MOLECULAR BASES OF DROUGHT STRESS RESPONSE IN ADAPTED ITALIAN TOMATO LANDRACES

Drought is one of the major abiotic stresses affecting plant growth and productivity. Among horticultural crops, tomato (Solanum lycopersicum) is generally classified as susceptible to drought stress. However, several local landraces adapted to arid Mediterranean conditions are successfully cultivated in low water input regimes. In order to identify the components involved in drought stress responses, we pursued an integrated approach where several biochemical and physiological parameters were evaluated together with transcriptome sequencing in two landraces ( Lucariello and Crovarese) from the Campania Region. Plants were grown in greenhouse and subjected to drought stress by water withhold for 13 days. Stomatal conductance and photosynthetic efficiency parameters showed similar profiles in both genotypes, with values greatly reduced after prolonged water withhold, indicative of a severe water stress condition. Similarly, ABA and proline levels greatly increased due to the stress treatment. Using RNA sequencing on leaf samples, we identified 2851 and 1867 genes upregulated upon stress in Crovarese and Lucariello, respectively. Furthermore, 4000 genes were downregulated in Crovarese and 2642 in Lucariello. Extensive overlapping was observed, with 1419 up- and 2297 down-regulated genes common to both genotypes. Gene Onthology (GO) enrichment analysis carried out on up-regulated and down-regulated genes revealed that several of the enriched categories were stress-related. A k-means clusterization was carried out. We identified in both genotypes 8 clusters (4 in upregulated dataset and 4 in downregulated dataset) of genes with similar expression profile. A new GO enrichment aimed at understanding the fuction of the clusters identified classes, which were peculiar for specific clusters. Among the genes showing the highest up-regulation in the stress treatment several members of the Late Embryogenesis Abundant protein (LEA) family were identified. This family was therefore selected for further studies. LEA proteins are a group of proteins that accumulate in response to cellular dehydration in many organism. Using multiple approaches for the discovery of LEA genes, we could identify 37 LEA proteins encoded in the tomato genome. We then built a dendrogram, which allowed the assignment of each gene to one of the known 5 LEA sub-family. Comparison of the dendrogram structure with transcriptome results highlighted that the 2 subfamilies "LEA 4" and "Dehydrin" in particular were upregulated following drought treatments in leaves. Altogether, our results contribute a broad understanding of the physiological, biochemical and molecular mechanisms employed by tomato to respond to water deprivation.