Phenols and lignin are involved in the defence response of banana (Musa) plants to Radopholus similis infection

Abstract

The role of lignin and phenols in plant defence ranges from preformed characteristic to inducible physical and chemical response against nematode infection. Our study shows the involvement of lignin and phenols in the defence of two newly identified resistant banana (Musa) genotypes to burrowing nematode Radopholus similis infection. Results were compared with reference resistant and susceptible banana cultivars. Histochemical analysis of root cross sections showed a more extensive secondary cell wall lignification of vascular bundles in R. similis-infected plants than in the nematode non-infected plants. Increased extensive lignification was not associated with the cortex cells that are directly attacked by the nematode. This showed that the increased lignification is a general defence response to protect the vascular bundle from damage rather than resisting the nematode development and reproduction at the root cortex. Histochemical localisation showed no preformed phenolic cells in the cortex of the non-infected, R. similis-resistant and -susceptible Musa genotypes. By contrast, phenolic substances were the major constituents of the nematode-infected necrotic cells. Phenols and lignin contents were also quantitatively assayed. The Folin-Ciocalteu assay confirmed the increase in phenol content of nematode-infected root cells. Phenol content in nematode-infected plants was twice the amount of phenol content in nematode non-infected plants at 3 weeks after infection. This is possibly due to the biosynthesis or accumulation of secondary metabolites such as phenolic phytoalexins in the nematode infection sites of all the banana genotypes. This study clearly demonstrates that phenols and lignin play an important role in the defence mechanisms of Musa to R. similis infection.