Two defined alleles of the LRR receptor kinase GmNARK in supernodulating soybean govern differing autoregulation of mycorrhization

Plants regulate the extent of nodulation and root colonization by arbuscular mycorrhizal fungi (AMF), a phenomenon named autoregulation of symbiosis. We tested AMF colonization in split roots of various soybean genotypes [Glycine max (L.) Merr. cv. Bragg, Enrei, Harosoy and Williams], where precolonization of one side of the split-root system by the AMF Glomus mosseae resulted in reduced mycorrhization of the other. AMF precolonization failed to control secondary mycorrhization in the supernodulating Bragg nonsense mutant nts1007 (Q106*), indicating that the GmNARK gene (predicted to encode a leucine-rich repeats (LRR) receptor kinase related to CLAVATA1 in Arabidopsis) is involved in autoregulation of the AMF symbiosis. Here, we tested whether the allelic En6500 nonsense supernodulating mutant (GmNARK K606*, derived from cv. Enrei) and supernodulating mutants of cv. Williams (Nod1-3 and Nod2-4) with yet-undefined genetic lesions exhibit a similar symbiotic phenotype in mycorrhizal split-root systems. Surprisingly, these supernodulating mutants retained their ability to autoregulate AMF. To examine possible differences between two allelic mutants, we determined levels of IAA, abscisic acid, coumestrol, daidzein and genistein in mycorrhizal and uninoculated control roots. Compared with wild-type plants, both mutants showed reduced IAA accumulation in mycorrhizal roots. Roots of cv. Enrei and En6500 exhibited high levels of isoflavonoids not seen in Bragg or nts1007. Taken together, these findings showed that supernodulation mutants, despite a common nodulation phenotype, differ in their ability to autoregulate AMF root colonization. This suggests either that the GmNARK gene product of some mutants is still partially functional (Q106* vs. K606*) or that varietal differences reflected in altered physiological responses suppress the loss of function

Nodulation in the legume biofuel feedstock tree Pongamia pinnata

The legume tree Pongamia pinnata (also called Millettia pinnata) is gaining importance as a biofuel feedstock tree because of the abundant annual production of oil-rich seeds, adaptation to a wide range of geoclimatic conditions and significant resistance to abiotic stress, such as water-deficit, salinity and acidity of soils. The major defining benefit of using pongamia as a biofuel feedstock is that it is a legume, enabling biological nitrogen fixation through symbiosis with soil bacteria, collectively called rhizobia, which results in root nodulation. Here, we report preliminary data, (i) indicating the range of rhizobia that can form nodules on pongamia, (ii) demonstrating the measurement of nitrogen fixation activity of pongamia nodules via the classical acetylene reduction assay, (iii) illustrating nodule morphology and development and (iv) demonstrating aspects of nodule regulation by external nitrate as well as internal autoregulation of nodulation. We note that in pongamia most nodulation-related characteristics are similar to those found in other annual crop legumes such as soybean