Detection of and Response to Signals Involved in Host-Microbe Interactions by Plant-Associated Bacteria

Abstract

Diverse interactions between hosts and microbes are initiated by the detection of host-released chemical signals. Detection of these signals leads to altered patterns of gene expression that culminate in specific and adaptive changes in bacterial physiology that are required for these associations. This concept was first demonstrated for the members of the family Rhizobiaceae and was later found to apply to many other plant-associated bacteria as well as to microbes that colonize human and animal hosts. The family Rhizobiaceae includes various genera of rhizobia as well as species of Agrobacterium. Rhizobia are symbionts of legumes, which fix nitrogen within root nodules, while Agrobacterium tumefaciens is a pathogen that causes crown gall tumors on a wide variety of plants. The plant-released signals that are recognized by these bacteria are low-molecular-weight, diffusible molecules and are detected by the bacteria through specific receptor proteins. Similar phenomena are observed with other plant pathogens, including Pseudomonas syringae, Ralstonia solanacearum, and Erwinia spp., although here the signals and signal receptors are not as well defined. In some cases, nutritional conditions such as iron limitation or the lack of nitrogen sources seem to provide a significant cue. While much has been learned about the process of host detection over the past 20 years, our knowledge is far from being complete. The complex nature of the plant-microbe interactions makes it extremely challenging to gain a comprehensive picture of host detection in natural environments, and thus many signals and signal recognition systems remain to be described

Rooteomics: The Challenge of Discovering Plant Defense-Related Proteins in Roots

“…Upon detection of these stimuli, there is a continuous process of response between plants and microorganisms that characterizes the plantmicroorganism interaction. Several plant factors such as pathogen recognition receptors and phenolic compounds as well as pathogen proteins and molecules including virulence factors and exopolysaccharides play important roles in plantmicroorganism relation and are determinants in the nature of the interaction [57].…”

Plant responses to plant growth-promoting rhizobacteria

“…Nod factors are perceived by plant root hairs and function in a hormone-like fashion to induce root nodules in which the Rhizobium bacterium can fix atmospheric nitrogen. The bacterium grows at the expense of carbohydrates from the host, but provides fixed nitrogen for amino acid biosynthesis in return (Brencic and Winans 2005;Gray and Smith 2005). This symbiosis is a prime example of an intimate relationship between a soil bacterium and its host plant, and illustrates the concept behind the term 'plant growthpromoting rhizobacteria' (PGPR): in nitrogen-poor environments the Rhizobium bacterium promotes legume plant growth by providing a limiting nutrient.…”

Using signal molecule genistein to alleviate the stress of suboptimal root zone temperature on soybean-Bradyrhizobium symbiosis under different soil textures

“…Signal molecules, i.e., genistein (4?,5,7-trihydroxyisoflavone) (Zhang and Smith 1995;Pan and Smith 2000) and jasmonates (jasmonic acid and methyl jasmonates) (Mabood and Smith 2005;Mabood et al 2006) in soybean, cause the expression of nodulation (nod) genes in (Brady)rhizobium (Broughton et al 2003;Peck et al 2006) resulting in the production of lipochitooligosaccharides (Nod factors), involved in morphogenic changes such as root hair curling, a necessary step for the bacteria entry, and eventually, nodule formation on legumes roots (Penmetsa et al 2003;Esseling et al 2004;Brencic and Winans 2005;Miransari et al 2006).…”