Functional Analysis of a Novel ABL (Abnormal Browning Related to Light) Gene in Mycelial Brown Film Formation of Lentinula edodes.

Lentinula edodes is a globally important edible mushroom species that is appreciated for its medicinal properties as well as its nutritional value. During commercial cultivation, a mycelial brown film forms on the surface of the sawdust growth medium at the late vegetative stage. Mycelial film formation is a critical developmental process that contributes to the quantity and quality of the mushroom yield. However, little is known regarding the genetic underpinnings of brown film formation on the surface of mycelial tissue. A novel causal gene associated with the formation of the mycelial brown film, named ABL (Abnormal browning related to light), was identified in this study. The comparative genetic analysis by dihybrid crosses between normal and abnormal browning film cultivars demonstrated that a single dominant allele was responsible for the abnormal mycelium browning phenotype. Whole-genome sequencing analysis of hybrid isolates revealed five missense single-nucleotide polymorphisms (SNPs) in the ABL locus of individuals forming abnormal partial brown films. Additional whole-genome resequencing of a further 16 cultivars showed that three of the five missense SNPs were strongly associated with the abnormal browning phenotype. Overexpression of the dominant abl-D allele in a wild-type background conferred the abnormal mycelial browning phenotype upon transformants, with slender hyphae observed as a general defective mycelial growth phenotype. Our methodology will aid the future discovery of candidate genes associated with favorable traits in edible mushrooms. The discovery of a novel gene, ABL, associated with mycelial film formation will facilitate marker-associated breeding in L. edodes.