibition of growth caused by the dgt mutation.