Abstract: Context and purpose of the study Merlot grapevine is the second wine cultivar most planted in the world and especially in the Bordeaux wine region. This cultivar has many advantages in producing high quality wine; however, in the last decade, climate change has increased the sugar concentration in berries at harvest and shortened the maturation cycle. If this has been up to now a great opportunity to improve wine quality profile, we are touching the tipping point. High sugar concentration at harvest induces high alcool content in wine which can negatively impact wine quality. There are many viticultural and oenological practices possible to limit this effect. In this study we focus on plant material through intra-varietal diversity of Merlot cultivar. Material and methods This study is based on three years of phenotyping a private collection of Merlot including 55 accessions and 3 ENTAV-INRA® registered clones. Each genotype is present in four blocks of 6 vines in the experimental plot. Among this collection, two extreme genotypes for sugar concentration, selected on 3 years of phenotyping, were phenotyped again on the fourth year at a larger scale. Separated individual berries were collected at 5 stages from green hard berries to mature berries. Sugar and organic acids were mearsured on each berry to compare both maturation cycles and to select triplicates of synchronized individual berries before transcriptomic analysis. Pair ended RNAseq reads were then aligned on high quality Merlot assembly and the counting table of gene expression was produced. Results Extreme genotypes have an average of 17g.L-1 difference in sugar concentration at harvest which approximately represents 1%vol of alcool in wine after fermentation. Whilst as expected thousands of genes were differentially expressed according to the advancement of ripening, statistical analysis only revealed 28 differentially expressed genes between the two genotypes throughtout all developmental stages and gives an insight on genes potentially involved in the control of sugar concentration. Selection among intra-varietal diversity therefore appears as a tool to immediate adaptation but also offers possibilities in gene detection of very specific agronomical traits.