Differential Induction of 3-deoxyanthocyanidin Phytoalexins in Relation to Colletotrichum falcatum Resistance in Sugarcane

Sugarcane secrets a reddish pigment presumed to contain phytoalexin compounds in the stalk tissues in response to infection of Colletotrichum falcatum, causing red rot. To identify the phytoalexin compounds involved in red rot resistance, detailed HPLC analyses were conducted using the phytoalexin standards in a set of sugarcane cultivars varying in red rot resistance, after pathogen inoculation. We found nine different 3-deoxyanthocyanidin phytoalexin compounds, which are differentially induced upon C. falcatum infection in sugarcane varieties with varying degrees of disease resistance. Among the nine compounds, four compounds were identified as luteolinidin, 5-methoxy luteolinidin, apigeninidin and arabinosyl-5-O-apigeninidin. Multifold induction of luteolinidin, apigeninidin and arabinosyl-5-O-apigeninidinin were recorded in the stalk tissues in disease resistant cvs Co 93009, BO 91 and Baragua, respectively after pathogen inoculation as compared to the susceptible cv CoC 671. In addition to the known compounds one to three unknown compounds were detected in the resistant cultivars. The susceptible cultivars exhibited a placid induction of phytoalexin compounds after pathogen infections. The results very clearly demonstrated a specific induction of 3-deoxyanthocyanidins in resistant cultivars at a higher level as compared to susceptible cultivars after C. falcatum infection in sugarcane. Total extracts of the induced phytoalexins in the calorimetric assays, also revealed similar results. Probably induction of either luteolinidin or apigeninidin compounds alone or in combination at higher concentrations in the resistant cultivars may enable the effective arrest of pathogen invasion and further development inside the stalk tissues.