Caracterización fenotípica, fisiológica y molecular de la respuesta a salinidad y sequía en tomate cultivado (Solanum Lycopersicum) y silvestre (S. pennellii)

Abstract

One priority objective for plant biologists is the development of new varieties better adapted to survive under abiotic stresses, such as salinity and drought, in economically-relevant agricultural crops, among them tomato (Solanum lycopersicum L.). To achieve this goal, one interesting approach is to study wild species related to cultivated tomato, as these generally show high levels of tolerance to abiotic stresses. Another promising approach is the identification and analysis of mutants affected in key genes involved in tolerance mechanisms. In this PhD thesis, phenotypical, physiological and molecular analyses were combined in order to characterize cultivated tomato, the wild species Solanum pennellii and two mutants obtained in both species. In the first chapter, a comparative study between cultivated tomato and S. pennellii was carried out under drought and salt stresses. The tolerance of S. pennellii to drought is related to the induction of genes involved in nitrogen metabolism, redox homeostasis and metabolism/signalling of jasmonate and ethylene. Moreover, we showed that the control of water loss through the leaves is a key determinant for drought and salinity tolerance in the wild species. In order to reduce water loss, the leaves of S. pennellii show several anatomical features, including the reduction of stomatal density and thickening of the cell wall. In addition, the maintenance of a high water content in S. pennellii leaves is related to the regulation of genes encoding aquaporins. Regarding the mechanisms controlling salinity tolerance, this study revealed that the coordinated role of SOS1 and HKT1;2 determine the high Na+ transport to the leaves of S. pennellii, where it is efficiently accumulated inside its huge vacuoles, as reflected by the higher expression of the genes NHX3 and NHX4. Finally, in the first chapter of this PhD work, we advanced in the characterization of the mutant pennellii salt hypersensitive (psh). The results suggest that the massive accumulation of water and Na+ observed in the shoot of the mutant is due, on the one hand, to the co-transport of both solutes mediated by aquaporins, such as PIP2;1, and on the other, to the alteration of HKT1;2 expression, which also contributes to the accumulation of Na+ in the aerial tissues of psh. In the second chapter of this PhD work, we first carried out the transcriptomic profiling of the res (restored cell structure by salinity) tomato mutant, in order to elucidate the molecular basis of the phenotypic alterations observed under non-stressful conditions and their recovery under salinity. The significant number of genes constitutively altered in res, especially in roots, revealed that the growth inhibition observed under non-stressful conditions is a consequence of an imbalance in the growth-defence trade-off. Moreover, several genes which may be important for salinity tolerance in tomato, including those related to photosynthetic efficiency, protein homeostasis and transcription factors, were identified. Subsequently, we discovered that the res phenotype is caused by a mutation in the gene SlDEAD39, which encodes for a member of the DEAD-box RNA helicase protein family. Moreover, our analyses revealed that SlDEAD39 is involved in the maturation of the rRNA 23S from the chloroplast ribosome, a function altered in the res mutant under control conditions but partially recovered under salt stress. Finally, a possible molecular mechanism to explain the recovery of 23S rRNA processing under salinity is discussed.