Potential physical and chemical barriers to infection by the burrowing nematode Radopholus similis in roots of susceptible and resistant banana (Musa spp.)

Abstract

Resistance in banana roots against the burrowing nematode Radopholus similis has been correlated in the past with the phenylpropanoid pathway of secondary metabolism, but quantitative chemical analyses to support histological data are lacking. Therefore, healthy and infected roots of two susceptible (Grande naine and Obino l’ewai) and three resistant cultivars (Yangambi km5, Pisang jari buaya and Calcutta 4) were extracted and chemically analysed for their lignin content and phenylpropanoid profile using a quantitative lignin assay, high performance liquid chromatography and liquid chromatography/mass spectrometry. Through histochemical staining phenylpropanoids were localized in root tissue. Compared to the susceptible cultivars, the resistant cultivars had constitutively significantly higher levels of lignin in the vascular bundle and cell‐wall bound ferulic acid esters in the cortex. Infection‐induced lignification was observed in the vascular bundles of all cultivars. The catecholamine dopamine was identified as a major metabolite in banana roots. Levels varied from 2·8 to 8·4 mg per g root fresh weight and were significantly higher in the resistant cultivars. Other compounds, tentatively identified as anthocyanidin‐related, were present in high quantities and may, besides dopamine, make up the substrates for polyphenol oxidation products in necrotic tissue.