Physiological Effect of Laccase, Secreted by Rigidoporus microporus on the Host Pathogen Interactions

Laccases (EC1.10.3.2) are multi-copper containing phenol oxidases that catalyse theoxidation of phenolic compounds and aromatic amines. They are widely distributed in higherplants and in many fungi. In fungi, they are involved in delignification, sporulation, pigmentproduction and plant pathogenesis and fruiting body formation. Rigidoporus microporus is awhite rot fungus efficient lignin degradation and causative agent of the most destructivewhite root disease. In this study, the production of laccase was detected qualitatively.Furthermore involvement of laccase in the host-pathogen interaction was also studied. Threeisolates of R. microporus were isolated from symptomatic Hevea brasiliensis (R1), Mucunabractiata (R2), and Artocarpus nobilis (R3). The level of laccase production was measuredqualitatively on solid media containing the colour indicator of guaiacol. 5 mm diameter ofmycelium discs from 7 day old cultures were innoculated into 2% PDA plates supplementedwith 0.2% glucose as a laccase degrading substrate with 0.02% Guaiacol (pH 5.5) andincubated at RT (28±20 C) under continuous dark conditions. After, an incubation period of 4days, the enzyme activity was examined by the decolorisation intensity. Morphology of thethree isolates was also evaluated on malt extract agar. Lignin degradation ability was studiedusing artificial inocula prepared by fresh root pieces of H. brasiliensis and incubated for fourmonths; thereafter they were splited longitudinally to observe the lignin degradation ability ofthe isolates. All the test isolates were laccase positive and R3 (A. nobilis) isolate producedhigher intensity showing comparatively larger diameter of the reddish brown halos indictinghigher level of laccase production. Moreover, the isolate R3 showed the formation ofrhizomorphs in vitro conditions while the other isolates did not show them. Formation ofrhizormorphs is associated with the laccase synthesis which has been reported to beresponsible for making polyphenolic glue that sticks the hyphae together. The degradation ofroots was observed to be comparatively faster by the isolate R3 leaving white pockets onroots by degrading cell wall component due to the removal of lignin in localized areas ofwood and observation can be attributed with the higher laccase enzyme. The root piecesinoculated with R1 and R2 did not show white pockets and a spongy nature until four monthincubation period. It has been revealed that the isolate R3 (A. nobilis) is a good laccaseproducer among the other isolates under investigation.Keywords: Laccase, Rhizormorphs, Rigidoporus microporus