Lanthanum: a novel inducer for enhancement of fungal laccase production by Shiraia bambusicola

Abstract Fungal laccases are industrially important inducible enzymes extensively used in the delignification of lignocellulosics, detoxification of environmental wastes and decolorization of textile dyes. The discovery of new inducers is crucial for laccase productivity by filamentous fungi. In this study, a novel laccase-producing strain S8 from a bambusicolous fungus Shiraia bambusicola was identified by using 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) as laccase secretion indicator and native polyacrylamide gel electrophoresis. The corresponding full-length cDNA of laccase was cloned and characterized. The effect of lanthanum (LaCl3) on extracellular laccase activity was tested at the concentration from 0.2 to 2.0 g/L and at different addition time (day 1–4) in the mycelium culture. In the presence of 1.0 g/L La3+ at the beginning of the mycelium cultures, the highest laccase activity (2.7 × 104 U/L) reached after 10 days of cultivation, about tenfold higher compared with non-induced cultures. La3+ was shown to be a very strong inducer for fungal laccase activity with no inhibitory effect on fungal growth at the optimized concentration. In vivo, La3+ added to the mycelium culture not only promoted a continuous and high expression of laccase gene (lcc1), but also induced a rapid generation of signaling molecules including reactive oxygen species (ROS) and nitric oxygen (NO). Both the NO donor sodium nitroprusside (SNP) and exogenous hydrogen peroxide (H2O2) potentiated La3+-induced laccase activity and increased membrane permeability of hyphal cells. Moreover, the scavenger such as 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) for NO and vitamin C for ROS suppressed the induction. These results suggest these signals could mediate La3+-induced laccase biosynthesis and its secretion. Our study provides a basis for understanding the induction mechanism of La3+ on laccase and a practical strategy for enhanced fungal laccase production.