Mutations affecting nodulation in grain legumes and their potential in sustainable cropping systems

Many spontaneous and a large number of induced mutants that show altered no dulation pattern have been isolated in pea, soybean, common bean, faba bean , chickpea, groundnut and pigeonpea. Available information on nodulation mu tants in these crops is summarised. The importance of nodulation mutants in basic studies on plant-microbe symbiotic interactions, nitrogen fixation a nd breeding of cultivars with higher yield and nitrogen fixation rate are e xamined. The nodulation mutants, after inoculation with specific bacterial strains or a number of different strains, show either: no nodulation (nod-) , few nodules (nod+/-), ineffective nodulation (fix-), hyper nodulation (no d++) or hypernodulation even in the presence of otherwise inhibitory nitrat e levels (nts). No spontaneous hypernodulation or nts mutants have been fou nd, all have been induced in independent experiments using different cultiv ars of pea, soybean and common bean after mutagenising seeds. Most nodulati on mutants show monogenic recessive inheritance, though semi-dominant and d ominant inheritance is also reported. Nodule number is controlled by a proc ess known as autoregulation; hypernodulating mutants show relaxed autoregul ation. By grafting shoots of hypernodulating soybean mutant on normal nodul ating soybean, mungbean and hyacinth bean, presence of a common, translocat able signal has been shown. Nodulation mutants have contributed to the unde rstanding of the genetic regulation of host-symbiont interactions, nodule d evelopment and N fixation. Initially, the hypernodulating mutants were foun d to be poor in yield. Using the induced hypernodulating mutant, a new soyb ean cultivar `Nitrobean 60', has been released in Australia. This cultivar is reported to have given 15% higher yield over cv. `Bragg,' and contribute d a higher amount of fixed N to the following cereal crop in rotation. Pros pects of using the nodulation mutants in developing grain legume cultivars that combine high yield with high residual N, within the bioenergetic const raints, for developing sustainable cropping systems are examined.