ABSTRACT


The effect of applied gibberellin (GA) and auxin on fruit-set and growth has been investigated in tomato (Solanum lycopersicum L.) cv Micro-Tom. Results indicate that this cultivar is a convenient model system to perform research on the hormonal regulation of tomato. It was found that to prevent competition between developing fruits, only one fruit per truss and two trusses should be left per plant. Unpollinated ovaries responded to GA3 > GA1 = GA4 > GA20 (but not to GA19) and to different auxins [indol-3-acetic acid, naphthaleneacetic acid, and 2,4-dichlorophenoxyacetic acid (2,4-D)], 2,4-D being the most efficient. GA- and auxin-induced fruits had different internal morphology, poor locular tissue development and empty locular cavities in the case of GA3, and pseudoembryos development and filled locular cavities in the case of 2,4-D. Also, GA3 produced larger cells in the internal region of the mesocarp correlated with higher mean C values (MCV), while 2,4-D promoted cell division and thus produced more cell layers in the pericarp than GA3 and pollinated treatments. Pollinated fruits had higher size and weight than GA3-treated fruits, but both were smaller compared to 2,4-D treatment. Pericarp thickness of GA3- and 2,4-D-induced fruits did not showed differences upon 20 dpa, reason why having fewer cells (GA3) could be compensated by a larger cell size. Two different inhibitors of GA biosynthesis [LAB 198999 and paclobutrazol (PCB)] decreased fruit growth and fruit-set, an effect reversed by GA3 application. LAB 198999 reduced GA1 and GA8 content, but increased that of their precursors GA53, GA44, GA19 and GA20 in pollinated fruits. This supports the hypothesis that GA1 is the active GA for tomato fruit growth. Both results demonstrate that fruit-set on tomato depends on GAs. The effect of pollination of GA biosynthesis genes was performed analyzing transcript levels of SlCPS, SlGA20ox1, -2 and -3, and SlGA3ox1 and -2. Pollination increased transcript content of SlGA20ox1, -2, and -3, and SlCPS, but not of SlGA3ox1 and -2. This summarized to the non-induction of fruit set by GA19, suggests that GA 20-oxidase activity was limiting in unpollinated ovaries. To investigate whether pollination also altered GA inactivation, full length cDNA clones of genes encoding enzymes catalyzing GA 2-oxidases (SlGA2ox1, -2, -3-, -4 and -5) were isolated and characterized. Transcript levels of these genes did not decrease early after pollination (5 dpa fruits), but transcript content reduction of all of them, mainly of SlGA2ox2, was found later (from 10 dpa). Thus, pollination mediates fruit-set by activating GA biosynthesis mainly through up regulation of GA20ox. A phylogenetic reconstruction of the GA2ox family was necessary to establish new genes position between subfamilies. It was showed the existence of three gene subfamilies maybe distinguished by its biochemical properties. Other aspect, is that fruit development induced by the auxins indol-3-acetic acid and 2,4-dichlorophenoxyacetic acid (2,4-D) were significantly reduced by simultaneous application of inhibitors of GA biosynthesis (paclobutrazol and LAB 198999), and that this effect was reversed by applied GA3. Parthenocarpic fruits induced by 2,4-D had higher contents of the active GA1 and of GA8 than pollinated fruits, while concentrations of the precursors (GA53, GA44, GA19, GA20) and GA29 were similar. Radioactive-labelled GAs applied to unpollinated ovaries, in presence or not of 2,4-D, showed that auxin induced the metabolism of [14C]GA12 to putative [14C]GA9, and of [14C]GA20 mainly to [14C]GA1, not to [14C]GA29 as occurred in control. [14C]GA1 metabolism to [14C]GA8 was much lower in 2,4-D-treated ovaries than in control. Transcript levels of genes encoding copalyldiphosphate synthase (SlCPS) and SlGA20ox1, -2 and -3 and SlGA3ox1, but not those encoding SlGA3ox2, were higher in unpollinated ovaries treated with 2,4-D, while transcript levels of SlGA2ox2 were lower in 2,4-D treated ovaries. Our results support the idea that auxins induce fruit-set and growth in tomato by enhancing GA biosynthesis through GA 20-oxidase, GA 3-oxidase and CPS, leading to higher GA1 content.