Endophytic Metarhizium robertsii promotes maize growth, suppresses insect growth, and alters plant defense gene expression

Abstract Fungi in the genus Metarhizium (Hypocreales: Clavicipitaceae) are entomopathogens that can establish as endophytes and benefit their host plant through growth promotion and suppression of insect pests. This study focuses on the Metarhizium-insect-plant interactions in greenhouse- and lab-based experiments. We inoculated seeds of maize (Zea mays L.) with spores of Metarhizium robertsii and evaluated V4 maize for endophytic colonization of leaves and roots, measured plant height, chlorophyll content and above-ground biomass, and relative growth rate of black cutworm, Agrotis ipsilon (Hufnagel). In addition, we studied the expression of selected plant defense genes. We recovered M. robertsii from 91% of plants grown from inoculated seeds. We detected M. robertsii more frequently in roots compared with leaves. Colonized plants were greater in plant height and above-ground biomass compared to control plants. Leaf tissue from colonized plants showed changes in the expression of genes involved in plant defense. In feeding bioassays, the relative growth rate of black cutworm was lower on leaves from endophytic plants compared to control plants. Few other studies have examined the defense response of plants to colonization by Metarhizium spp. Endophytic colonization of maize plants by M. robertsii promoted plant growth and altered defense gene expression in maize, and suppressed growth rate of black cutworm larvae.