Lignin biosynthesis as a key mechanism to repress Polystigma amygdalinum, the causal agent of the red leaf blotch disease in almond

The red leaf blotch (RLB) of almond, caused by the fungus Polystigma amygdalinum, is considered as one of the most important leaf diseases of this fruit tree. Differential cultivar susceptibility to the RLB has been described based on field observations, while its molecular and biochemical bases remain unknown to date. We aimed to explore the plant defence mechanisms related to the cultivar susceptibility by identifying some relevant physical and chemical strategies for the pathogen control. Thus, we studied the regulation of seven defence-related genes as well as the lignin deposition in two almond cultivars with highly differential response to RLB: the highly tolerant 'Mardia' and the susceptible 'Tarraco' cultivars. 'Mardia' displayed an up-regulation of the CAD and DFN1 genes at early stages of RLB symptom expression, with further lignin deposition in the fungal-colonized area that was visualized by microscopy. Thus, 'Mardia' uses both physical and chemical responses to effectively repress the pathogen. In contrast, 'Tarraco' triggered the up-regulation of HQT and LDOX genes, related to chlorogenic acid and anthocyanin biosynthesis pathways, respectively, while lignin deposition was not clearly noticed. This strategy recorded in 'Tarraco' at later stages of RLB symptoms failed to control the fungal infection and colonization. Our results suggested a major role of the phenylpropanoids pathway in the defence response against RLB, by showing that an early production of lignin might be a major mechanism to control the spread of P. amygdalinum within the host leaf tissues.