Global Regulator PhoP is Necessary for Motility, Biofilm Formation, Exoenzyme Production and Virulence of Xanthomonas citri Subsp. citri on Citrus Plants.

Chudan Wei,Qiongguang Liu,Changqing Chang,Chengpeng Yu,Xingwei Li,Tian Ding

doi:10.3390/genes10050340

Abstract

Citrus canker caused by Xanthomonas citri subsp. citri is one of the most important bacterial diseases of citrus, impacting both plant growth and fruit quality. Identifying and elucidating the roles of genes associated with pathogenesis has aided our understanding of the molecular basis of citrus-bacteria interactions. However, the complex virulence mechanisms of X. citri subsp. citri are still not well understood. In this study, we characterized the role of PhoP in X. citri subsp. citri using a phoP deletion mutant, ΔphoP. Compared with wild-type strain XHG3, ΔphoP showed reduced motility, biofilm formation, as well as decreased production of cellulase, amylase, and polygalacturonase. In addition, the virulence of ΔphoP on citrus leaves was significantly decreased. To further understand the virulence mechanisms of X. citri subsp. citri, high-throughput RNA sequencing technology (RNA-Seq) was used to compare the transcriptomes of the wild-type and mutant strains. Analysis revealed 1017 differentially-expressed genes (DEGs), of which 614 were up-regulated and 403 were down-regulated in ΔphoP. Gene ontology functional enrichment and Kyoto Encyclopedia of Genes and Genomes pathway analyses suggested that the DEGs were enriched in flagellar assembly, two-component systems, histidine metabolism, bacterial chemotaxis, ABC transporters, and bacterial secretion systems. Our results showed that PhoP activates the expression of a large set of virulence genes, including 22 type III secretion system genes and 15 type III secretion system effector genes, as well as several genes involved in chemotaxis, and flagellar and histidine biosynthesis. Two-step reverse-transcription polymerase chain reaction analysis targeting 17 genes was used to validate the RNA-seq data, and confirmed that the expression of all 17 genes, except for that of virB1, decreased significantly. Our results suggest that PhoP interacts with a global signaling network to co-ordinate the expression of multiple virulence factors involved in modification and adaption to the host environment during infection.

Highlights

Citrus crop canker, caused by Xanthomonas citri subsp. citri and generally associated with a characteristic embossment of necrotic lesions on infected leaves, stems, and fruit [1,2], is consideredGenes 2019, 10, 340; doi:10.3390/genes10050340 www.mdpi.com/journal/genesGenes 2019, 10, 340 one of the most serious crop diseases worldwide [3]
To determine whether mutation of phoP affects the growth and proliferation of X. citri subsp. citri, the growth of the wild-type, mutant, and complementation strains was compared in YEB medium
Little is known about the PhoQ/PhoP proteins in the important plant pathogen X. citri subsp. citri

Summary

Introduction

Citrus crop canker, caused by Xanthomonas citri subsp. citri and generally associated with a characteristic embossment of necrotic lesions on infected leaves, stems, and fruit [1,2], is consideredGenes 2019, 10, 340; doi:10.3390/genes10050340 www.mdpi.com/journal/genesGenes 2019, 10, 340 one of the most serious crop diseases worldwide [3]. Citrus crop canker, caused by Xanthomonas citri subsp. Citri is an important model Xanthomonas pathogen and is used in studies investigating plant-microbe interactions and virulence mechanisms. Previous studies have characterized the major pathogenicity and virulence genes responsible for secretion systems such as the T3SS components and effector molecules, as well as bacterial adhesins, extracellular enzymes, toxins, surface structural elements, and rpf (regulation of pathogenicity factors)-encoded cell-cell signaling proteins [7,8,9,10,11,12]. PthA, an effector of the T3SS, is a critical factor for citrus canker symptoms [13], while GalU, a UTP-glucose-1-phosphate uridylyltransferase, contributes to the growth of X. citri subsp. Genome sequencing of X. citri subsp. citri has greatly increased our understanding of X. citri subsp. citri-citrus plant interaction [7,15]

Methods

Results

Conclusion