New insights on the role of SlDMR6-1 in drought avoidance in tomato

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Abstract
The DOWNY MILDEW RESISTANCE 6 (DMR6) protein is a 2-oxoglutarate (2OG) and Fe(II)-dependent oxygenase, involved in salicylic acid (SA) metabolism, and its inactivation in tomato was found to increase SA levels and to confer disease-resistance against several pathogens. SA is also recognized as an abiotic stress-tolerance enhancer, and we tested the resistance to drought stress in Sldmr6-1 tomato mutants generated by the CRISPR/Cas9 technique. Wild-type (WT) tomato cultivar ‘San Marzano’ and its Sldmr6-1 mutants were subjected to water deprivation for 7 days. At the end of the period, while WT plants exhibited severe wilting, the T 2 Sldmr6-1 mutant plants showed turgid leaves and maintained higher Soil Relative Water Content (SRWC). Sldmr6-1 mutants adopted a water saving behaviour reducing transpiration rate (E) by decreasing stomatal conductance (Gs). Assimilation rate (A) decreased in parallel to E under drought stress, resulting in no alteration of the CO 2 concentration in the sub-stomatal chamber (Ci) and increasing the Water Use Efficiency (WUE, A/E). Defence mechanisms of the photosynthetic machinery triggered in Sldmr6-1 mutants, that under drought stress showed up-regulation of the genes SlAPX and SlGST (anti-oxidant related) as well as down-regulation of SlCYP707A2 gene, which is involved in ABA catabolism. Our results suggest that the disabling of SlDMR6-1 in tomato plants leads to a drought-avoidance strategy through tight control of stomatal closure controlling water loss. In addition, it was highlighted, for the first time in tomato, that Sldmr6-1 mutants showed reduced susceptibility to Phytophthora infestans , the causal agent of Late Blight.
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