Interaction of Rhizobia with Soil Suppressiveness Factors

Abstract

There has been an increasing awareness of the importance of soil quality and soil health in sustainable agricultural production and of the role played by the soil microbiota. More recently the impact of rhizobia on soil suppressiveness has been recognised. Unfortunately despite an initial flurry of research in the 1990s, little further exploration has been carried out. Much of this lack of study may be due to (a) the significant reclassification of the rhizobia resulting in lack of clarity in terms, classification and nomenclature of rhizobial strains and (b) the complexity of the interactions between rhizobia, other soil microbes and host and non-host plants. The ability of rhizobia to form symbiotic N-fixing nodules on compatible legume roots is usually mediated by a plasmid pSym which carries nod and nif genes responsible for nodule formation and nitrogen fixation, respectively. The establishment of N-fixing nodules is a complex interactive process during which the plant root and the rhizobia both produce a range factors and compounds. Suppressive effects of rhizobia against fungi, nematodes and parasitic weeds have long been recognised and may be attributed directly to effects of these factors and compounds and/or to direct competition effects or indirectly through improved plant growth and/or induced resistance responses. There is clearly scope to develop optimised rhizobial inoculant strains that could be used to boost crop growth and reduce disease; however, a better understanding of how rhizobia suppress disease will be required.