On the “priming” steps: initial results on the effect of PEG-induced drought stress upon the oxidative status and related gene expression in Citrus rootstocks

Desertification and overexploitation of water represent detrimental issues strongly challenging a sufficient crop yield that should be able to meet the food needs of an ever-growing world population. Especially in citrus, water scarcity negatively affects the overall tree growth and fruit quality. A key factor in the overall attempt to sustain and withstand the negative effects of drought stress is the use of grafting, as rootstocks effectively modulate the physiological performance of the trees. It has been suggested that plants can induce some stress “memory” following a first stress exposure that leads to acclimation to later exposure to stress. To establish the adequate genotype and conditions for creating a priming-induced drought stress tolerance, plants of two different citrus rootstock genotypes (Carrizo citrange, Bitters C22) were grown in aerated half-strength Hoagland’s nutrient solution for seven days. Afterwards, the plants were exposed to a nutrient solution containing 0 (control) or 15% PEG-8000 (PEG) for 10 days. To assess the level of oxidative stress caused by the imposed water stress condition, the H₂O₂ leaf content was measured. Furthermore, we analysed the expression levels of genes involved in ROS quenching to evaluate the plant response to the imposed stressful conditions. Our results indicate that Carrizo citrange is much more sensitive to PEG treatment than Bitters (C22) as plants exhibited higher levels of H₂O₂ and physiological alterations. Moreover, gene expression analysis highlighted apxS and gpx as candidate genes for H₂O₂ scavenging in response to drought stress in the Carrizo citrange. Our results, within the PON “Ricerca e Innovazione” project Water4AgriFood, lay the basis for the current analysis aimed at identifying the genetic and epigenetic mechanisms of priming in the Citrus genus.