1.
Bacillus velezensis tolerance to the induced oxidative stress in root colonization contributed by the two-component regulatory system sensor ResE.
Zhang, H, Liu, Y, Wu, G, Dong, X, Xiong, Q, Chen, L, Xu, Z, Feng, H, Zhang, R
Plant, cell & environment. 2021;(9):3094-3102
Abstract
Efficient root colonization of plant growth-promoting rhizobacteria is critical for their plant-beneficial functions. However, the strategy to overcome plant immunity during root colonization is not well understood. In particular, how Bacillus strains cope with plant-derived reactive oxygen species (ROS), which function as the first barrier of plant defence, is not clear. In the present study, we found that the homolog of flg22 in Bacillus velezensis SQR9 (flg22SQR9 ) has 78.95% identity to the typical flg22 (flg22P.s. ) and induces a significant oxidative burst in cucumber and Arabidopsis. In contrast to pathogenic or beneficial Pseudomonas, live B. velezensis SQR9 also induced an oxidative burst in cucumber. We further found that B. velezensis SQR9 tolerated higher H2 O2 levels than Pst DC3000, the pathogen that harbours the typical flg22, and that it possesses the ability to suppress the flg22-induced oxidative burst, indicating that B. velezensis SQR9 may exploit a more efficient ROS tolerance system than DC3000. Further experimentation with mutagenesis of bacteria and Arabidopsis showed that the two-component regulatory system, ResDE, in B. velezensis SQR9 is involved in tolerance to plant-derived oxidative stress, thus contributing to root colonization. This study supports a further investigation of the interaction between beneficial rhizobacteria and plant immunity.
2.
Sensing environmental and developmental signals via cellooligomers.
Oelmüller, R
Journal of plant physiology. 2018;:1-6
Abstract
Roots respond to a cocktail of chemicals from microbes in the rhizosphere. Infochemicals in nmol concentrations activate receptor-mediated signal pathways, which reprogram the plant responses to environmental changes. The microbial signals have to pass the cell wall to activate pattern recognition receptors at the surface of the plant plasma membrane. The structure of the cell wall is not only a barrier for the signaling molecules, but also changes permanently during growth and development, as well as in response to microbial attacks or abiotic stress. Recently, cellooligomers (COMs) were identified as novel chemical mediators in Arabidopsis thaliana, which inform the cell about the alterations in and around the cell wall. They can be of microbial and plant origin and represent novel invasion patterns (Cook et al., 2015). COMs initiate Ca2+-dependent signaling events that reprogram the cell and adjust the expression and metabolite profiles as well as innate immunity in response to changes in their rhizosphere environment and the state of the cell wall. COMs operate synergistically with other signals or their recognition machineries and activates local and systemic responses in the entire plant. They also adjust the performance of the areal parts of the plant to signals perceived by the roots. Here, I summarize our current knowledge about COMs and propose strategies for future investigations.
3.
Rhizobia: a potential biocontrol agent for soilborne fungal pathogens.
Das, K, Prasanna, R, Saxena, AK
Folia microbiologica. 2017;(5):425-435
Abstract
Rhizobia are a group of organisms that are well known for their ability to colonize root surfaces and form symbiotic associations with legume plants. They not only play a major role in biological nitrogen fixation but also improve plant growth and reduce disease incidence in various crops. Rhizobia are known to control the growth of many soilborne plant pathogenic fungi belonging to different genera like Fusarium, Rhizoctonia, Sclerotium, and Macrophomina. Antagonistic activity of rhizobia is mainly attributed to production of antibiotics, hydrocyanic acid (HCN), mycolytic enzymes, and siderophore under iron limiting conditions. Rhizobia are also reported to induce systemic resistance and enhance expression of plant defense-related genes, which effectively immunize the plants against pathogens. Seed bacterization with appropriate rhizobial strain leads to elicitation and accumulation of phenolic compounds, isoflavonoid phytoalexins, and activation of enzymes like L-phenylalanine ammonia lyase (PAL), chalcone synthase (CHS), peroxidase (POX), polyphenol oxidase (PPO), and others involved in phenylpropanoid and isoflavonoid pathways. Development of Rhizobium inoculants with dual attributes of nitrogen fixation and antagonism against phytopathogens can contribute to increased plant growth and productivity. This compilation aims to bring together the available information on the biocontrol facet of rhizobia and identify research gaps and effective strategies for future research in this area.
4.
The role of phenotypic variation in rhizosphere Pseudomonas bacteria.
van den Broek, D, Bloemberg, GV, Lugtenberg, B
Environmental microbiology. 2005;(11):1686-97
Abstract
Colony phase variation is a regulatory mechanism at the DNA level which usually results in high frequency, reversible switches between colonies with a different phenotype. A number of molecular mechanisms underlying phase variation are known: slipped-strand mispairing, genomic rearrangements, spontaneous mutations and epigenetic mechanisms such as differential methylation. Most examples of phenotypic variation or phase variation have been described in the context of host-pathogen interactions as mechanisms allowing pathogens to evade host immune responses. Recent reports indicate that phase variation is also relevant in competitive root colonization and biological control of phytopathogens. Many rhizospere Pseudomonas species show phenotypic variation, based on spontaneous mutation of the gacA and gacS genes. These morphological variants do not express secondary metabolites and have improved growth characteristics. The latter could contribute to efficient root colonization and success in competition, especially since (as shown for one strain) these variants were observed to revert to their wild-type form. The observation that these variants are present in rhizosphere-competent Pseudomonas bacteria suggests the existence of a conserved strategy to increase their success in the rhizosphere.