MtLAX2, a Functional Homologue of the Arabidopsis Auxin Influx Transporter AUX1, Is Required for Nodule Organogenesis.

Most legume plants can form nodules, specialized lateral organs that form on roots, and house nitrogen-fixing bacteria collectively called rhizobia. The uptake of the phytohormone auxin into cells is known to be crucial for development of lateral roots. To test the role of auxin influx in nodulation we used the auxin influx inhibitors 1-naphthoxyacetic acid (1-NOA) and 2-NOA, which we found reduced nodulation of This suggested the possible involvement of the AUX/LAX family of auxin influx transporters in nodulation. Gene expression studies identified , a paralogue of Arabidopsis () , as being induced at early stages of nodule development. is expressed in nodule primordia, the vasculature of developing nodules, and at the apex of mature nodules. The promoter contains several auxin response elements, and treatment with indole-acetic acid strongly induces expression in roots. mutants displayed root phenotypes similar to Arabidopsis mutants, including altered root gravitropism, fewer lateral roots, shorter root hairs, and auxin resistance. In addition, the activity of the synthetic DR5-GUS auxin reporter was strongly reduced in roots. Following inoculation with rhizobia, roots developed fewer nodules, had decreased DR5-GUS activity associated with infection sites, and had decreased expression of the early auxin responsive gene Our data indicate that MtLAX2 is a functional analog of Arabidopsis AUX1 and is required for the accumulation of auxin during nodule formation in tissues underlying sites of rhizobial infection.