ABSTRACT
Plant roots are exposed to very large numbers of soil microorganisms for which, being the main source of soil organic matter, they form a prime source of nutrients. It is understandable that microorganisms should have evolved a number of different methods for interacting with plants and the environment to gain access to the nutrients that plants can provide. Similarly, to some extent, plants have developed mechanisms to inhibit (pathogens) or to encourage (symbiotic N2 fixing) microorganisms in the rhizosphere. For most microorganisms, interactions with growing plants extend no further than the colonization of the root surface, where exudates are available, but others can produce a vast range of metabolites that can modify the rhizosphere properties. These changes in the root zone can influence plant growth, directly or indirectly.
