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

Authors: Zhu, Yanmin; ZHOU, ZHE - Chinese Academy Of Agricultural Sciences

Submitted to: Fruit Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/2021
Publication Date: 11/23/2021
Citation: Zhu, Y., Zhou, Z. 2021. The genotype-specific laccase gene expression and lignin deposition patterns in apple root during Pythium ultimum infection. Fruit Research. 1. Article 12. https://doi.org/10.48130/FruRes-2021-0012.
DOI: https://doi.org/10.48130/FruRes-2021-0012

Interpretive Summary: Plant roots, as an underground organ, are constantly under the pathogenic pressure. Apple replant disease (ARD) is incited by a soilborne pathogen complex, and Pythium ultimum is one of the primary components. Understanding the molecular mechanisms underpinning the resistance versus susceptibility is essential for utilizing the innate resistance to manage ARD efficiently and sustainably. As a necrotrophic pathogen, P. ultimum is known to have multiple pathogenic factors which can disrupt cellular functions of apple roots. On the host plant side, maximized defense output is required to thwart the attach from necrotrophic pathogen like P. ultimum through coordinated cellular activities. It has long been proposed that the efficient lignin biosynthesis and deposition, as a physical barrier at the initial stage of infection, empower a full development of a more robust cellular defense activation process. Our recent transcriptome datasets over the defense activation towards P. ultimum infection to apple root have identified a multi-phased and multi-layered defense network including several laccase encoding genes. In current study four apple laccase genes (or MdLACs) were characterized for their roles during defense activation in apple root. Their bioinformatic features, genotype-specific expression and lignin deposition pattern during pathogen infection were investigated between two apple rootstock genotypes, i.e., a resistant O3R5-#161 and a susceptible O3R5-#132. The bioinformatics analyses confirmed the authentic identity as plant laccase encoding genes in apple. Except MdLAC7a, other three laccase genes contain the conserved cu-oxidase domains and characteristic gene structures. The gene expression data showed that both MdLAC3 and MdLAC5 had a limited induction to P. ultimum infection. However, MdLAC7a and MdLAC7b demonstrated a high-level inducibility. In particular, MdLAC7b exhibited elevated upregulation specifically in the roots of resistant O3R5-#161. The higher inducibility of MdLAC7b in the roots of O3R5-#161 is corresponded to an enhanced lignin deposition upon P. ultimum infection. Further study including the transgenic manipulation of its expression and biochemical analysis of altered phenotypes should provide more definitive evidence on its potential contribution to the resistance trait of apple roots to P. ultimum infection.

Technical Abstract: Plant resistance responses against invading pathogens require the coordinated 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 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 potential roles during defense activation in apple root. Their bioinformatic features, 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 of 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 the 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 comparing 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.