Cluster thinning effect on Pinot noir berry ripening.

The impact of yield on fruit and wine quality has been studied extensively; vine response to increased yield is well known, but responsible mechanisms – including the regulation of pathways responsible for color, aroma and mouthfeel – have not been characterized at the level of genetic regulation. To examine the impact of crop load on transcriptomic and metabolomic changes during grape berry maturation vines from two cluster thinning treatments (50% or 75% of clusters removed following fruit set) were compared to control vines (no cluster thinning). Treatments were applied to mature Pinot noir grapevines over three consecutive vintages. Agronomic parameters, including yield per vine and the yield to pruning weight ratio, as well as fruit and wine composition, were determined at harvest. Fruit transcriptomic and metabolomic changes were measured in berry samples collected from each treatment at 7 to 10 day intervals from fruit set to harvest. The patterns of berry development by weight, sugar accumulation, and malic acid degradation indicated that vines responded differentially to crop load reduction by accelerating berry maturation progress. RNA-Seq analysis, and untargeted GC-MS and UHPLC-QTOF-MS -based metabolomic approaches were used. The indication of the acceleration of maturation transcriptional programs and of metabolite accumulation in the treated vines was suggested by multivariate analyses. To identify the transcriptional key triggers of this response we applied a two-regression step statistical approach which identified genes modulated over development dependent on the crop load. Comparing vintages we were able to estimate the environmental influence on the crop load treatments among years. The application of this multifaceted approach allowed the construction of robust models describing the impact of crop load on the genetic regulation of grape maturation. Overall design: Cluster thinning treaments were applied removing 50% or 75% of clusters following fruit set. Grape berries were collected at ten-day intervals in 2012, and weekly in 2013 and 2014 beginning at fruit set until harvest (24.5 °Brix) at the same time of day (8:00 am). Samples were collected in randomized 8-vine block designs along nine rows (3 rows/treatment). Each sample entailed the collection of 26 clusters from each vine block. 60 berries, selected from 6 isolated clusters randomly selected from the vine blocks, were ground under liquid nitrogen. Seeds were removed in the grinding process. Frozen powder was aliquoted in 100mg and 400 mg samples for metabolite and RNA extractions respectively. Transcriptomic and metabolomic approaches were used to monitor the expression of all grapevine genes and to elucidate grape berry metabolome during development.