Ecotypes of the fungus Bipolaris oryzae with various responses of the mycochrome system

Abstract

Two types of fungi based on the light required for induction of conidiophore formation are known as ‘photo‐induced’ and ‘non‐photo‐induced’ sporulators. Conidial development in a ‘photo‐induced’ strain of Bipolaris oryzae (formerly, Helminthosporium oryzae) is controlled by antagonistic action of blue and near‐UV radiation mediated through the ‘mycochrome’ system at two developmental stages: conidiophore induction and conidiophore maturation. Otherwise, conidial development in a ‘non‐photo‐induced’ strain of B. oryzae, unlike that in the ‘photo‐induced’ strain, is controlled by antagonistic action of blue and near‐UV radiation at the conidiophore maturation stage alone. In this study, we collected fungi, B. oryzae, from brown lesion spots on rice leaves cultivated in paddy fields, and investigated the possible existence of a new ecotype different from known ‘photo‐induced’ and ‘non‐photo‐induced’ sporulators. We also investigated the sensitivity of conidial development of each isolate to blue and near‐UV radiation.The isolates were identified as B. oryzae by morphology of hyphae and conidia and the symptomatic brown lesion spots on rice leaves caused by each isolate. There was no distinction between ‘photo‐induced’ and ‘non‐photo‐induced’ strains on rDNA base. Moreover, the existence of a new type of ‘non‐photo‐induced’ sporulator different from the one presently known was confirmed. In this newly identified sporulator, conidial development was not affected by light irradiation. Among 153 isolates, one isolate belonged to a previously known ‘non‐photo‐induced’ sporulator, five isolates belonged to a new ‘non‐photo‐induced’ sporulator ecotype, and over 95% of the isolates were “photo‐induced” sporulators. Among ‘photo‐induced’ strains, there were several subtypes having different sensitivities to near‐UV radiation required for inducing conidiophore formation.