The Mannose Binding Lectin gene FaMBL1 is involved in the resistance of unripe strawberry fruits to Colletotrichum acutatum

The fungal pathogen Colletotrichum acutatum is the causal agent of strawberry (Fragaria × ananassa) anthracnose. Despite the fungus can infect strawberry fruits both at unripe or ripe stages, the symptoms appear only on red ripe ones. On white unripe fruits, the pathogen becomes quiescent as melanized appressoria after 24 h of interaction. Previous transcriptome analysis indicated that a Mannose Binding Lectin (MBL) gene is the most upregulated gene in 24 h-infected white strawberries, suggesting a role for this gene in the low susceptibility of unripe stages. A time course analysis of the expression of this MBL gene, named FaMBL1 (Fragaria x ananassa MBL 1a), was undertaken to monitor its expression profile in white and red fruits at early interaction times: FaMBL1 resulted expressed exclusively in white fruit after 24 h, when the pathogen becomes quiescent. Agrobacterium-mediated transient transformation was used to silence and overexpress the FaMBL1 gene in 24 h-infected white and red strawberries, respectively. FaMBL1-silenced unripe fruits showed an increase in susceptibility to C. acutatum. These 24 h-infected tissues contained subcuticular hyphae, indicating pathogen penetration and active growth. In contrast, overexpression of FaMBL1 in ripe fruits decreased susceptibility; here, 24 h-infected tissues showed high percentage of ungerminated appressoria, as if pathogen slowed down its growth. These data suggest that FaMBL1 plays a crucial role in the resistance of unripe strawberry fruits to C. acutatum.