Exoenzymes improve penetration and colonization of potato plants by endophytic entomopathogenic Metarhizium brunneum

Abstract: Biocontrol with endophytic entomopathogenic fungi is a promising option to protect plants systemically from insect herbivores. Yet, current applications are limited especially by low plant colonization. Inspired by penetration mechanisms of plant pathogenic fungi, we aimed at enhancing Metarhizium brunneum strain Cb15 potato plant penetration and colonization by supplementation of plant cell-wall degrading enzymes pectinase, hemicellulase or cellulase. As part of a pre-conditioning strategy of “endophytically competent” biomass, we further looked into the possibility of inducing these enzymes during submerged cultivation of Metarhizium brunneum mycelial biomass. We found that after hemicellulase or cellulase supplementation to mycelial biomass on potato tubers, the number of plants with endophytic fungal colonization in the roots was enhanced by 20% and even by 60%, respectively. This correlated with a rise in fungal endophytic DNA levels by 4% after hemicellulase and by 25% after cellulase treatments. Pectinase had no beneficial effect on plant colonization. Supplementation of cellulose derivatives during submerged cultivation of Metarhizium brunneum led to induction of cellulase activity. Maximum enzymatic activity depended significantly upon the water solubility of the cellulosic materials. Our study provides first evidence that refined formulations of endophytic entomopathogenic fungi could contribute to a more effective use of these fungi in strategies priming plants against pests and diseases.