Transcriptional profiles of MdWRKY33 in apple root in response to infection by Pythium ultimum, abiotic stresses and chemical treatments

Authors: Zhu, Yanmin; Saltzgiver, Melody

Submitted to: International Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2019
Publication Date: 12/19/2019
Citation: Zhu, Y., Saltzgiver, M.J. 2019. Transcriptional profiles of MdWRKY33 in apple root in response to infection by Pythium ultimum, abiotic stresses and chemical treatments. International Journal of Plant Pathology. 8(3):87-100. https://doi.org/10.33687/phytopath.008.03.2996.
DOI: https://doi.org/10.33687/phytopath.008.03.2996

Interpretive Summary: As plants combat pathogen infection, massive transcriptional reprogramming occurs to redirect cellular activities and resources from normal growth to defense responses. Transcriptional regulation of gene expression is one of the foremost adjustments to initiate defense activation to pathogen aggression. Transcription factors (TFs) are among the most prominent components in gene expression network to regulate hundreds of target genes. Based on our previous high-throughput transcriptome analyses, Member of a TF family named WRKYs (so called because of the presence of the conserved motif of W-R-K-Y amino acid residues) were shown to be associated with the process of defense activation and resistance responses in the root tissues of apple rootstock genotypes. One of the highly expressed members, MdWRKY33, appeared to be an orthologue of AtWRKY33, a well-researched counterpart in Arabidopsis that is crucial for resistance to the foliar fungal pathogen Botrytis cineara. In this study, tissue-specific expression patterns of MdWRKY33 indicated it primarily expressed in the vegetative tissue of root and leaf. Its responsiveness to abiotic stresses and pharmacological treatments added evidences for its functional identity as a stresses related TF. The consistent upregulation of MdWRKY33 in eight apple rootstock genotypes in response to P. ultimum infection validated its essential roles in defense activation in apple roots. While it is a foregone conclusion due to the polygenetic nature of plant resistance to necrotrophic pathogens that many genetic factors are likely to play a fundamental role in resistance traits, the specifically earlier and stronger induction patterns of MdWRKY33 in the roots of resistant genotypes indicated its status as a key contributing factor to apple root resistance to P. ultimum infection. Transgenic manipulation of this gene is under way to provide more definitive evidences related to the functions of MdWRKY33 in resistance to P. ultimum infection in apple roots.

Technical Abstract: Plant resistance responses to pathogen infection involve massive transcriptional reprograming and widespread redirection of cellular pathways to activate defense responses. Transcription factors (TFs) function at the major regulating points of gene expression, and specific TFs are known to play central roles in plant defense activation. Molecular defense activation in apple root from infection by oomycete necrotrophic pathogen Pythium ultimum, a primary component in a pathogen complex inciting apple replant disease, has not been investigated until recently. Based on our recent transcriptome analyses, members of apple WRKY gene family have been identified as primary candidates in regulating defense response in apple root. Among them, MdWRKY33, an orthologue of AtWRKY33 in apple genome, demonstrated high-level expression and genotype-specific induction patterns during P. ultimum infection. The sequence features of MdWRKY33 and its tissue-specific expression, as well as its responses to abiotic and pharmacological treatments added to the evidences for its functional roles in stress response and pathogen resistance in apple root. In response to P. ultimum infection, MdWRKY33 was consistently upregulated in all tested eight apple rootstock genotypes at all timepoints. Between genotypes, the earlier and stronger induction patterns in resistant genotypes suggested its crucial roles of contributing to apple root resistance, although other genetic factors must be needed given the fact of polygenetic nature of quantitative plant resistance to necrotrophic pathogens. Transgenic manipulation of MdWRKY33 is under way to provide more definitive evidences for its functional identity in contributing to apple root resistance to P. ultimum infection.