Plant-Microbe Partnerschips

Abstract

Associations between plants and microorganisms are very complex and are subject of an increasing number of studies. Although it is well documented that plants create specific niches for their associated bacteria by providing them nutrients in the rhizosphere, the phyllosphere as well as inside the plant, the role and importance of the microbes in these plant-microbe partnerships still are sometimes underestimated. At first, plant-associated bacteria can enhance plant growth and development in a direct or indirect way. Direct plant growth promoting mechanisms may involve nitrogen fixation, the supply of unavailable nutrients such as phosphorus and other mineral nutrients, production of plant growth regulators such as auxins, cytokinins and gibberellines, and suppression of ethylene synthesis by 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. Plant-associated bacteria can indirectly benefit the plant by preventing the growth or activity of plant pathogens through competition for space and nutrients, antibiosis, production of hydrolytic enzymes, inhibition of pathogen-produced enzymes or toxins and through induction of plant defense mechanisms. Beside the application of plant growth promoting bacteria to increase biomass production in function of food, feed and raw materials for industrial processes, a very promising area for the exploitation of plant-microbe partnerships is the remediation of contaminated soils and (ground) water. In case of phytoremediation of metal contaminated soils and (ground) water, plant-associated bacteria possessing a metal-sequestration system can reduce metal phytotoxcicity and enhance translocation to the upperground plant parts. Besides, rhizosphere bacteria producing siderophores and/or organic acids can increase plant availability of metals. During phytoremediation of some organic contaminants, plants rely on their associated microorganisms possessing the appropriate degradation pathway to obtain an efficient degradation of organic contaminants resulting in decreases of both phytotoxicity and evapotranspiration of volatile contaminants and/or degradation intermediates to the atmosphere. Therefore a systems biology approach to understand the synergistic interactions between plants and their beneficial bacteria represents an important field of research, which is facilitated by the sequencing of the genomes of model plant species and their associated beneficial bacteria.