Soil bacteria as sources of virulence signal providers promoting plant infection by Phytophthora pathogens.

Ping Kong,Chuanxue Hong

doi:10.1038/srep33239

Abstract

Phytophthora species are known as “plant destroyers” capable of initiating single zoospore infection in the presence of a quorum of chemical signals from the same or closely related species of oomycetes. Since the natural oomycete population is too low to reach a quorum necessary to initiate a disease epidemic, creation of the quorum is reliant on alternate sources. Here, we show that a soil bacterial isolate, Bacillus megaterium Sb5, promotes plant infection by Phytophthora species. In the presence of Sb5 exudates, colonization of rhododendron leaf discs by 12 Phytophthora species/isolates was significantly enhanced, single zoospores of P. nicotianae infected annual vinca and P. sojae race 25 successfully attacked a non-host plant, Nicotiana benthamiana as well as resistant soybean cultivars with RPS1a or RPS3a. Sb5 exudates, most notably the fractions larger than 3 kDa, promoted plant infection by improving zoospore swimming, germination and plant attachment. Sb5 exudates also stimulated infection hypha growth and upregulated effector gene expression. These results suggest that environmental bacteria are important sources of virulence signal providers that promote plant infection by Phytophthora species, advancing our understanding of biotic factors in the environmental component of the Phytophthora disease triangle and of communal infection of plant pathogens.

Highlights

That release metabolites into the environment[25,26]
We focus on a soil isolate of Bacillus megaterium, Sb5, and demonstrate that environmental bacteria are sources of signals for Phytophthora virulence as well as fitness, and communal infection is a life strategy of these pathogens
In an initial assay with P. nicotianae isolate 1B11, soil water extract (SWE) and its five major bacterial component species (Sb1, 3 to 6) were compared with sterile distilled water (SDW) and surface rinsate of potato dextrose (PDA) agar medium used to grow bacteria (SDWms) for their impact on pathogen colonization. This was evaluated by submerging leaf discs of Rhododendron, a universal suscept, overnight in zoospore suspensions prepared with autoclaved bacterial cell suspensions of individual isolates in PDA plates, autoclaved SWE (ASWE), SDW or SDWms

Summary

Introduction

That release metabolites into the environment[25,26]. Some of metabolites play important roles in the life cycle of Phytophthora species. They enhance the growth and reproduction of Phytophthora species[27,28,29,30] They may act as plant hormone mimics stimulating the opening of plant stomata to allow pathogens to enter the apoplast and impair defense gene expression[31,32,33,34]. Plant colonization by Phytophthora species is associated with abundant bacterial populations in the rhizosphere[36] Such beneficial impacts of environmental microorganisms on plant pathogens are largely overlooked in current research[24]. Infection of immunocompromised patients by Cryptococcus neoformans is attributed to virulence precursors provided by Klebsiella aerogenes bacterial microflora[43] These studies have provided unparalleled insights into pathogen ecology, pathogenicity, and disease epidemiology, facilitating our understanding of how plant infections occur under natural conditions and ways to counteract. We focus on a soil isolate of Bacillus megaterium, Sb5, and demonstrate that environmental bacteria are sources of signals for Phytophthora virulence as well as fitness, and communal infection is a life strategy of these pathogens

Methods

Results

Conclusion