Phenotypic key factors, genetic regions and genes associated to cluster architecture in grapevine (Vitis vinifera)

Abstract

Cultivated grapevine (Vitis vinifera) is one of the most widely grown fruit crops in the world and held in high regard for its nourishing fruits, sweet juices and iconic wines. Global viticulture predominantly utilizes Vitis vinifera varieties, because they convey sensory attributes corresponding to the current consumer ideal of product quality. However, they are also highly susceptible to fungal pathogens, and therefore require intense applications of plant protection products with adverse side effects. Newly bred varieties with resistances against fungal pathogens reduce the necessity for fungicide application by 60%. For grey mold, a severe threat in viticulture, an active resistance mechanism is still not feasible. Therefore, grapevine-breeding aims at introducing fungi-static physical properties. The central hub of these physical barriers is a loosely clustered variety. The enhanced available space between the berries provides the framework for the effective formation of a firm berry surface and waxy cover and is restricting the time-span with favorable moisture conditions for fungal infections.

The experimental design of this thesis draws on different sources of natural variance: Firstly, the F1 generation of the cross (‘Calardis Musqué’ × ‘Villard Blanc’) and secondly, somatic variants of the variety ‘Pinot Noir’ showing significantly different cluster compactness. The phenotypic assessment in both sources of natural variation identified six main drivers for cluster compactness. Cluster weight, berry number, berry volume, rachis length, shoulder length and pedicel length are main drivers of cluster compactness.

The genetic approach exposed eight overlapping regions with up to four quantitative trait loci for important architecture sub-traits that are physically co-located on the grapevine reference genome. In addition, several molecular markers with strong linkage to these cluster architecture sub-traits could be proposed. It was possible to exploit three of these markers for MAS against unwanted compactly clustered individuals. So, the number of undesirable compact clustered genotypes could be reduced by 29 % without selecting a single false positive in the investigated population. The gene expression of 15 candidate genes consistently correlates to cluster architecture variations of ‘Pinot Noir’ clones in a multi environmental experiment. The transcription factor gene PRE6 and six genes related to auxin metabolism, cell wall loosening, brassinosteroids and strigolactones showed differential expression in a further extended set of phenotypically divergent individuals from a genetically diverse background. The genetic approach and the gene expression experiments provide multiple lines of evidence for the reported candidate genes. Thus, the candidate genes presented here may have the capacity to be successfully involved in marker development with the aim of selecting cluster architecture traits in MAS enabling breeders to identify optimized breeding material with physical resilience to fungal pathogens such as Botrytis cinerea.