Abstract
Previous studies showed that the number of berries is a major component of the compactness level of the grapevine clusters. Variation in number of fruits is regulated by events occurring in the fruitset, but also before during the flower formation and pollination, through factors like the initial number of flowers or the gametic viability. Therefore, the identification of the genetic bases of this variation would provide an invaluable knowledge of the grapevine reproductive development and useful tools for managing yield and cluster compactness. We performed the phenotyping of four clones (two compact and two loose clones) of the Tempranillo cultivar with reproducible different levels of cluster compactness over seasons. Measures of reproductive performance included flower number per inflorescence, berry number per cluster, fruitset, coulure, and millerandage indices. Besides, their levels of several hormones during the inflorescence and flower development were determined, and their transcriptomes were evaluated at critical time points (just before the start and at the end of flowering). For some key reproductive traits, like number of berries per cluster and number of seeds per berry, clones bearing loose clusters showed differences with the compact clones and also differed from each other, indicating that each one follows different paths to produce loose clusters. Variation between clones was observed for abscisic acid and gibberellins levels at particular development stages, and differences in GAs could be related to phenotypic differences. Likewise, various changes between clones were found at the transcriptomic level, mostly just before the start of flowering. Several of the differentially expressed genes between one of the loose clones and the compact clones are known to be over-expressed in pollen, and many of them were related to cell wall modification processes or to the phenylpropanoids metabolism. We also found polymorphisms between clones in candidate genes that could be directly involved in the variation of the compactness level.
Highlights
Cluster compactness, the density of the berries in the cluster, is a primary aspect of grapevine (Vitis vinifera L.) selection programs
We described here for the first time in grapevine intracultivar differences at three levels: phenotypical, hormonal, and transcriptional, among four clones, two producing compact clusters and two producing loose clusters
Evolution of hormonal levels during inflorescence development have been shown in Tempranillo, with clear differences between hormones and few differences between the clones
Summary
The density of the berries in the cluster, is a primary aspect of grapevine (Vitis vinifera L.) selection programs. Berries inside the cluster are organized as a thyrse, which is unique to grapevine among economically important crops. The thyrse, etymologically derived from Dionysus’s staff, is a type of mixed compound inflorescence. It is a raceme inflorescence type where flowers are replaced by a cyme (Hickey and King, 2000; Gerrath et al, 2017), described as a panicle (Pratt, 1971). All treatments that reduce cluster compactness are expected to lead to a lower predisposition of clusters to early and severe incidence of pests and diseases (Molitor et al, 2012). Loose cluster is a favorable factor for table grape appearance
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