Characteristics of an ideotype acid tolerant pasture legume symbiosis in Mediterranean agriculture

A series of experiments has led to the following concept of eight characteristics being required in an acid-tolerant pasture legume symbiosis for use in ley-farming:In the bulk soil i -microsymbiont (preferably of the Bradyrhizobium genus) capable of maintaining high numbers into autumn, through processes which allow saprophytic function at low pH such as regulation of its internal pH, ii -microsymbiont with a carboxylated cell surface electrochemistry stable under the influence of ambient pH in its interactions with soil colloids, minerals and root surfaces.In the rhizosphere iii -microsymbiont capable of appreciable growth in response to substrate availability, iv icrosymbiont able to recognise root exudates allowing interaction with its nodD gene protein, v -microsymbiont whose nodABC gene products (nod metabolites) are pH stable and which induce cortical cell division and root-hair curling in the host, vi -microsymbiont whose surface polysaccharides and proteins are pH stable to allow attenuation of the cell at the root surface,At the root surface vii-host apoplast function unaffected by low pH such that it may (a) produce pH stable exudates capable of interacting with the rhizobial nodD gene protein and (b) receive rhizobial nod metabolites and respond physiologically, viii-root able to produce pH stable organic acids for linkage with rhizobial cell surface structures.These ideotype characteristics reflect our current understanding of the mechanisms of acid tolerance in the nodulation phase of species such as Medicago polymorpha and M. murex, and acid soil tolerance in rhizobial inoculant strains such as WSM540.