Phenotypic and genetic characterization of the pistillate mutation in tomato

Abstract

Many floral phenotypes have been described in decades of tomato genetics, but for very few of them the underlying genes have been identified so far. Because the increasing availability of genome sequence data will facilitate forward genetics in tomato, novel descriptive and map information will help the attribution of genes to phenotypes. In this contribution, we present our work on pistillate (pi), a genotype that directly recalls mutations affecting class B MADS-box genes, but that has not been further characterized after the first description. Plants homozygous for the pi allele appear with Mendelian proportions and, compared to wild-type, show delayed flowering, a frequently modified sympodial segment, higher occurrence of compound inflorescences, and reversion of the floral meristem to vegetative identity. In pi mutant flowers, the most striking aberration is the homeotic transformation of stamens into carpels. Ultrastructural analysis also reveals more or less subtle sepaloid features in the three inner floral whorls, mainly based on the presence, distribution and amount of glandular and non glandular trichomes. In the ovary, a 'flower within flower' phenotype was seldom observed; in one instance such phenotype was coupled with the setting of a parthenocarpic fruit, that reiterated the differentiation of a new flower. Mapping experiments positioned PI on the distal end of the long arm of chromosome 3. This position was not compatible with any class B or E MADS box gene; differently, the PI genetic window contained the FALSIFLORA (FA) gene, the tomato orthologue of LEAFY (LFY). The pi defects in flowering time and inflorescence development are in agreement with a direct involvement of the floral meristem identity gene. The class B- and E-like phenotypes shown by pi mutant plants are likely an indirect consequence because FA, as LFY, is reported as a positive regulator of homeotic MADS-box genes. Because fa mutant plants do not form complete flowers, the pi mutation deserves a particular interest, producing four-whorled, although modified, flowers useful to study the functional linkage between flower induction and flower organ identity specification.