Improving production and fruit quality of tomato under abiotic stress: Genes for the future of tomato breeding for a sustainable agriculture

Climate change and a rising world population make generation of crops with enhanced abiotic stress tolerance and higher nutritive value a priority objective in agriculture. In this review we have summarized recent advances on tomato (Solanum lycopersicum), the most important horticultural species regarding production and nutritional quality. Firstly, the different and complex processes evolved in abiotic stress tolerance are summarized, with special attention on responses to osmotic, oxidative and ionic and nutritional stress, as well as the anatomical changes involved in plant adaptation to abiotic stress. In order to avoid or reduce production loss and improve fruit quality under abiotic stress, it is of utmost importance the identification of tomato key genes involved in abiotic stress long-term tolerance, that is, genes which simple variation determines a significant change not only in vegetative but also in reproductive growth. The functions of the tomato genes known to date affecting fruit yield under abiotic stress are described, including those involved in signal sensing, perception and transduction of abiotic stress, transcription factors and abiotic stress responsive genes. Together with production, fruit quality is crucial in tomato given its so high worldwide consumption per capita. Nutritional compounds as carotenoids and vitamin E are well known antioxidants and abiotic stress induces a secondary oxidative stress to which plants have to react. Although the increased metabolites production under abiotic stress may be due to physiological adaptation processes, advances in this field will go in parallel to the identification of genes improving fruit quality when the plant is grown subjected to stressful conditions. The key genes involved in improvement of nutritional quality of tomato fruit under abiotic stress are reviewed, in particular the main ones involved in accumulation of crucial metabolites in this aspect, such as carotenoids and tocopherols. One of the main bottlenecks in breeding programs nowadays is the strong reduction of phenotypic and genetic diversity in modern germplasm. Therefore, searching of new genetic resources for improvement of abiotic stress tolerance and fruit quality is critical, including sources of natural variation, like wild relatives and traditional varieties or landraces, as well as spontaneous and induced mutants with beneficial effects on these traits. All of them will expand our knowledge on the processes involved in abiotic stress tolerance and fruit quality in tomato.