MOLECULAR MECHANISMS UNDERLYING MORPHO- PHYSIOLOGICAL ADAPTATION TO COMBINED SALT/LOW NUTRIENT STRESS IN TOMATO

Tomato (Solanum lycopersicum L.) is a high value horticultural crop and an important dietary source of nutrients, vitamins and antioxidants. The cultivation is often subjected to drought, salinity and N-limited conditions. Thus, understanding the responses of tomato to salt and low N conditions will be instrumental to improve yields in stress-prone environments. In this study, physiological and molecular responses of three Italian landraces (TRPO0040, TRPA0130 and TRPO0670) to salt stress and low nitrate, alone or in combination were analysed. The experimental set up allowed four different treatments named: Control (13.5 mM NO3- - 0 mM NaCl), Salt stress (13.5 mM NO3- - 80 mM NaCl), N stress (3.4 mM NO3-- 0 mM NaCl) and Combined stress (3.4 mM NO3- - 80 mM NaCl). The treatments had different effects on plants: low N caused chlorosis, while salt stress inhibited growth causing loss of basal leaves. Leaf relative water content was affected by both salt and low nitrate, depending on the genotype, while proline content was dramatically increased by salt stress but not under combined stress. Growth parameters and fruit yield were reduced by single stress condition, and the highest reduction was observed in the combined stress. TRPO0040 genotype showed high Nitrogen Use Efficiency and Nitrogen Utilization Efficiency under salt, indicating that nitrogen allocation to fruits was not affected by this stress. We evaluated the impact of single and combined stresses on tomato transcriptome by performing RNAseq on roots and leaves of TRPO0040 after long-term exposure to the treatments. In leaves, a more extensive transcriptome reorganization was observed in response to N stress, while salt induced a significant variation in gene expression of a handful of genes. Parallel to the proline levels, expression of P5CS gene, encoding the rate limiting proline biosynthetic enzyme, was induced by salt, while the catabolic proline dehydrogenase was suppressed. Similarly, several genes encoding nitrate transporters were induced in N stress, including NRT2;2 and others. By contrast, in roots several thousands of differentially expressed genes (DEGs) DEGs in salt stress and only few hundreds in N stress were identified. Consistent with results gathered in Arabidopsis, SLAH1, a gene encoding a root specific anion transporter involved in the long distance transport of MOLECULAR MECHANISMS UNDERLYING MORPHO- PHYSIOLOGICAL ADAPTATION TO COMBINED SALT/LOW NUTRIENT STRESS IN TOMATO chloride ions, was upregulated in roots in salt stress condition, while a NRT2;4 like and AMT1 were induced by low nitrate. In the combined stress, a more extensive overlap of the DEGs was observed with the N stress rather than the salt stress for both roots and leaves. This indicates that nutrient availability may have a higher and longer- term impact on gene expression, compared to salt stress, which may induce rapid and transient responses, attenuated, particularly in leaves, when plant adaptation occurs. Altogether, we show that different cultivation regimes affect metabolic and transcriptomic profiles as well as growth in tomato, identifying a set of physiological and molecular targets specifically influenced by single or combined stress.