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Fire blight is a devastating plant disease caused by the bacterium Erwinia amylovora, and its control is frequently based on the use of copper-based compounds whose mechanisms of action are not well known. Consequently, in this article, we investigate the response of E. amylovora to copper shock by a whole-genome microarray approach. Transcriptional analyses showed that, in the presence of copper, 23 genes were increased in expression; these genes were classified mainly into the transport and stress functional categories. Among them, the copA gene was strongly induced and regulated in a finely tuned manner by copper. Mutation of copA, soxS, arcB, yjcE, ygcF, yhhQ, galF and EAM_3469 genes revealed that tolerance to copper in E. amylovora can be achieved by complex physiological mechanisms, including: (i) the control of copper homeostasis through, at least, the extrusion of Cu(I) by a P-type ATPase efflux pump CopA; and (ii) the overcoming of copper toxicity caused by oxidative stress by the expression of several reactive oxygen species (ROS)-related genes, including the two major transcriptional factors SoxS and ArcB. Furthermore, complementation analyses demonstrated the important role of copA for copper tolerance in E. amylovora, not only in vitro, but also in inoculated pear shoots. [ABSTRACT FROM AUTHOR]

Bacterial biofilms are multicellular aggregates encased in an extracellular matrix mainly composed of exopolysaccharides (EPSs), protein and nucleic acids, which determines the architecture of the biofilm. Erwinia amylovora Ea1189 forms a biofilm inside the xylem of its host, which results in vessel plugging and water transport impairment. The production of the EPSs amylovoran and levan is critical for the formation of a mature biofilm. In addition, cyclic dimeric GMP (c-di-GMP) has been reported to positively regulate amylovoran biosynthesis and biofilm formation in E. amylovora Ea1189. In this study, we demonstrate that cellulose is synthesized by E. amylovora Ea1189 and is a major modulator of the three-dimensional characteristics of biofilms formed by this bacterium, and also contributes to virulence during systemic host invasion. In addition, we demonstrate that the activation of cellulose biosynthesis in E. amylovora is a c-di-GMP-dependent process, through allosteric binding to the cellulose catalytic subunit BcsA. We also report that the endoglucanase BcsZ is a key player in c-di-GMP activation of cellulose biosynthesis. Our results provide evidence of the complex composition of the extracellular matrix produced by E. amylovora and the implications of cellulose biosynthesis in shaping the architecture of the biofilm and in the expression of one of the main virulence phenotypes of this pathogen. [ABSTRACT FROM AUTHOR]

Biofilms are complex bacterial assemblages with a defined three-dimensional architecture, attached to solid surfaces, and surrounded by a self-produced matrix generally composed of exopolysaccharides, proteins, lipids and extracellular DNA. Biofilm formation has evolved as an adaptive strategy of bacteria to cope with harsh environmental conditions as well as to establish antagonistic or beneficial interactions with their host. Plant-associated bacteria attach and form biofilms on different tissues including leaves, stems, vasculature, seeds and roots. In this review, we examine the formation of biofilms from the plant-associated bacterial perspective and detail the recently-described mechanisms of genetic regulation used by these organisms to orchestrate biofilm formation on plant surfaces. In addition, we describe plant host signals that bacterial pathogens recognize to activate the transition from a planktonic lifestyle to multicellular behavior. [ABSTRACT FROM AUTHOR]

Many plant-pathogenic bacteria suppress pathogen-associated molecular pattern ( PAMP)-triggered immunity by injecting effector proteins into the host cytoplasm during infection through the type III secretion system ( TTSS). This type III secretome plays an important role in bacterial pathogenicity in susceptible hosts. Xanthomonas axonopodis pv. manihotis ( Xam), the causal agent of cassava bacterial blight, injects several effector proteins into the host cell, including TALE1 Xam. This protein is a member of the Transcriptional Activator- Like effector ( TALE) protein family, formerly known as the AvrBs3/ PthA family. TALE1 Xam has 13.5 tandem repeats of 34 amino acids each, as well as two nuclear localization signals and an acidic activation domain at the C-terminus. In this work, we demonstrate the importance of TALE1 Xam in the pathogenicity of Xam. We use versions of the gene that lack different domains in the protein in structure-function studies to show that the eukaryotic domains at the 3′ end are critical for pathogenicity. In addition, we demonstrate that, similar to the characterized TALE proteins from other Xanthomonas species, TALE1 Xam acts as a transcriptional activator in plant cells. This is the first report of the identification of a TALE in Xam, and contributes to our understanding of the pathogenicity mechanisms employed by this bacterium to colonize and cause disease in cassava. [ABSTRACT FROM AUTHOR]