Laccase Directed Lignification Is One of the Major Processes Associated With the Defense Response Against Pythium ultimum Infection in Apple Roots.

Yanmin Zhu,Gennaro Fazio,Awais Khan,Jugpreet Singh,Melody Saltzgiver,Rui Xia,Guanliang Li

doi:10.3389/fpls.2021.629776

Abstract

Apple replant disease (ARD), incited by a pathogen complex including Pythium ultimum, causes stunted growth or death of newly planted trees at replant sites. Development and deployment of resistant or tolerant rootstocks offers a cost-effective, ecologically friendly, and durable approach for ARD management. Maximized exploitation of natural resistance requires integrated efforts to identify key regulatory mechanisms underlying resistance traits in apple. In this study, miRNA profiling and degradome sequencing identified major miRNA pathways and candidate genes using six apple rootstock genotypes with contrasting phenotypes to P. ultimum infection. The comprehensive RNA-seq dataset offered an expansive view of post-transcriptional regulation of apple root defense activation in response to infection from P. ultimum. Several pairs of miRNA families and their corresponding targets were identified for their roles in defense response in apple roots, including miR397-laccase, miR398-superoxide dismutase, miR10986-polyphenol oxidase, miR482-resistance genes, and miR160-auxin response factor. Of these families, the genotype-specific expression patterns of miR397 indicated its fundamental role in developing defense response patterns to P. ultimum infection. Combined with other identified copper proteins, the importance of cellular fortification, such as lignification of root tissues by the action of laccase, may critically contribute to genotype-specific resistance traits. Our findings suggest that quick and enhanced lignification of apple roots may significantly impede pathogen penetration and minimize the disruption of effective defense activation in roots of resistant genotypes. The identified target miRNA species and target genes consist of a valuable resource for subsequent functional analysis of their roles during interaction between apple roots and P. ultimum.

Highlights

Apple replant disease (ARD) is defined by the stunted growth or possible death of newly planted trees in replant sites, where continuous cultivation of apples or closely related species leads to pathogen inoculum accumulation
Based on the results from a systematic phenotyping study (Zhu et al, 2018b), six apple rootstock genotypes, i.e., three resistant and three susceptible genotypes, were identified and included in this study. These genotypes originated from an “Ottawa 3” × “Robusta 5” (O3R5) apple rootstock F1 population developed in the mid-1970s
Six apple rootstock genotypes included in this study were previously identified to have contrasting resistance traits based on repeated root infection assays with P. ultimum, as shown in Figure 1A (Zhu et al, 2018a,b)

Summary

Introduction

Apple replant disease (ARD) is defined by the stunted growth or possible death of newly planted trees in replant sites, where continuous cultivation of apples or closely related species leads to pathogen inoculum accumulation. ARD form a soilborne pathogen complex that includes necrotrophic soilborne oomycetes (Phytophthora and Pythium) and fungi (Ilyonectria and Rhizoctonia) (Jaffee et al, 1982; Mazzola, 1998), has been a serious threat for the establishment of economically viable orchards. Pythium ultimum is a major component of this pathogen complex in orchard soils worldwide (Mazzola, 1997; Tewoldemedhin et al, 2011). Management of ARD depends almost exclusively on pre-plant fumigation of orchard soils to eradicate ARD pathogens, which inadvertently eliminates other beneficial soil microbiota. Use of these chemical fumigants is under increasing regulatory restriction due to environmental and human health concerns. Maximized exploitation of host resistance through investigation of the molecular regulation controlling defense responses of apple roots, and thereafter development and deployment of resistant apple rootstocks can offer a cost-effective, environment friendly and durable strategy for ARD management (Zhu et al, 2014; Zhu and Saltzgiver, 2020)

Objectives

Methods

Results

Conclusion