Bacterially produced rice thaumatin-like protein shows in vitro antifungal activity

Thaumatin-like proteins (TLPs) are pathogenesis-related (PR) proteins of group 5 believed to be involved in plant defense mechanisms. A full-length TLP cDNA clone was previously isolated from a rice cDNA library prepared from RNA isolated from Rhizoctonia solani-infected rice plants. The cDNA region corresponding to the putative mature TLP was expressed in Escherichia coli as a Histagged fusion protein and purified by Ni-NTA resin column chromatography. The purified 23-kDa rice TLP inhibited the mycelial growth of Fusarium oxysporum f. sp. cubense, F. graminearum, Botrytis cinerea, Curvularia lunata, Drechslera oryzae, Sarocladium oryzae and Rhizoctonia solani in vitro at a concentration of 5 μg per disc. The fungi Aspergillus niger, Alternaria alternata and Sclerotium oryzae were inhibited only at 10 μg level. TLP had no effect on the growth of Macrophomina phaseolina and Sclerotinia sclerotiorum even at 10 μg concentration. The purified TLP also inhibited spore germination and germ tube growth of some fungi. The sensitive fungi exhibited growth abnormalities including swelling of hyphal tips and excessive branching of hyphae.