Plant-growth promoting effect of newly isolated rhizobacteria varies between two Arabidopsis ecotypes.

Various rhizobacteria are known for their beneficial effects on plants, i. e. promotion of growth and induction of systemic resistance against pathogens. These bacteria are categorized as plant growth promoting rhizobacteria (PGPR) and are associated with plant roots. Knowledge of the underlying mechanisms of plant growth promotion in vivo is still very limited, but interference of bacteria with plant hormone metabolism is suggested to play a major role. To obtain new growth promoting bacteria, we started a quest for rhizobacteria that are naturally associated to Arabidopsis thaliana. A suite of native root-associated bacteria were isolated from surface-sterilized roots of the Arabidopsis ecotype Gol-1 derived from a field site near Golm (Berlin area, Germany). We found several Pseudomonas and a Microbacterium species and tested these for growth promotion effects on the Arabidopsis ecotypes Gol-1 and Col-0, and for growth-promotion associated traits, such as auxin production, ACC deaminase activity and phosphate solubilization capacity. We showed that two of the bacteria strains promote plant growth with respect to rosette diameter, stalk length and accelerate development and that the effects were greater when bacteria were applied to Col-0 compared with Gol-1. Furthermore, the capability of promoting growth was not explained by the tested metabolic properties of the bacteria, suggesting that further bacterial traits are required. The natural variation of growth effects, combined with the extensive transgenic approaches available for the model plant Arabidopsis, will build a valuable tool to augment our understanding of the molecular mechanisms involved in the natural Arabidopsis - PGPR association.

Microbacterium yannicii sp. nov., isolated from Arabidopsis thaliana roots.

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 G72T 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 G72T possessed a peptidoglycan type based on B2ß 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-C15:0, anteiso-C17:0 and iso-C16:0 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 G72T is considered to represent a novel species, for which the name Microbacterium yannicii sp. nov. is proposed. The type strain is G72T (=DSM 23203T=LMG 25521T).

A naturally associated rhizobacterium of Arabidopsis thaliana induces a starvation-like transcriptional response while promoting growth.

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.