Plant growth promotion by rhizobacteria is a known phenomenon but the underlying mechanisms are poorly understood. We searched for plant growth-promoting rhizobacteria that are naturally associated with Arabidopsis thaliana to investigate the molecular mechanisms that are involved in plant growth-promotion. We isolated a Pseudomonas bacterium (Pseudomonas sp. G62) from roots of field-grown Arabidopsis plants that has not been described previously and analyzed its effect on plant growth, gene expression and the level of sugars and amino acids in the host plant. Inoculation with Pseudomonas sp. G62 promoted plant growth under various growth conditions. Microarray analysis revealed rapid changes in transcript levels of genes annotated to energy-, sugar- and cell wall metabolism in plants 6 h after root inoculation with P. sp. G62. The expression of several of these genes remained stable over weeks, but appeared differentially regulated in roots and shoots. The global gene expression profile observed after inoculation with P. sp. G62 showed a striking resemblance with previously described carbohydrate starvation experiments, although plants were not depleted from soluble sugars, and even showed a slight increase of the sucrose level in roots 5 weeks after inoculation. We suggest that the starvation-like transcriptional phenotype - while steady state sucrose levels are not reduced - is induced by a yet unknown signal from the bacterium that simulates sugar starvation. We discuss the potential effects of the sugar starvation signal on plant growth promotion.
Bacterial strain G72T was isolated from surface-sterilized roots of Arabidopsis thaliana growing in its natural habitat. This Gram-positive, rod-shaped, non-motile, microaerophilic and aerobically growing isolate was characterized by using a polyphasic approach. On the basis of 16S rRNA gene sequence similarity, strain G72 T belongs to the genus Microbacterium. However, reassociation values in a DNA-DNA hybridization analysis with closely related strains were between 45.1 and 15.9 %. The DNA G+C content was 70.1 mol%. Strain G72 T possessed a peptidoglycan type based on B2b with partial substitution of glutamic acid by 3-hydroxy glutamic acid (Glu/Hyg-Gly-D-Orn). The isolate contained MK-13, MK-12 and MK-11 as major respiratory quinones and anteiso-C 15 : 0 , anteiso-C 17 : 0 and iso-C 16 : 0 as predominant fatty acids. The major polar lipids were diphosphatidylglycerol, phosphatidylglycerol and three unidentified glycolipids. Based on its physiological and biochemical traits, as well as genotypic results, strain G72 T is considered to represent a novel species, for which the name Microbacterium yannicii sp. nov. is proposed. The type strain is G72Plants are generally colonized by many microbes. Apart from pathogenic bacteria, there are also bacteria that improve the growth and health of their host plants (Lugtenberg & Kamilova, 2009). To date, the beneficial effects of bacteria on plant growth have been mainly investigated in crop plants, but little is known about the molecular and physiological mechanisms that ultimately result in growth promotion. In our study, we isolated bacteria from roots of Arabidopsis thaliana, which is a model organism in plant research. Therefore, a wide array of molecular and genetic techniques is available to perform further research, which should lead to a better understanding of the mechanism leading to bacterially induced plant growth promotion. Here we describe the isolation and characterization of a bacterial strain, designated G72 T , isolated from surface-sterilized roots of A. thaliana growing in the field, with the subsequent aim to investigate its effect on plant growth. As a result of pheno-and genotypic characterization, we propose that strain G72T represents a novel species of the genus Microbacterium, for which we propose the name Microbacterium yannicii sp. nov. Strain G72T was isolated from roots of A. thaliana (thale cress) collected from a field in Golm (52 u 249 570 N 12 u 589 180 E, Potsdam, Germany; soil texture: loamy sand). Roots were surface-sterilized with 70 % ethanol for 1 min and 2.5 % sodium hypochlorite for 3 min. After rinsing three times in sterile distilled water, roots were ground using a sterile mortar and pestle. The ruptured root tissue was incubated in 1/4 strength Ringer solution (Merck) for 45 min with shaking at room temperature. After 5 min of sedimentation, dilutions of the suspension were spread on modified tryptic soy agar (TSA) medium adapted from the method of Jacxsens et al. (2003). To simulate nutrient availability in Arabidopsis, thi...
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