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[EN] Gibberellins (GA) and auxin (indole-3-acetic acid, IAA) are considered the main compounds involved in the induction of fruit set. Citrus trees flower profusely but exhibit dramatically low fruit set rates and, in particular, seeded orange cultivars also require pollination for fruit to adequately set. Consequently, they represent an excellent model to investigate the interactions between both hormones and their effect on fruit set and development. Unpollinated ovaries from 'Pineapple' sweet orange trees were treated with IAA and pollinated ones with TIBA (2,3,5-triiodobenzoic acid, inhibitor of auxin transport), and changes in ovaries were registered shortly after the treatments. The highest IAA levels were found in unpollinated ovaries treated with auxin (twofold increase compared to pollinated ones), and the lowest corresponded to pollinated ones supplemented with TIBA (30% reduction). GA content also differed substantially among samples. In general, expression of the GA-biosynthetic gene GA20ox2 in the ovule and pericarp paralleled the changes in GA(20) content in both tissues, and also expression of GA3ox1 and GA(1) content but only in the ovule. The levels of these GA in unpollinated ovaries were promoted in response to exogenous IAA, whereas expression of the GA-inactivation gene GA2ox1 and the concentration of the GA-catabolite GA(8) were reduced by this treatment. Significantly, treatments with GA(3) or IAA to unpollinated ovaries recovered fruit set to the level reached by free pollinated ones. Our study demonstrates that IAA alters GA metabolism in citrus leading to marked changes in the active GA(1) levels in ovules and pericarp, mainly through the regulation of GA-biosynthetic genes and the inhibition of the catabolic pathway.

[EN] In this work, we study the capacity to biosynthesize gibberellins (GA) of ovules (either fertilised or unfertilised), developing seeds and pericarp from fruitlets and their relation with fruit set capacity. Experiments were performed in adult, 12-year-old trees of seeded (Pineapple) and seedless parthenocarpic (Washington navel) sweet orange [Citrus sinensis L. Osbeck] cultivars. The activity of GA20-, GA3- and GA2-oxidases and gibberellin levels were measured in the ovules and pericarp of fruitlets in different development states. The results indicate that ovules are the main sites of gibberellin synthesis in fruitlets during the post-anthesis period. The most intense GA(1) synthesis-coincident with the highest expression of GA20ox2, GA3ox1 and GA2ox1-was detected in the ovules of the seeded cultivar, probably induced by fecundation and associated with low early fruitlet abscission rates. By contrast, the low activity detected in the sterile cultivar appears to be rather developmentally or constitutively regulated. As a fruitlet develops, the GA(1) concentration is augmented in the pericarp in comparison to ovules or developing seeds, and levels therein did not exhibit noticeable differences between varieties. Furthermore, developing seeds from pineapple had higher GA(1) content than the unfertilised abortive ovules from Washington navel. Taken together, data suggest a main role for this hormone in the control of fruitlet abscission, and also demonstrate a function in seed development.