ABSTRACT
Trace metals contamination due to natural and anthropogenic sources is a global environmental concern. A number of concerted efforts have been made to decontaminate the polluted biosphere. Keeping in view the nonbiodegradability of majority of inorganic environmental contaminants, microbial metals bioremediation is an ef—cient strategy due to its low cost, high ef—ciency, and eco-friendly nature. Plant-associated microorganisms include endophytic, phyllospheric, and rhizosphere microorganisms. Endophytic microorganisms can be de—ned as microorganisms that colonize the internal tissue of the plant without causing visible external sign of infection or a negative effect of the host (Weyens et al. 2009). Because endophytic microorganisms (including bacteria and fungi) can proliferate within the plant tissue, they are likely to interact closely with their host and therefore face less competition for nutrients and are more protected from adverse changes in the environment than bacteria in the rhizosphere and phyllosphere. Endophytic microorganisms exhibit tremendous diversity in both plant hosts and microbial taxa such as bacteria and fungi. The phyllosphere is the external regions of plant parts that are above ground, including leaves, stems, blossoms and fruits. Because the majority of the surface area available for colonization is located on the leaves (the dominant tissue of the phyllosphere), the external region of plant regions was called phyllosphere parts. Microorganisms residing in the phyllosphere are exposed to large and rapid ¬uctuations in temperature, solar radiation and water availability. In the review, the phyllosphere microorganisms were not included. The unique ability of hyperaccumulator plants to accumulate excessive amounts of metals or metalloids are related to transport systems of the root tissues (Wenzel et al. 2009). Root exudates are believed to have a major in¬uence on the diversity of microorganisms (Weyens et al. 2009). More recently, attention has concentrated on the plant-growth promoting capacity of endophytes and a close relationship exists between microorganisms living in the rhizosphere and those inside the plant roots (endophytes). Plant roots are the main site of endophytic
18.1 Introduction .......................................................................................................................... 345 18.2 Bacterial and Fungal Communities of Brassica Oilseeds ....................................................346 18.3 Properties of Rhizosphere and Endophytic Microorganisms ...............................................348 18.4 Microbial Effects on Metals-/Metalloids-Bioavailability in the Rhizosphere ..................... 350 18.5 Rhizosphere and Endophytic Microorganisms Impact on Oilseed Rape Growth and
Phytoremediation Potential ................................................................................................... 351 18.6 Conclusions and Future Prospects ........................................................................................ 356 Acknowledgments .......................................................................................................................... 357 References ...................................................................................................................................... 357
colonization. Root colonization by microorganisms was described to involve several stage (Taghavi et al. 2010): in the initial step, bacteria move toward the plant roots either passively via soil water ¬uxes, or actively via speci—c induction of ¬agellar activity by plant-released compounds; second, a nonspeci—c adsorption of bacteria to roots occurs, followed by anchoring that results in —rm attachment of bacteria to the root surface. Speci—c or complex interactions between the bacterium and the host plant, such as the secretion of root exudates may arise resulting in the induction of bacterial gene expression. Finally, endophytes can enter the plant at sites of tissue damage, which naturally arise as the result of plant growth through root hairs and at epidermal conjunction. In addition, plant exudates given off through these wounds provide a nutrient source for the colonization microorganisms and thus create favorable conditions. After entering the plant, endophytes must establish themselves once established, they can induce plant resistance to pathogen and insects or promote plant growth by producing plant growth regulating substances such as indole acetic acid (IAA), cytokinins or metabolize the stress ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). The plant-microbe partnerships are bene—cial for increasing biomass production of host plants. The rapeseed meal is high in nitrogen (6% wt/wt), stimulates 100-fold increases in populations of resident Streptomyces species, and suppresses fungal infection of roots subsequently cultivated in the amended soil (Cohen et al. 2004).
