Comparative gametogenesis and genomic signatures associated with pollen sterility in the seedless mutant of grapevine

Seedless grapes are in high demand for fresh and dry fruit consumption. Seedlessness in grapes (Vitis vinifera L.) is triggered by two different mechanisms: stenospermocarpy and parthenocarpy. However, the key regulators of seed development and their targets in grapes are not well characterized. The present study used the seeded grape hybrid ARI 516 and its seedless mutant to understand the molecular mechanisms controlling the seedless phenotype in grapes.

Gametogenesis studies demonstrated that the seedless mutant exhibits pollen sterility due to abnormal pollen morphology, significantly low viability, and a complete lack of germination ability. The macrogametophyte in the seedless mutant was significantly smaller than in ARI 516. Transcriptomic comparisons were performed during three developmental stages, including pre-flowering stage E-L 15, anthesis stage E-L 23, and berry formation E-L 31, to study altered developmental processes in the seedless mutant and ARI 516. Genes downregulated in the seedless mutant were enriched in the male gametophyte development-related pathways, which may cause pollen sterility. The RNAseq results were validated by qRT-PCR. Genome sequence data was also used to identify induced mutations in the seedless mutant, which revealed three homozygous and 25 heterozygous InDels in the genes related to male gametophyte development. RNAseq and genome sequencing data collectively indicate parthenocarpic seedless phenotype due to aberrant developmental and physiological processes involved in pollen formation, maturation and germination in the seedless mutant of ARI 516.

The study showed the downregulation of transcription factors and their target genes involved in cell division, gibberellin biosynthesis and signalling, cell wall development, organization, and pollen germination. This study represents a comprehensive attempt to identify putative candidate genes associated with parthenocarpic pollen sterility in grapes using genomic approaches.