Involvement of auxin distribution in root nodule

S (PH D THESIS) Involvement of auxin distribution in root nodule development of Lotus japonicus (Graduate School of Agriculture, Laboratory of Plant Gene Expression, RISH, Kyoto University) Kojiro TAKANASHI Legumes (Fabaceae) constitute the third largest plant family with around 700 genera and 20,000 species. Legume plants form root nodules through symbiosis with a soil microbe called rhizobia. This plant-microbe symbiosis in nodules mediates a harmonized exchange of chemical signals between host plants and rhizobia. Nodules are biologically divided into two different groups, i.e., indeterminate nodules and determinate nodules. Indeterminate nodules, represented by Trifolium repens (white clover) and Medicago truncatula, are initiated from the inner cortex to form a persistent nodule meristem, which allows continuous growth, and leads to the formation of elongated nodules, whereas in determinate legumes, nodules are mostly developed from outer cortical cells and form spherical nodules. Auxin is one of the most important regulators for nodule development. Since the possible involvement of auxin in nodule formation was first reported by Thimann, auxin distribution during nodulation has been studied in particular with indeterminate nodules. However, little is known about auxin involvement in determinate nodule formation. To evaluate auxin functions in the determinate nodulation of legume plants, I performed an auxin-responsive promoter analysis in detail. Using GH3:GUS transformed Lotus japonicus, detected auxin signals throughout the nodulation process, e.g., at the basal and front part of the nodule primordia, circumjacent to the infection zone of the young developing nodules, and at the nodule vascular bundle in mature nodules. I also investigated the effect of several auxin inhibitors, including newly synthesized auxin antagonist PEO-IAA on the nodulation of L. japonicus, and revealed that auxin was required for forming a nodule vascular bundle and lenticels (Fig. 1). Figure 1. The effect of auxin inhibitor on nodule surface. (A) Typical mature nodule of L. japonicus at 21 dpi. Lenticels are pointed out by arrowheads. (B) The treatment of auxin inhibitor (NPA 100 M) inhibited lenticel formation on the nodule surface. In indeterminate legumes, auxin is accumulated at the site of rhizobia inoculation. This is caused by the inhibition of polar auxin transport by accumulation of flavonoids around the infection site, which are known