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 necessityto improve the tolerance to lower water inputs is crucial and it could deliver a substantialcontribution to saving of water. Several local genotypes are available within the rich Italian tomatogermplasm, whose potential as source of superior adaptive alleles has not been fully exploited sofar, due to the complexity of the molecular processes involved. The goal of our research was thecharacterization of physiological, biochemical and molecular processes occurring in tomatogenotypes with different levels of tolerance to limiting water supplies, in order to dissect thecomplexity 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 incontrolled 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 secondcycle of stress imposed for 8 days (D2). Several physiological, biochemical and molecularparameters were monitored during the experiment to gain a deeper understanding of the response oftomato to water limitations.Stomatal conductance (gs) and CO2 assimilation (A) steeply decreased in all genotypes duringthe 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 wasconstrained by stomatal closure rather than by a permanent impairment of the photosyntheticmachinery. While plant gas exchanges did not highlight a different behaviour between the testedgenotypes, striking differences were noticed in leaf ABA and proline content, with M82accumulating high levels of both metabolites during D1. Despite the induction of the nced1 (9-cisepoxycaratenoiddioxygenase) gene, plants of IL9.2.5 did not accumulate ABA during both cyclesof drought stress.The physiological and metabolic adjustments were related to changes in expression of genesknown as key components of the response to water deficit. At the molecular level, a dramatic upregulationof a set of genes was observed in D1. After re-watering, the expression of all target genesdropped 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 (starchphosporilase) and nced1, while the expression of aco (1-aminocyclopropane-1- carboxylic acidoxidase) and erd15 (early responsive to dehydration) was not induced, indicating that relatedmetabolic pathways were not up-regulated. Transcriptomic changes elicited by drought andrehydration conditions were further being explored by a deep sequencing approach.Altogether, the results presented indicate that differences exist in the response of differentgenotypes of tomato to drought stress conditions and that genomic tools could be used to furtherexploit tomato genetic variability for developing drought-tolerant cultivars.