Mediterranean Tomato Landraces Exhibit Genotype-Specific Transcriptomic Responses to Water Stress.

Drought stress is a critical limitation to crop production, particularly in Mediterranean climates. The molecular response of drought adaptation in crops is variable and coordinated in leaves and roots. To understand the molecular basis of drought tolerance in tomato, the transcriptomic responses of two drought-tolerant Mediterranean landraces ('de Ramellet' from Spain and 'Lucariello' from Italy) were compared with two commercial varieties ('Moneymaker' and 'Ailsa Craig'). Plants were grown under well-watered and water stress conditions, performing RNA sequencing in leaf and root tissues. Differential expression analysis denoted that the transcriptomic response to water stress was mostly genotype-dependent and tissue-dependent. Only 0.9% of leaf DEGs and 5.1% of root DEGs were shared across all genotypes, including pathways like response to salt, response to heat, and photosynthesis. Commercial genotypes regulated a wide array of metabolic pathways, including plant-defense, indicative of a response to a general metabolic impairment. In contrast, landraces exhibited a more targeted transcriptional response, specifically triggering drought- and stress-related pathways. The 'de Ramellet' landrace exhibited a highly divergent leaf response, characterized by increased expression of osmotic stress-related genes and heat stress pathways, while 'Lucariello' regulated salt transmembrane transporters and antioxidant defenses, particularly in roots. These findings highlight the distinct molecular strategies employed by landraces and commercial cultivars under water stress, emphasizing the potential of Mediterranean landraces as genetic resources for improving drought resilience in tomato breeding programs.