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Abstract
Plant resistance responses to pathogen infection involve massive transcriptional reprograming and widespread redirection of cellular pathways to adjust the plant from growth to defense. Transcription factors (TFs) function at the major regulating points of gene expression, and specific TFs are known to play crucial 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 in detail. Base on previous transcriptome analyses, members of apple WRKY gene family have been identified as the primary candidates in regulating defense response in apple root. Among them, MdWRKY33, an orthologue of AtWRKY33 in apple genome, demonstrated as a highly-expressed WRKY with 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 evidence for its functional roles in defense activation in apple root. In response to P. ultimum infection, MdWRKY33 was consistently upregulated in all eight tested apple rootstock genotypes at all timepoints. Between genotypes, the stronger induction patterns at the earlier stage of infection in resistant genotypes suggest its essential roles of contributing to apple root resistance, although plant resistance to necrotrophic pathogens is polygenetic quantitative resistance in nature. Transgenic manipulation of this gene is underway to provide more definitive functional identity in contributing to apple root resistance to P. ultimum infection
Highlights
Coordinated regulation of gene expression is essential for plant growth, development, and response to environmental factors
Identification of MdWRKY33 during P. ultimum infection of apple root by transcriptome analyses: Our recent transcriptome analyses using the large-scale and high-throughput RNA-seq platform have identified many specific transcriptome changes associated with apple root defense activation during P. ultimum infection, and the global features of transcriptome changes have been reported previously (Shin, Zheng, et al, 2016; Zhu et al, 2017; Zhu et al, 2019)
Transcription factors (TF), through sequence-specific DNA-binding to the promoter of target genes, control one of the primary regulation points on the level and/or duration of transcriptional expression of various functional categories of genes
Summary
Coordinated regulation of gene expression is essential for plant growth, development, and response to environmental factors. Members from several plant TF families are known to directly participate in transcriptional regulation of plant defense responses under pathogenic pressure (Buscaill and Rivas, 2014; Birkenbihl et al, 2017). WRKY TFs bind to the specific cis-element of W-box, or (T)(T)TGAC(C/T) sequence, in the promoter of target genes (Eulgem et al, 2000). Roles of WRKYs in apple root and soilborne necrotrophic pathogen Pythium ultimum interaction has not been investigated. Comparative transcriptome analyses identified four major TF families, WRKY, MYB, BHLH and AP2/ETF among many other factors, which are believed to actively participate in the defense activation during interaction between apple root and P. ultimum (Zhu et al, 2019; Shin, Zheng, et al, 2016). The expression profiles MdWRKY33, a highly-expressed WRKY member in apple root, were studied in detail
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