INVESTIGATING THE GENETIC BASIS OF DROUGHT STRESS RESPONSE IN GRAPE ROOTSTOCKS

Consequences of climate change are becoming markedly worrying, since average surface temperatures are constantly going up and extreme climatic events are getting more frequent and intense, posing a considerable threat to worldwide viticulture. Among different abiotic stresses, drought is the factor that has a greater influence on plant physiology with a drastic impact on grape yield and quality. To overcome the deleterious effects of drought, plants adopt a multitude of physiological, biochemical and molecular mechanisms at cellular and systemic levels. Therefore, understanding the complexity of plant’s response to water deficit represents a major challenge for sustainable winegrowing. Especially, the development of strategies to reduce water consumption and to improve water-use efficiency (WUE) in vines will be fundamental in future years. Furthermore, the regulation of water use is particularly influenced by rootstocks, on which cultivars are generally grafted to cope with phylloxera infestations. The adaptation to drought indeed seems to be a cooperative action between scions and rootstocks, by means of hydraulic conductivity, chemical signalling and exchange of genetic material. However, a very few number of works were focused on identifying the genetic regions of grape rootstocks responsible for drought tolerance mechanisms. In this regard, the present research aimed to identify genetic determinism of phenotypic traits associated with drought tolerance. A genome-wide association study (GWAS) approach has been applied on an ‘ad hoc’ association mapping panel including different Vitis species, in order to dissect the genomic bases of transpiration-related traits and to identify genetic regions of grape rootstocks involved in drought tolerance, thereby potentially relevant for crop improvement. The panel was first genotyped with the commercial GrapeReSeq Illumina 20K SNP array and infrared thermography has been applied to estimate stomatal conductance values and to assess water status during progressive water stress and re-watering in two years. Some significant marker-trait associations were detected and a good list of candidate genes with a feasible role in drought response were identified. The physiological responses to drought were further investigate in four commercial rootstocks, 101.14 Millardet et de Grasset (V. riparia x V. rupestris), Selection Oppenhiem 4 (V. riparia x V. berlandieri), 110 Richter (V. rupestris x V. berlandieri) and Riparia Gloire de Montpellier (V. riparia). Differences were observed among genotypes and between water stress experiments that were performed in pots and in hydroponics. Furthermore, the application of osmotic stress in a hydroponic system has proved to be a useful method to evaluate the short-term stress response, especially for a rapid screening of stomatal sensitivity. In addition, a pilot study on a reduced subset of Vitis sylvestris genotypes exposed to water deficit treatment was carried out to evaluate their drought tolerance, because they represent a source of natural genetic diversity that could be exploited for future breeding programs. Taken together, a step forward to understand the basis of genetic variability of the response to water deprivation in grape rootstocks has been done in the present research. Moreover, it has been proved that different phenotyping approaches may help to dissect a highly complex trait such as water stress response.