The Medicago truncatula NIP/LATD Transporter Is Essential for Nodulation and Appropriate Root Architecture

Legumes are capable of forming two types of lateral organs on their roots, lateral roots (LRs) and symbiotic root nodules. Mutants in the Medicago truncatula NIP/LATD gene have defects in both types of LR organs. When bioavailable nitrogen is limiting and specific rhizobia are present, the severe Mtnip-1 and Mtlatd mutants develop nodule primordia with rhizobia encased in infection threads. These mutants also have striking LR defects. The weaker Mtnip-3 mutant forms Fix+/- nodules containing the characteristic developmental zones and more wild-type-like LRs. MtNIP/LATD was cloned and shown to encode a protein in the broad-substrate NRT1(PTR) transporter family. Functional studies in Xenopus laevis oocytes demonstrated that MtNIP/LATD is a high-affinity nitrate transporter. In addition, constitutive expression of MtNIP/LATD in Arabidopsis thaliana mutants defective in the dual-affinity major nitrate transporter NRT1(CHL) gene restored its chlorate susceptibility, a proxy assessment of nitrate transport, implying that MtNIP/LATD transports nitrate in planta. Experimental evidence suggests that MtNIP/LATD has a second function. These results are discussed in the context of MtNIP/LATD's potential roles in nodulation.