The tomato response to water deficit stress

Anthropogenic climate change is reshaping global hydrological patterns, posing significant challenges to agricultural productivity, particularly in regions like the Mediterranean, where droughts are expected to intensify. Tomato (Solanum lycopersicum) is one of the major crop species grown in this region, meaning its cultivation could be disproportionally affected. Thus, there is a strong need for the development of varieties with enhanced drought tolerance. The work in this thesis focuses on understanding the molecular and physiological responses of tomato plants to water deficit (WD) stress, with the goal of contributing to the development of drought-resilient crops. The work described in this thesis first focuses on a transcriptomic approach to track gene expression changes in tomato plants subjected to gradual and prolonged WD. By aligning these changes with physiological thresholds, such as stomatal closure, wilting, and the onset of vascular embolism, I identify key transcription factors (TFs) involved in both ABA-dependent and independent pathways. Notably, this work identifies distinct phases of the water deficit response, likely centered on stomatal closure and leaf wilting. In addition, this work uncovers key differences in WD response between young and mature leaves, offering insights into the apparent resilience of younger tissues. Next, this work delves into the functional characterization of the NAC transcription factor SlRD26, specifically pertaining its role in the response to WD. I found that SlRD26 likely acts as a positive regulator of the response to WD, as overexpression of SlRD26 leads to typically water-saving adaptations, such as shorter stature and smaller leaves. However, the modulation of SlRD26 expression, through either constitutive overexpression, or knockdown via artificial micro-RNA, did not enhance the maintenance of yield under water limiting conditions, nor does it decrease transpiration rate under moderate WD conditions. Instead, RNA-Seq analysis suggests that SlRD26 plays a role in regulating lignin biosynthesis, contributing to structural- and water saving-adaptations of water transporting tissues. Ectopic lignification, or disproportionate lignification of water-transporting tissues in the overexpression mutant, could in this case prove detrimental to WD adaptation and not lead to a resilient phenotype. Collectively, these findings advance our understanding of the genetic and physiological networks that underpin tomato’s response to WD. By identifying critical genes and pathways, this research provides insights for the future development of crops with enhanced drought resilience, a crucial step in addressing the agricultural challenges posed by climate change.