Identifying an Elite Panel of Apple Rootstock Germplasm with Contrasting Root Resistance to Pythium ultimum

Abstract

Apple replants disease (ARD), incited by a soil borne pathogen complex, is a major obstacle to establishing an economically viable apple orchard at replant sites. The predominant control method is pre-plant chemical fumigation of orchard soil, which is expensive and comes with environmental and regulatory concerns. To maximize the exploitation of host resistance for ARD management, high quality resistance phenotypes in apple roots to ARD pathogen infection are required for elucidating the underlying resistance mechanisms. In this study, root resistance responses to Pythium ultimum infection were systematically evaluated among the 'Ottawa 3' × 'Robusta 5' (O3R5) F1 progeny. Tissue culture-based micro propagation was employed to generate genetically-defined and age-equivalent apple plants for repeated infection assays. A wide range of plant survival rates were observed, with fewer than 30% for the susceptible genotypes and over 80% for the resistant ones. The levels of root and shoot biomass reduction among the surviving plants varied substantially between the most resistant genotypes and the most susceptible genotypes. Contrasting necrosis patterns were demonstrated along the infected roots between resistant and susceptible genotypes using a novel glass-box pot for continuous microscopic observation. Swift necrosis occurred across the entire root system within 24 hours for the susceptible genotypes; in sharp contrast, evidently deterred root necrosis was observed for the resistant genotypes. A well-defined boundary separating healthy and necrotic root tissues were often accompanied with the infected roots of resistant genotypes, while the profuse growth of P. ultimum hyphae was specifically associated with the infected roots of susceptible genotypes. The results from this study represented the first comprehensive and detailed effort undertaken to define the genotype-specific resistance responses in apple roots as they are challenged by a soil-borne pathogen.

Understanding Root Rot Disease in Agricultural Crops

“…Similar observations were discernible from transcriptomic and microscopic data in the susceptible B.9 roots [27,366,368]. Microscopy data on the pathogen growth progress revealed a swift development of root necrosis in the most susceptible genotypes, with the entire root system becoming necrotic within a period of 24 h after initial infection [368]. The necrosis progression could be delayed for several days without the whole root tissues being engulfed for the most resistant genotypes.…”

“…This has been observed in various transcriptome analysis studies against root rots [295,296,299,353]. Similar observations were discernible from transcriptomic and microscopic data in the susceptible B.9 roots [27,366,368]. Microscopy data on the pathogen growth progress revealed a swift development of root necrosis in the most susceptible genotypes, with the entire root system becoming necrotic within a period of 24 h after initial infection [368].…”

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

“…Based on the results from a systematic phenotyping study ( Zhu et al, 2018b ), six apple rootstock genotypes, i.e., three resistant (or R group: R-#58, R-#161, and R-#164) and three susceptible genotypes (or S group: S-#106, S-#115, and S-#132), 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.…”

“…A detailed description of the resistance phenotypes for included six genotypes has been reported previously ( Zhu et al, 2018b ). Briefly, inoculated plants were allowed to grow for 4 weeks in autoclaved potting mix.…”

“…In contrast, deficient detoxification, weakened effector-triggered immunity (ETI), or existence of a susceptibility ( S ) gene may contribute to a disrupted defense activation process in the roots of the susceptible genotype. Parallel to these transcriptome analyses, a systematic phenotyping effort has recently identified a panel of apple rootstock germplasm with contrasting and reliable resistance traits against P. ultimum infection ( Shin et al, 2016a ; Zhu et al, 2018a , b ). Besides the expression patterns of the protein coding genes, elucidating post-transcriptional control of genotype-specific defense responses may facilitate the identification of key genes underpinning apple roots resistance traits.…”