Saturday, 15 July 2006

Characterization of Bacterial Communities Isolated from Rhizosphere Soil of Field Grown Chinese Cabbage (Brassica campestris ssp pekinensis) by Cultivation Dependent Methods and Screening of Potential Plant Growth Promotion.

Selvaraj Poonguzhali1, Munusamy Madhaiyan1, Kyounga Kim1, Daesuck Suh2, Bumki Park2, Jaijoung Kim3, and Tongmin Sa1. (1) Chungbuk National Univ, 12, Gaesin-dong, Heungduck-ku, Cheongju, South Korea, (2) Namhae Chemicals Corp, 60-1, 3GA, Chungmu Ro, Seoul, South Korea, (3) Chungbuk National University, 12, Gaesin-dong, Heungduck-ku, Cheongju, South Korea

The rhizosphere is the portion of the soil that forms the complex habitat of plant roots, having a marked influence on microbial communities that play an important role for the functioning of the ecosystem, both in relation to direct interactions with plants (both beneficial and deleterious to growth) and with regard to nutrient and organic matter cycling and the composition of rhizosphere is altered by root activity. On the other hand, influence of the rhizosphere microorganisms on root biology is so important that is critical to evaluate how the basic nutritional and developmental processes of the plant are affected. Investigating the diversity of rhizosphere microorganisms is important for understanding the impacts of new agricultural technologies upon soil ecology, nutrient transformations and plant succession and to manipulate these associations to increase crop production. Microorganisms that colonize the rhizosphere can be classified according to their effects on plants and the way they interact with roots, some being pathogens whereas other trigger beneficial effects. Beneficial rhizobacteria are often referred to as Plant Growth Promoting Rhizobacteria (PGPR) that inhabit plant roots and exert a positive effect ranging from direct influence mechanisms to an indirect effect. Bacteria of several taxonomic classes are found in crop rhizosphere and in soil and they can increase plant growth and productivity. Our study aimed at detecting the root associated bacterial communities of Chinese cabbage (Brassica campestris ssp pekinensis) under different fertilization regimes using cultivation dependent methods. The cultivable population was studied using agar plating assay, Fatty Acid Methyl Ester (FAME) analysis and carbon substrate utilization using BIOLOG™ plates. Taxonomical identification of the isolates by FAME resulted in about 83% identification and they represented 9 and 14 different known bacterial genera from the rhizosphere and root interior respectively from Proteobacteria (a, b, and g), firmicutes (actinobacteria and the Bacillus groups) and Bacteroidetes. Pseudomonas and Bacillus were associated with the plants grown under all the fertilized conditions and actinobacteria could be observed only in rhizosphere of plants grown on unfertilized plots. The principal component analysis of the FAME profiles of the bacterial (rhizosphere and endophytic) isolates resulted in apparently four groupings regardless of treatment, and no treatment exerted a dominating influence on the bacterial diversity. But Principal Component Analysis (PCA) on the BIOLOG substrate utilization of the isolates revealed the metabolical dissimilarity of the isolates. The isolates that were the same strains according to the FAME analysis were not necessarily metabolically similar. The diversity, as revealed by the diversity indices was greater among the isolates obtained from unfertilized samples than that of fertilized ones. The isolates analyzed for different plant growth promoting effects revealed variations depending on the fertilization regimes. The highest percentage of phosphate solubilizing bacteria (PSB) and 1-aminocyclo propane-1-carboxylate (ACC) utilizers was recorded in chemical fertilizer treated samples, followed by the organic fertilizer treated. Clearly, the bacterial diversity is influenced by the soil physical and chemical characteristics and descriptions of both structural and functional diversity are a key to understanding microbial ecology and ecosystem function. This study provides an insight into the root associated bacterial communities that can be cultivated from Chinese cabbage under different fertilization regimes. It is clear from this study that some species fluctuate between endophytic and epiphytic niche and the functional and the physiological diversity of the bacterial communities vary with the soil management practices ie. fertilizer application. Future studies addressing these factors could expand the knowledge on the rhizosphere microbiology.

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