Proceedings of the XLVII Italian Society of Agricultural Genetics - SIGA Annual Congress

Verona, Italy - 24/27 September, 2003

ISBN 88-900622-4-X

Poster Abstract - 1.38

ARE CLASS B HOMEOTIC GENES INVOLVED IN THE EXPRESSION OF PARTHENOCARPY IN TOMATO?

I. OLIMPIERI, F. SILIGATO, R. CACCIA, A. MAZZUCATO

Dipartimento di Agrobiologia e Agrochimica, Sezione di Genetica, Università degli Studi della Tuscia, Via S.C. de Lellis snc, 01100 Viterbo, Italy

floral mutants, homeotic genes, parthenocarpy, tomato

In flowering plants, the ovary develops into a fruit only after pollination and fertilization, when the requirements for seed development are thoroughly fulfilled. The genetic and molecular mechanisms that negatively control ovary development before pollination are not well understood. A recently proposed theory postulates that stamens act as regulatory factors towards the fourth floral whorl, by repressing the development of the ovary until fertilization has taken place. Accordingly to this theory, the silencing of a class B homeotic gene has been shown to be the cause of parthenocarpy in apple and a correlation between stamen disruption and parthenocarpy has been reported in transgenic tomatoes.

In tomato, natural or induced parthenocarpic mutants offer a useful method to regulate fruit production and a suitable experimental system to study ovary and fruit development. Among them, the parthenocarpic fruit (pat) mutation deserves particular interest because of its strong expressivity and the pleiotropic effects that affect anther and ovule phenotype. In such mutant in fact, the anthers show homeotic transformation into carpel-like structures, seldom bearing external ovules on the adaxial surface and ovules show arrested integument growth with absence of meiosis and consequent unviability. To date, no research has tested the hypothesis that stamen aberrations and parthenocarpy are driven by cues coming from the altered expression of class B genes in this genotype.

In this research, we address this possibility by constructing double mutants between pat and tomato mutations putatively involved in the B function, such as stamenless-2 (sl-2, that shows homeotic conversion of stamens into carpels) and pistillate-2 (pi-2, that shows strong stamen and subtle petal aberrations). In addition, the expression patterns of tomato class B (LeAP3 and TM6) and class B-related (TM8 and TM29) genes have been studied in time courses of tomato WT and pat mutant ovaries.

F₁ hybrids between the pat mutant and sl-2 or pi-2 showed wild-type (WT) phenotype, indicating that the mutations affect different loci. In F₂, whereas pat and sl-2 segregated independently and numerous double mutants could be recovered, for pat and pi-2 no double mutant could be unambiguously recognized out of 200 progeny plants, thus suggesting tight linkage. pat and sl-2 showed mainly additive effects in double mutant segregants, while some phenotypes of the pat mutation were counteracted by a single dose of the sl-2 allele. These results indicated that pat and sl-2 mutants might share only partially overlapping genetic determinants.

RT-PCR based expression analysis revealed that all the four assayed genes were down-regulated in the pat mutant ovary or fruit in comparison with the WT. For those transcripts characterized as being specifically transcribed in ovules (AP3), such lower expression in the mutant could be merely explained by the presence of very few normal ovules inside the pat ovary. However, the down-regulation of LeAP3 might also be directly involved in the parthenocarpic phenotype, because this pattern was also observed in the pat-2 mutant, a different tomato mutation for parthenocarpy that shows normal ovule development. Similarly, the lower expression of TM29 in the pat mutant can be directly related to parthenocarpy, because this gene is normally expressed in the young ovary pericarp.