Challenges and progress in evaluating apple root resistance responses to Pythium ultimum infection

Authors: Zhu, Yanmin; ZHOU, ZHE - Institute Of Animal Science (CAAS)

Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/2023
Publication Date: 12/21/2023
Citation: Zhu, Y., Zhou, Z. 2023. Challenges and progress in evaluating apple root resistance responses to Pythium ultimum infection. American Journal of Plant Sciences. 14:1410-1429. https://doi.org/10.4236/ajps.2024.153016.
DOI: https://doi.org/10.4236/ajps.2024.153016

Interpretive Summary: Because of the hidden nature of roots in soils, it is obviously more challenging to study their resistance phenotypes and defense responses as compared to those of the aerial organs. At the same time, it is self-evident that root health is fundamental to a plant’s entire life and productivity. It is also well acknowledged that root function, physiology, morphology, and architecture are constantly impacted by the complex soil environment including both biotic and abiotic factors. This report summarizes and updates the challenges and progresses in evaluating resistance responses of apple root to infection from a necrotrophic oomycete pathogen Pythium ultimum. Several obstacles impede the progress of investigating apple root resistance traits including the difficulties in direct and real-time evaluation and the lack of continuous supply of apple plants for repeated infection assays. Therefore, the systematic and detailed analyses were made possible only by implementing micropropagation procedure for continuingly generating uniform apple plants. The progress is also attributed to formulating a standardized inoculation method and applying several innovative methodologies to discern the subtle variations of responses between genotypes. The comprehensive datasets encompass several aspects of resistance responses of apple root to P. ultimum infection: from genotype-specific survival rates at population and whole plant level to anatomic or subcellular changes under pathogenic pressure. Aa a result, we identified an elite panel of apple rootstock germplasm with distinct resistance levels. These apple rootstock genotypes with well-defined resistance levels are the much-needed plant materials for subsequent genomics and transgenics analyses to define the functional roles of selected candidate genes. Between resistant and susceptible genotypes, the contrasting necrosis progression patterns were uncovered for the first time through careful microscopic examination which reveal the potential mechanisms underlying resistance traits. The current investigation at subcellular variations and biochemical responses such as lignin deposition patterns is aiming at establishing the direct connection between candidate genes and pathways with the observed resistance traits. Our continuing research will provide a clearer view regarding the genetic elements regulating resistance traits in apple root to P. ultimum infection.

Technical Abstract: Due to the hidden nature of roots in the soils, it is more challenging to study their resistance traits and defense responses as compared to those of the aerial organs. At the same time, it is self-evident that root health is fundamental to a plant’s entire life and productivity. It is also easily conceivable that root function, physiology, morphology, and architecture are constantly impacted by the complex soil environment including both biotic and abiotic factors. This report summarizes and updates the challenges and progresses in evaluating resistance responses of apple root to infection from a necrotrophic oomycete pathogen Pythium ultimum. Several obstacles impede the progress of investigating apple root resistance traits including the difficulties of direct and real-time evaluation and the lack of continuous supply of apple plants for repeated infection assays. The systematic and detailed analyses were made possible by implementing micropropagation procedure for continuingly generating uniform apple plants for the tested genotypes. The progress is also attributed to formulating a standardized inoculation method and applying several innovative methodologies to discern the subtle variations of responses between genotypes. The comprehensive datasets encompass several aspects of resistance responses of apple root to P. ultimum infection: from genotype-specific survival rates at population and whole plant level to anatomic or subcellular responses. Aa a result, an elite panel of apple rootstock germplasm with distinct resistance levels were identified. These apple rootstock genotypes with well-defined resistance levels are the much-needed plant materials for subsequent genomics and transgenics analyses to define the functional roles of specific candidate genes. Careful microscopic examination revealed the contrasting necrosis progression patterns between resistant and susceptible genotypes, which shed light on the potential mechanisms underlying resistance traits. The on-going investigation at subcellular and biochemical responses such as lignin deposition patterns may allow to connect specific candidate genes and pathways with observed resistance traits. Our continuing research will provide a clearer view regarding the genetic elements regulating resistance traits in apple root to P. ultimum infection.