The genotype-specific laccase gene expression and lignin deposition patterns in apple root during Pythium ultimum infection

Plant resistance responses against invading pathogens require a coordinated set of cellular processes to optimize the effective defense output. Previous transcriptome analyses have identified a multi-phase and multi-layered defense strategy in apple root towards infection from a necrotrophic oomycete pathogen Pythium ultimum. Among the identified apple genes, members of the laccase gene family represent an actively regulated group at both transcriptional and posttranscriptional levels. In this study, several apple laccase genes were selected for further analyses on their sequence features and potential roles during defense activation in apple roots. Their bioinformatic specifics, genotype-specific induction, and lignin deposition patterns during pathogen infection were examined between two apple rootstock genotypes, a resistant O3R5-#161 and a susceptible O3R5-#132. The sequences these laccase genes contain the conserved cu-oxidase domains and the characteristic gene structures with MdLAC7a as an exception. While MdLAC3 and MdLAC5 showed a partial induction to P. ultimum infection, both MdLAC7a and MdLAC7b genes demonstrated consistent and high-level inducibility. Moreover, MdLAC7b exhibited a differential expression pattern, with a higher expression in the resistant O3R5-#161. Lignin deposition appeared to be stronger in the infected root of the resistant genotype compared to that of the susceptible one. The efficient lignin biosynthesis and deposition at the initial stage of infection is crucial for impeding the progression of this fast-growing necrotrophic pathogen. Future study regarding the role of MdLAC7b, including the transgenic manipulation and biochemical analysis, should provide more definitive evidence for its contribution to resistance to P. ultimum infection.