Mapping QTLs involved in trichome development in tomato and understanding their role in drought and herbivory resistance

The work described in this thesis aimed to identify new structural and regulatory genes involved in trichome development in tomato (Solanum lycopersicum). Trichomes are hairs that cover the aerial epidermis of most terrestrial plants, and they confer resistance to abiotic and biotic stresses. In species with glandular trichomes, such as tomato, trichomes can produce a vast array of specialised metabolites with diverse biological activities. I explored the genetic variation present in the wild tomato species Solanum pennellii, which has high trichome density and is tolerant to biotic and abiotic stresses, to find genes involved in trichome development. I determined the trichome phenotype of the S. pennellii x S. lycopersicum cv. M82 introgression lines over two generations and identified genomic regions potentially containing genes involved in trichome development. I tested candidate gene by transient virus-induced gene silencing (VIGS) in tomato and could identify three genes with a role in trichome formation: SlMIXTA-like, SlMX2 and SlCycB2. I generated transgenic lines overexpressing SlMIXTA-like as well as SlMIXTA-like knock-out mutants using the CRISPR/Cas9 system to characterise this gene functionally. These results, combined with promoter analysis and comparisons between the MIXTA-like structure and function in S. pennellii and S. lycopersicum, showed that SlMIXTA-like is a negative regulator of trichome initiation and a key factor in epidermal patterning in tomato. I used the CRISPR/Cas9 system to generate knock-out mutants of known regulators of trichome development in tomato (SlMX1, Woolly, Hairless, CD2 and DWARF). Analyses of these knock-out mutants contributed to clarifying their precise function in trichome development and led to the establishment of a better model for trichome initiation and morphogenesis in tomato. My research also showed a clear relationship between trichome density and tolerance to drought stress and whitefly herbivory, indicating that trichome-related traits can be targeted for agricultural improvement of tomato cultivars.