Analysis of the origin of parthenocarpy in grapevine cultivar Corinto Bianco

Abstract

Seedless fruits appear spontaneously in grapevine (Vitis vinifera L.) as a result of somatic variation. Stenospermocarpic and parthenocarpic seedlesness are known. The first type is widely used in table grapes production because seed development aborts after fertilization giving rise to seed traces and almost normal size berries. In contrast, small berries without seed traces develop in absence of fertilization in parthenocarpic cultivars that are appreciated for raisin production. The cultivar Corinto Bianco is a parthenocarpic somatic variant of the Spanish seeded cultivar Pedro Ximénez. Morphological and molecular comparison of flower development and gametogenesis between both genotypes were directed to understand the genetic and molecular basis of this parthenocarpic phenotype. Histological analyses showed that ovules developed similarly in both genotypes. However, macrogametogenesis was altered in Corinto Bianco showing disorders during mitosis after mother cell meiotic reduction. Microgametogenesis was also altered in Corinto Bianco, which pollen was 100% sterile in 2012 and 2013, compared with Pedro Ximénez pollen that showed only 8.47 and 21.64% of sterility in those years. The average number of seeds per berry was 1.35 in Pedro Ximénez berries, which weight was six times higher than in Corinto Bianco parthenocarpic berries with zero seeds. In addition, we observed partial phenotypic reversion in 2.6% of Corinto Bianco berries that carried one seed and displayed a comparable size to that of Pedro Ximénez berries. These seeds were unable to germinate in soil under normal conditions. Fortunately, following an in vitro germination protocol we were able to rescue 41 seedlings derived from revertant berries for further analyses. Gene expression alterations between Pedro Ximénez and Corinto Bianco, that could lead to the identification of candidate genes responsible for the phenotypic change, were analyzed by comparing closed flowers of both genotypes at 50% bloom time using the NimbleGen Vitis HX12 microarray. We identified 441 genes upregulated and 949 downregulated in the parthenocarpic mutant (¿2-fold change and 5% FDR). Interestingly, genes related with cell cycle and gametogenesis were downregulated in Corinto Bianco, including a cyclin (CYB1;2- like) and a MADS-box gene (AGL66-like), whose function in Arabidopsis is related with mitosis and pollen development, respectively. The expression of these genes could be related with defects in gametogenesis in Corinto Bianco. These preliminary results suggest the presence of defects in the meiotic mechanisms central to the process of gametogenesis in Corinto Bianco, providing clues for further characterization of the origin of parthenocarpy in this cultivar.