Identifying an elite panel of apple rootstock germplasm with contrasting root resistance to Pythium ultimum

Authors: Zhu, Yanmin; ZHAO, JINGXIAN - Hebei Academy Of Agriculture & Forestry; ZHOU, ZHE - Chinese Academy Of Agricultural Sciences

Submitted to: Journal of Plant Pathology & Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2018
Publication Date: 11/23/2018
Citation: Zhu, Y., Zhao, J., Zhou, Z. 2018. Identifying an elite panel of apple rootstock germplasm with contrasting root resistance to Pythium ultimum. Journal of Plant Pathology & Microbiology. 9(11):1000461. https://doi.org/10.4172/2157-7471.1000461.
DOI: https://doi.org/10.4172/2157-7471.1000461

Interpretive Summary: Control of soilborne disease, such as apple replant disease (ARD), is difficult. The primary control method for ARD is soil chemical fumigation broad-spectrum biocides, which is under increasing regulatory restriction due to its environmental and health concerns. Conversely, it is well-acknowledged that development and deployment of resistant or tolerant rootstocks may offer a cost-effective, ecologically friendly, and durable approach for ARD management. To maximize the exploitation of the innate resistance, identifying reliable resistance phenotypes in apple root is the prerequisite for careful and meaningful genetic study. Devised molecular tools, based on the elucidated molecular resistance mechanism, for genetic-informed breeding promises the accurate and efficient incorporation of resistant traits into new apple rootstock varieties. In the current study, root resistance responses to Pythium ultimum infection were systematically evaluated among more than sixty F1 progeny of the ‘Ottawa 3’ x ‘Robusta 5’ (O3R5) cross population. Tissue culture-based micropropagation was employed to generate genetically-defined and age-equivalent apple plants for repeated infection assays. A wide range of plant survival rates was observed, with under 30% for the susceptible genotypes to over 80% for the resistant ones. A panel of genotypes was identified which showed the contrasting resistance traits at biological and microscopic levels. The degree of root and shoot biomass reduction among the surviving plants varied substantially between the more resistant genotypes and the more susceptible genotypes. Contrasting patterns of necrosis progression between resistant and susceptible genotypes were documented for the first time using an innovative glass-box pot for continuous microscopic observation. Swift necrosis occurred across entire root system within 24 hours for the susceptible genotypes was in sharp contrast with the evidently deterred root necrosis for the resistant genotypes. Resistant genotypes often demonstrated well-defined boundaries separating healthy and necrotic root tissues along the infected root, while the profuse growth of P. ultimum hyphae was specifically associated with the infected roots of susceptible genotypes. The identification of this panel of apple rootstock genotypes with contrasting and reproducible resistance phenotypes is pivotal for the subsequent genetic studies to unravel the underlying controls over the observed resistance traits.

Technical Abstract: Apple replant disease (ARD), incited by a soilborne 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 in ARD management, high quality resistance phenotypes in apple roots to ARD pathogen infection are critical for elucidating the underlying resistance mechanisms. In this study, root resistance responses to Pythium ultimum infection were systematically evaluated among the ‘Ottawa 3’ x ‘Robusta 5’ (O3R5) F1 progeny. Tissue culture-based micropropagation was employed to generate genetically-defined and age-equivalent apple plants for repeated infection assays. A wide range of plant survival rates was observed, with under 30% for the susceptible genotypes and over 80% for the resistant ones. The level 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 along the infected roots were demonstrated 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 with the evidently deterred root necrosis 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 along, while the profuse growth of P. ultimum hyphae was specifically associated with infected roots of the susceptible genotypes.