AN INTEGRATED VIEW OF GENE EXPRESSION, PHYSIOLOGICAL AND METABOLIC CHANGES TRIGGERED IN TOMATO BY DROUGHT AND REHYDRATION

Drought stress in plants is among the environmental stresses causing major yield losses. Tomato has the highest acreage devoted of any vegetable crop in the world, therefore the necessity to improve the tolerance to lower water inputs is crucial and it could deliver a substantial contribution to saving of water. Several local genotypes are available within the rich Italian tomato germplasm, whose potential as source of superior adaptive alleles has not been fully exploited so far, due to the complexity of the molecular processes involved. The goal of our research was the characterization of physiological, biochemical and molecular processes occurring in tomato genotypes with different levels of tolerance to limiting water supplies, in order to dissect the complexity of the molecular events occurring in response to drought. 40-d-old potted plants of the genotype M82 and the introgression line IL.9.2.5 were grown in controlled conditions in a greenhouse and subjected to drought by withholding water for 16 days (D1), then allowed to recover by re-watering for 7 days (RW) and finally subjected to a second cycle of stress imposed for 8 days (D2). Several physiological, biochemical and molecular parameters were monitored during the experiment to gain a deeper understanding of the response of tomato to water limitations. Stomatal conductance (gs) and CO2 assimilation (A) steeply decreased in all genotypes during the two cycles of drought stress and recovered to control levels when stressed plants were rewatered. The complete recovery of the photosynthetic rate upon re-watering indicated that A was constrained by stomatal closure rather than by a permanent impairment of the photosynthetic machinery. While plant gas exchanges did not highlight a different behaviour between the tested genotypes, striking differences were noticed in leaf ABA and proline content, with M82 accumulating high levels of both metabolites during D1. Despite the induction of the nced1 (9-cisepoxycaratenoid dioxygenase) gene, plants of IL9.2.5 did not accumulate ABA during both cycles of drought stress. The physiological and metabolic adjustments were related to changes in expression of genes known as key components of the response to water deficit. At the molecular level, a dramatic upregulation of a set of genes was observed in D1. After re-watering, the expression of all target genes dropped to levels comparable to those of control plants. A second cycle of drought stress (D2), however, elicited a great up-regulation of lea (late embryogenesis abundant protein), phosph (starch phosporilase) and nced1, while the expression of aco (1-aminocyclopropane-1- carboxylic acid oxidase) and erd15 (early responsive to dehydration) was not induced, indicating that related metabolic pathways were not up-regulated. Transcriptomic changes elicited by drought and rehydration conditions were further being explored by a deep sequencing approach. Altogether, the results presented indicate that differences exist in the response of different genotypes of tomato to drought stress conditions and that genomic tools could be used to further exploit tomato genetic variability for developing drought-tolerant cultivars.