Bacterial elicitation and evasion of plant innate immunity

Abstract

Recent research on plant responses to bacterial attack has identified extracellular and intracellular host receptors that recognize conserved pathogen-associated molecular patterns and more specialized virulence proteins, respectively. These findings have shed light on our understanding of the molecular mechanisms by which bacteria elicit host defences and how pathogens have evolved to evade or suppress these defences.Plants are a rich source of nutrients and water for microbes, and they are infected by many bacterial pathogens from both the Proteobacteria and Actinobacteria phyla (Supplementary information 1 (table)). Because of their broad host range, serious economic consequences and experimental tractability, the most intensively studied bacteria are members of the Proteobacteria phylum (such as Agrobacterium, Erwinia, Pseudomonas, Ralstonia and Xanthomonas) [1][2][3][4] . These pathogens are spread by wind, rain, insects or cultivation practices. They enter plant tissues either by wounds or through natural openings such as lenticels, hydathodes or stomata 5 , and they occupy the inter cellular spaces (apoplast) of various plant tissues or the xylem.Plant-pathogenic members of the Proteobacteria cause diverse disease symptoms, including specks, spots, blights, wilts, galls and cankers, and they can cause host-cell death in roots, leaves, flowers, fruits, stems and tubers (FIG. 1). These symptoms affect both yield and quality of agricultural crops and bacterial diseases can have serious economic, social and even political consequences [6][7][8] . Control of bacterial diseases is only partially effective and consists of copper-based sprays, antibiotics, biocontrol strategies, large-scale removal of infected plants and, most importantly, host genetic resistance 5 . Therefore, research on bacterial diseases of plants helps to elucidate fundamental aspects of microbial pathogenesis and associated host responses and also to develop more effective and sustainable disease-control methods.Correspondence to G.B.M. gbm7@cornell.edu. Competing interests statementThe authors declare no competing financial interests. DATABASES NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptPlant-pathogenic bacteria use virulence strategies that are either specialized to plant tissues or are broadly conserved among pathogens of both plants and animals (FIG. 1). Bacterial virulence is manifested as increases in the rate of growth or final population size, as well as by enhanced disease symptoms, which promote the spread of the pathogen through the plant or the broader environment.Unlike mammals, plants have a complex cell wall that bacteria must surmount to gain access to water and nutrients. Bacteria attack this barrier with extracellular virulence factors, such as cell wall degrading enzymes, and bypass it by the secretion of cell wall-permeable toxins 5 . However, perhaps the most effective virulence strategy, and one shared with animal bacterial pathogens, is to breach the wall by use of the type III s...

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

“…One hypothesis is that solidification of the infection thread matrix, as a result of this crosslinking, generates a mechanical force that helps the ingrowth of the infection thread to proceed 48 . The production of ROS by plants is often part of a defence response against invading pathogens and it can also serve as an activating signal for other plant defences (see below) 52 . One intriguing possibility is that the facilitation of infection thread progression by ROS is one way that rhizobial bacteria have adapted a plant defence response for their own benefit.…”

“…MAMPs and host-specific elicitors-Microbial factors that induce innate immune responses in plants are classified as either general or host-specific elicitors 52 . General elicitors include flagellin, elongation factor Tu (EFTu), cold shock protein (CSP), chitin and LPS [123][124][125] .…”

“…These compounds are conserved across multiple groups of bacteria, allowing plants to perceive and respond to an epitope that is common to many bacteria [123][124][125] . Plants respond to general elicitors with basal defence responses that can impart a measure of resistance to infection 52 . Basal defences include increases in extracellular pH, ethylene, ROS and reactive nitrogen species, phenolic compounds and changes in intracellular Ca 2+ , as well as cell signalling cascades and transcriptional changes 52 .…”

“…Plants respond to general elicitors with basal defence responses that can impart a measure of resistance to infection 52 . Basal defences include increases in extracellular pH, ethylene, ROS and reactive nitrogen species, phenolic compounds and changes in intracellular Ca 2+ , as well as cell signalling cascades and transcriptional changes 52 . Many plant species mount basal defence responses to the broadly conserved flg22 peptide found in bacterial flagellin 123 .…”

“…Unlike general elicitors, most host-specific elicitors are effector proteins that are injected by pathogens into plant cells by bacterial type 3 secretion systems (T3SSs) and which cause a hypersensitive response and programmed cell death 52 . These responses are more extreme and cause more tissue damage than basal defence responses 52 .…”