A unique ecological role of the entomopathogenic fungus Metarhizium koreanum in infecting plant peltate glands

Abstract Metarhizium koreanum , a known entomopathogenic fungus previously reported only from insect hosts, was discovered colonizing the peltate glands on the abaxial leaf surface of Clerodendrum japonicum var. japonicum in Taiwan. Field surveys consistently showed that the fungus was exclusively associated with sand-colored glands, whereas uninfected glands remained transparent. The transition from transparent to yellow–brown coloration was found to result from fungal infection. This phenomenon had previously been misinterpreted as an inherent trait of the host plant. Morphological examination and multilocus phylogenetic analyses (ITS, TUB , TEF-1α , and RPB1 a) confirmed the identity of the isolates as M . koreanum . Endophyte isolation assays did not recover the fungus from surface-sterilized leaf lamina segments lacking infected peltate glands, indicating that M. koreanum does not occur as an endophyte. Pathogenicity tests further confirmed the ability of M . koreanum to infect the foliar glands. This study represents the first report of a Metarhizium species infecting a flowering plant, although colonization appears limited to external glandular structures. This unique association raises intriguing questions about potential plant–fungus–insect interactions, especially given the overlapping geographic distribution of C. japonicum var. japonicum and M . koreanum in Japan, South Korea, Taiwan, and Thailand.