Tomato fruit development in the auxin-resistant dgt mutant is induced by pollination but not by auxin treatment

In tomato (Solanum lycopersicum Mill.), auxin is believed to play a pivotal role in controlling fruit-set and early ovary growth. In this paper we investigated the effect of the reduced auxin sensitivity exhibited by the diageotropica (dgt) tomato mutant on ovary growth during early stage of fruit development. Here we show that in hand-pollinated ovaries fruit-set was not affected by the dgt lesion while fruit growth was reduced. This reduction was associated with a smaller cell size of mesocarp cells, with a lower mean C values and with a lower gene expression of the expansin gene LeExp2. When a synthetic auxin (4-CPA, chlorophenoxyacetic acid) was applied to the flowers of wild type plants, parthenocarpic ovary growth was induced. On the contrary, auxin application to the flowers of dgt plants failed to induce parthenocarpy. Hand-pollinated ovaries of dgt contained higher levels of IAA compared to wild type and this was not associated with high transcript levels of genes encoding a key regulatory enzyme of IAA biosynthesis (ToFZYs) but with lower expression levels of GH3, a gene involved in the conjugation of IAA to amino acids. The expression of diverse Aux/IAA genes and SAUR (small auxin up-regulated RNA) was also altered in the dgt ovaries. The dgt lesion does not seem to affect specific Aux/IAA genes in terms of transcript occurrence but rather in terms of relative levels of expression. Transcript levels of Aux/IAA genes were up regulated in auxin-treated ovaries of wild-type but not in dgt. Together, our results suggest that dgt ovary cells are not able to sense and/or transduce the external auxin signal, whereas pollinated dgt ovary cells are able to detect the IAA present in fertilized ovules promoting fruit development.