SUMMARYA grapevine model system is described that is suitable for rapid forward and reverse genetic studies in small controlled environments. It is based on the Vvgai1 mutant allele that confers a dwarf stature, short generation cycles and continuous flowering ('microvine'). Black and white berry microvine genotypes were developed that can be transformed by Agrobacterium tumefaciens. Near-homozygous lines were created for efficient bi-allelic single nucleotide polymorphism (SNP) marker mapping and mutagenesis studies. A genetic mapping strategy based on picovine-derived microvine progeny populations was used to rapidly phenotype and map the flower sex and fleshless berry loci and identify a new lethal recessive locus, Vvlrl1. The microvine provides a unique model system for rapid genetic studies of grapevine by changing the perennial long life cycle of the plant to one with features and advantages similar to an annual plant.

Plant genetic sex determinants that mediate the transition to dioecy are predicted to be diverse, as this type of mating system independently evolved multiple times in angiosperms. Wild Vitis species are dioecious with individuals producing morphologically distinct female or male flowers; whereas, modern domesticated Vitis vinifera cultivars form hermaphrodite flowers capable of self-pollination. Here, we identify the VviPLATZ1 transcription factor as a key candidate female flower morphology factor that localizes to the Vitis SEX-DETERMINING REGION. The expression pattern of this gene correlates with the formation reflex stamens, a prominent morphological phenotype of female flowers. After generating CRISPR/Cas9 gene-edited alleles in a hermaphrodite genotype, phenotype analysis shows that individual homozygous lines produce flowers with reflex stamens. Taken together, our results demonstrate that loss of VviPLATZ1 function is a major factor that controls female flower morphology in Vitis.