ABSTRACT
Although the term rhizosphere was coined by Hiltner (1) to describe specifically the interaction between the roots of legumes and bacteria, the usage of the term has broadened. Today we consider the rhizosphere to be the zone of influence of all plant root systems, and it includes the various cell layers of the root itself (the endorhizosphere), which microorganisms can colonize; the root surface (the rhizoplane); and the region surrounding the root (the ectorhizosphere). There has been steady progress in our understanding of the rhizosphere during the last 100 years and an increasing realization that it can have many influences on crop productivity (Table 1). However, there has been a sudden increase in interest and investment in rhizosphere research in the past few years. This is largely due to the growth of biotechnology in relation to agriculture and, in particular, our increasing ability to manipulate plants by using recombinant deoxyribonucleic acid (DNA). Even with the genetic information of the plant modified, however, there will still be a highly influential microbial population associated with the roots. Therefore, the possibility exists that the balance between beneficial and harmful rhizosphere microorganisms might be improved not only by modification of the plant genotype but also by inoculation with useful and compatible microorganisms. The source of these useful bacteria and fungi could be either other soils and rhizospheres, hence amplifying the natural population, or introduction of “foreign” organisms that have been genetically modified to elevate their useful properties and/or increase their competence to colonize the rhizosphere. Given current public opinion, the latter is unlikely in the near future.
