Salicylic acid and methyl jasmonate activate key genes of plant-defense pathways conferring partial protection to Polystigma amygdalinum in a susceptible almond cultivar

Red leaf blotch (RLB) of almond, caused by Polystigma amygdalinum, is an economically important foliar disease affecting almond crops. This study explored the hormonal responses of two almond cultivars, namely ‘Tarraco’ (highly susceptible) and ‘Mardía’ (highly tolerant), to P. amygdalinum. Hormonal profiling and gene expression analyses were conducted to examine the roles of salicylic acid (SA), jasmonic acid (JA), and 1-aminocyclopropane-1-carboxylic (ACC) acids, and methyl jasmonate (MeJA) in plant defense mechanisms. Results showed a significant accumulation of SA in symptomatic leaves of both cultivars, suggesting a SA-mediated defense response to the pathogen. However, no substantial changes in JA and ACC levels were observed. In ‘Tarraco’, expression of SA-responsive genes (PR1 and PR5) and ET/JA-associated genes (ACO and ERF1) increased, but the cultivar remained susceptible. In contrast, symptomatic ‘Mardía’ leaves exhibited increased expression in CAD, linked to lignin biosynthesis, while other hormone-related genes (ACO, ERF1, PR1, and PR5) did not show significant changes. Thus, ‘Mardía’ could be following a different defense strategy against RLB. Exogenous applications of SA and MeJA significantly reduced RLB incidence and severity in young ‘Tarraco’ trees, with MeJA enhancing ERF1 expression and SA increasing both ERF1 and CAD expression. MeJA also inhibited plant growth. These findings reveal contrasting defense mechanisms between the two almond cultivars, suggesting a possible protection against RLB through lignin biosynthesis. Furthermore, the protective role of SA would be associated with CAD, indicating a connection between SA signaling and the phenylpropanoid pathway.