Abstract
Phytophthora sojae is an oomycete pathogen that causes the disease known as root and stem rot in soybean plants, frequently leading to massive economic damage. Additionally, P. sojae is increasingly being utilized as a model for phytopathogenic oomycete research. Despite the economic and scientific importance of P. sojae, the mechanism by which it penetrates the host roots is not yet fully understood. It has been found that oomycetes are not capable of penetrating the cell wall solely through mechanical force, suggesting that alternative factors facilitate breakdown of the host cell wall. Pectin methylesterases have been suggested to be important for Phytophthora pathogenicity, but no data exist on their role in the P. sojae infection process. We have scanned the newly revised version of the annotated P. sojae genome for the presence of putative pectin methylesterases genes and conducted a sequence analysis of all gene models found. We also searched for potential regulatory motifs in the promoter region of the proposed P. sojae models, and investigated the gene expression levels throughout the early course of infection on soybean plants. We found that P. sojae contains a large repertoire of pectin methylesterase-coding genes and that most of these genes display similar motifs in the promoter region, indicating the possibility of a shared regulatory mechanism. Phylogenetic analyses confirmed the evolutionary relatedness of the pectin methylesterase-coding genes within and across Phytophthora spp. In addition, the gene duplication events that led to the emergence of this gene family appear to have occurred prior to many speciation events in the genus Phytophthora. Our results also indicate that the highest levels of expression occurred in the first 24 hours post inoculation, with expression falling after this time. Our study provides evidence that pectin methylesterases may be important for the early action of the P. sojae infection process.
Highlights
The common soybean, Glycine max, is a widespread food crop both in the United States and around the world
Using reference genes from Aspergillus nidulans, Fusarium fujikuroi, and Arabidopsis thaliana obtained from the CAZyme database, 16 candidate genes were found in the P. sojae genome, Table 1
Gene models that meet minimum criteria for functionality, in particular the presence of a PFAM domain for pectin methylesterase, has been reduced to 16, which still is a very large number for a gene family, and equals the size of Carbohydrate Esterases (CE) family 5 reported elsewhere [27]. These 16 gene models are distributed in five scaffolds, with scaffolds 1 and 8 containing the majority of pectin methylesterases (PME) gene models found in P. sojae
Summary
The common soybean, Glycine max, is a widespread food crop both in the United States and around the world. Much work has centered on genes encoding effector proteins, which tend to modulate the plant defense response Such proteins are characterized by the presence of an amino acid motif with the sequence RxLR, which was first discovered in the genomes of P. sojae and P. ramorum and has since been suggested to be essential to the entry of the pathogen into the cell [4], this remains controversial [5,6,7,8]. Because of its potential role in oomycete pathogenicity, a group of genes encoding enzymes involved in carbohydrate metabolism has been the target of multiple of investigations [9] These carbohydrate-active enzymes ( known as CAZymes) are classified into superfamilies (each composed of multiple families) referred to as Glycoside Hydrolases (GH), Glycosyl Transferases (GT), Polysaccharide Lyases (PL), or Carbohydrate Esterases (CE) [10]. We analyzed the PME gene expression levels in P. sojae throughout the early course of infection of susceptible soybean plants
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