Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2016
Publication Date: 12/9/2016
Citation: Zhu, Y., Shin, S., Mazzola, M. 2016. Genotype-specific responses of apple roots to pathogenic infection by Pythium ultimum. Canadian Journal of Plant Pathology. 38(4):483-491.
Interpretive Summary: Apple replant disease (ARD) occurs when young trees are planted on a site that has a history of previous cultivation of apple or closely related species. In the absence of control, the effects of ARD can be experienced over the entire lifetime of the orchard in the form of decreased fruit yields. Control of ARD has traditionally relied upon pre-plant chemical fumigation of orchard soils, which is either with short-lived effect or ineffective and with increased environmental concern. Development and deployment of resistant rootstock cultivars could offer an important component of a cost-effective, ecologically friendly and durable ARD management strategy. However, resistance mechanisms employed by apple root to defend against soilborne necrotrophic pathogens are poorly understood; and currently there is no available protocol to phenotype the resistance response of apple rootstock to the components of ARD pathogen complex. Previous field tests indicated that apple rootstock cultivar G.935 is more tolerant to ARD compared to B.9, but the underlying cellular and molecular mechanisms are unknown. In this study, the resistance responses of G.935 and B.9 to P. ultimum infection were compared under controlled environment conditions using tissue culture derived plant materials of equivalent age. It was found that plant survival rate, shoot and root biomass and patterns of root tissue necrosis differed considerably. Therefore, an effective functional resistance mechanism seems to exist in the root of tolerant G.935, but not in that of B.9. Differential expression patterns of select defense response genes, which were identified through previous transcriptome profiling of the apple root/P. ultimum interaction added molecular justification for pursuing study to understand the underlying mechanisms of the contrasting rootstock reactions to P. ultimum infection. The observed phenotypes between the resistance responses of G.935 vs. susceptibility from G.9 root in this study are consistent with tree growth performance observed in field trials. The current study resulted in preparation of plant materials for future studies employing comparative transcriptomic and metabolomic analyses to uncover the functional mechanisms underlying the observed differential resistance responses between these rootstocks.
Technical Abstract: Resistance mechanisms employed to defend against soilborne necrotrophic pathogens are poorly understood, particularly with respect to perennial tree fruit crops such as apple. Pythium ultimum is a component of the pathogen complex that incites apple replant disease (ARD). Different levels of tolerance to ARD among apple rootstocks had been demonstrated by field evaluation, but the existence of functional resistance and underlying cellular and molecular mechanisms are unknown. In this study, a tissue culture based micro-propagation procedure was implemented to generate plants of uniform genetic background and equivalent developmental stage. Genotype-specific resistance responses in apple root to P. ultimum infection were investigated at the biological, microscopic and molecular level using tolerant (G.935) and susceptible (B.9) rootstocks. Distinct responses in terms of survival rate, root biomass, shoot biomass and maximum root length were observed between G.935 and B.9. Contrasting dynamics in symptom development were detected through microscopic observations. B.9 demonstrated an inability to limit pathogen progression, resulting in rapid development of discoloration and necrosis across the entire root system; while effective constraint of pathogen progression was shown in roots of G.935, as indicated by limited or localized root necrosis. Differential expression profiles for candidate genes with annotated functions in plant defense responses suggested that discrete molecular and biochemical processes were functional in the roots of these two cultivars. The differential behavior of these two rootstocks in response to P. ultimum infection deserves a more thorough study via genomic approaches.