Mild water stress-induced priming enhance tolerance to Rosellinia necatrix in susceptible avocado rootstocks

Abstract

Background: White root rot (WRR) disease caused by Rosellinia necatrix is one of the most important threats affecting avocado orchards in temperate regions. The eradication of WRR is a difficult task and environmentally friendly control methods are needed to lessen its impact. Priming plants with a stressor (biotic or abiotic) can be a strategy to enhance plant defense/tolerance against future stress episodes but, despite the known underlying common mechanisms, few studies use abiotic-priming for improving tolerance to forthcoming biotic-stress and vice versa (‘ cross-factor priming’ ). To assess whether cross-factor priming can be a potential method for enhancing avocado tolerance to WRR disease, ‘Dusa’ avocado rootstocks, susceptible to R. necatrix , were subjected to two levels of water stress (mild-WS and severe-WS) and, after drought-recovery, inoculated with R. necatrix . Physiological response and expression of plant defense related genes after drought-priming as well as the disease progression were evaluated. Results: Water-stressed avocado plants showed lower water potential and stomatal limitations of photosynthesis compared to control plants. In addition, NPQ and q N values increased, indicating the activation of energy dissipating mechanisms closely related to the relief of oxidative stress. This response was proportional to the severity of the water stress and was accompanied by the deregulation of pathogen defense-related genes in the roots. After re-watering, leaf photosynthesis and plant water status recovered rapidly in both treatments, but roots of mild-WS primed plants showed a higher number of overexpressed genes related with plant defense than severe-WS primed plants. Disease progression after inoculating primed plants with R. necatrix was significantly delayed in mild-WS primed plants. Conclusions: These findings demonstrate that mild-WS can induce a primed state in the WRR susceptible avocado rootstock ‘Dusa’ and reveal that ‘ cross-factor priming’ with water stress (abiotic stressor) is effective for increasing avocado tolerance against R. necatrix (biotic stressor), underpinning that plant responses against biotic and abiotic stress rely on common mechanisms. Potential applications of these results may involve an enhancement of WRR tolerance of current avocado groves and optimization of water use via low frequency deficit irrigation strategies.

Elicitors and Plant Defence

“…Moreover, qRT-PCR quantification of the defence protease inhibitor gene, showed a 4-and 5-fold increases following elicitation with salicylate and jasmonate, respectively, in root samples after 12 hours (Clara Pliego, IFAPA-Málaga, unpublished results, May, 18, 2020). Previous investigations, also in avocado plants, had shown that exposure to mild water stress improved plant response to White Root Rot (cross-factor priming) [29]; the improved performance of avocado plants was linked to a general increased expression of plant defence genes, in particular those coding for NPR1 protein and NAC domain containing protein 72, leading these authors to conclude that adequate irrigation management could be an useful strategy to control fungal infections in commercial avocado orchards. In rice, Samota [30] observed that seed elicitation with methyl jasmone induced changes in expression of drought responsive genes RD1 and RD2 and plants derived from the elicited seeds would respond better to water stress than those from control seeds.…”