Response of potato to coronofacic acid a virulence factor in Pectobacterium

Abstract

Pectobacterium atrosepticum (Pba) and Pectobacterium carotovorum subsp. brasiliensis (Pbr) are necrotrophic bacterial plant pathogens that cause blackleg disease on potato stems during the growing season and soft rot of tubers post-harvest. Coronafacic acid (CFA), encoded by the cfa gene cluster, is one among many pathogenicity determinants that have been identified in Pba SCRI1043 and Pbr NZEC1. CFA is a component of coronatine (COR), which in the hemibiotrophic pathogen Pseudomonas syringae functions as a molecular mimic of Jasmonic Acid (JA) during pathogenicity on host plants to suppress the Salicylic Acid (SA) signalling pathway. The SA pathway is essential for defence against P. syringae, and other hemibiotrophic and biotrophic pathogens.

Studies on Arabidopsis thaliana and tomato have identified that genes related to JA signalling and wound response are differentially transcribed upon infection with P. syringae carrying the cfa gene cluster. Studies to date on Pba have confirmed the influence of CFA in pathogenicity on potato plants, but it’s mode of action in Pectobacteirum spp. still remains unclear.

In this study, a model plant-pathogen system was developed for studying the interaction of Pectobacterium spp. with potato. Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) was then used to investigate the expression of target genes upon infection with Pbr NZEC1 and a CFA knockout mutant, identified from the previous studies in tomato and A. thaliana using COR. As an alternative approach, Illumina-based RNA sequencing (RNA seq) was used to investigate the global transcription of potato in response to this pathogen to identify novel genes differentially expressed in this pathosystem.

Although qRT-PCR showed no significant differential expression of candidate genes, RNA seq identified the differential expression of a multitude of genes upon infection with Pbr NZEC1 and the CFA knockout mutant. Of particular note were the large proportion of differentially expressed genes related to ethylene (ET) biosynthesis and the JA pathway. These pathways are essential for production of defensin, which is central to plant defence against necrotrophic pathogens such as Pectobacterium. The research also provided new information on the isoforms of these defence related genes that are active in tubers providing novel insights into how potato may respond to tuber infection in general.