Kielo Haahtela and PhD Pasi Haansuu - General Microbiology

Bacteria in Rhizosphere: Diversity, Plant-Growth Promoting Activities, and Molecular Interactions with Plant Cells

Professor Kielo Haahtela, PhD Pasi Haansuu

Plants are thought to selectively enrich the soil neighbouring the roots (rhizosphere) for
microorganisms that are well adapted to utilisation of released root exudates. An understanding of the factors influencing colonisation of the rhizosphere is essential to detect and identifity population in soil and rhizosphere.

Detection and identification of bacterial populations are of greatest interest for understanding the microbial ecology of the soil. Therefore a great diversity of bacteria has been isolated and identified from forest soils and birch and grass roots inoculated with humus suspensions. Many of these bacteria (Frankia, Streptomyces, Bacillus, Pseudomonas) have been shown to produce antibiotics, siderophores, and volatiles (e.g. hydrogen cyanide) that inhibit the growth of many root pathogenic bacteria and fungi (e.g. Rhizoctonia).

Some others fix atmospheric nitrogen or produce root growth- promoting hormones thus acting like biological fertiliz!   ers. A novel nitrogen-fixing species, Paenibacillus borealis, was isolated from the humus and the roots. We also study the effects of plant-growth promoting bacteria on plants under stress e.g. clear-cutting.

Phytoremediation of hazardous waste compounds, such as polyaromatics (PAH) and heavy metals is studied using silver birch (Betula pendula). Lead (Pb) is considered as highly toxic metal, which induces serious health risks to humans as well as to the environment. The main objective of our present study is to investigate the effects of different lead concentrations on bacterial diversity in humus soil and in rhizosphere. To perform this, we use both cultivation-
dependent (e.g. plate-count, Biolog ®) and -independent (e.g. DGGE) methods. We focus on screening bacteria with plant growth promoting properties together with the ability to tolerate high lead concentrations. It is also in our interest to learn more about the ecological role and function of bacteria in lead contaminated soil.