Peroxidase and Pectin Methylesterase Activities in Cocoyam (Xanthosoma sagittifolium L. Schott) Roots upon Pythium myriotylum Inoculation

Cocoyam (Xanthosoma sagittifolium) is an important source of energy food for humid and semihumid tropic populations and consists of three cvs distinguished by the colour of their flesh, their productivity and their root rot disease scores: one sensitive (white), one intermediate (red) and one resistant (yellow). Changes in peroxidase (POD; EC 1.11.1.7) and pectin methylesterase (PME; EC 3.1.1.11) activities from roots of the three cvs were examined by isoelectrofocusing (IEF), polyacrylamide gel electrophoresis (PAGE) and sodium dodecyl sulphate (SDS)-PAGE upon inoculation with Pythium myriotylum. The evaluation of total POD activity shows the existence of a correlation between the sensitivity to P. myriotylum and POD activity. After 2 days of inoculation, POD activity of yellow cv. increased by 41.5%, while in the white and the red cvs the increase was only 13.4% and 7.5% respectively. In the soluble fractions of the resistant cv., the patterns of POD isozymes showed three new bands (Rf ¼ 0.25; 0.38 and 0.85) and in ionically bound fraction, a new acidic (Rf ¼ 0.2) specifically in inoculated root. The study of substrate specificity of POD with chlorogenic acid (C-POD), ferulic acid (F-POD) and indole acetic acid (IAA-POD) showed an induction of C-POD and F-POD and a decrease in IAA-POD in cocoyam roots after inoculation with P. myriotylum. Constitutively, ionically bound C-POD and F-POD activities were higher in roots of the yellow (11.6 and 11.7 EU/g Fw) than in those of the red (5.3 and 7.0 EU/g Fw) and the white (2.3 and 4.2 EU/ g Fw) cvs. In addition, isoelectric-focusing analysis of PME revealed an alteration of their profile correlated with inoculation. After inoculation, the appearance of a strong (pI: 9.7) and low (pI: 7.6) PME activities in the roots of yellow cv. were detected. Our findings suggest that POD and PME might be involved in the defence mechanism of cocoyam against P. myriotylum