Multi-dimensional characterization of grape berry cuticles and their usability for an improved breeding of Botrytis bunch rot resilience

Abstract

For one of the major fungal diseases in cool-climate viticulture, Botrytis bunch rot (BBR), management practices as well as plant protection possibilities reach their limits. In contrast to downy and powdery mildew or black rot, no active defense mechanisms by major R-genes are available conferring resistance towards BBR. However, the cuticle and its epicuticular waxes proved to be one of the most powerful physical barriers increasing BBR resistance. Therefore, berry impedance (BI) was previously identified as a sensor-based indicator for high-throughput and reliable phenotyping of cuticle and berry surface characteristics. Aiming at the development of molecular markers for BBR- associated traits, a fine mapping approach was done based on the F1 progeny of ‘Dakapo’ x ‘Cabernet-Sauvignon’ (DxC) using 364 genotypes, 348 SSR marker mapped on 19 linked groups (LG) and three-years of phenotypic data. This resulted in QTLs on the chromosomes 6, 11, 15 and 17 with narrowed confidence intervals. The data further indicated that the presence of QTLs for BI seems to have an additive effect, i.e., genotypes with combinations of several QTLs show much higher berry impedance values.

However, as BI is naturally affected by the thickness and permeability of the cuticle as well as the hydrophobicity of epicuticular waxes, berry surfaces of varieties differing in BBR susceptibility and BI were investigated regarding the microscopic structure of epicuticular waxes and corresponding metabolomic profile. On the basis of Cryo Scanning Electron Microscopy in two consecutive seasons, five different epicuticular wax classes (EWC) could be identified. The EWC of different varieties is hereby correlated to BI (r = 0.65) and is comparable between years. In addition, the chemical composition of berry cuticles from the same varieties were analyzed by high-resolution mass spectrometry. By this, one specific compound was for instance detected in Calardis Blanc, a white variety with high BI values and BBR resilience. In addition, genotypes (F1 progeny of DxC) with very high and very low impedance values were selected for further metabolomic analysis of the berry cuticle, aiming to identify molecular markers linked to crucial chemical compounds. All of these findings will contribute to a better understanding of BBR resilience and allow the integration of measurable parameters into the breeding process to enhance the natural protection of grapevines against BBR.