Medicago truncatula MtLAX2, an orthologue of the AtAUX1 auxin influx transporter, mediates auxin control of nodulation
S. ROY (1), J. Murray (2), A. Downie (2), J. Lilley (2), G. Oldroyd (2), D. Cousins (2), C. Bone (3), S. Walker (4), J. Sun (2) (1) The Samuel Roberts Noble Foundation, U.S.A.; (2) John Innes Centre, United Kingdom; (3) University of Sheffeild, United Kingdom; (4) Babraham Institute, United Kingdom

Nodules are specialized root lateral organs in legumes which accommodate symbiotic nitrogen-fixing bacteria called rhizobia. The plant hormone auxin has been implicated in nodule organogenesis but the molecular machinery involved is presently unknown. Using pharmacological assays we found that both auxin influx and efflux transport inhibitors were sufficient to decrease nodule numbers; suggesting that the auxin transport machinery plays an important role in nodulation. Quantitative RT-PCR revealed that, of the five membered family of AUX-LAX auxin influx transporters, MtLAX2, an orthologue of the Arabidopsis AUX1 gene is the first to transcriptionally respond at the site of rhizobial inoculation. MtLAX2 spatial and temporal expression patterns, detected using promoter-GUS fusions in Medicago were associated with the expanding nodule primordia but were restricted to the meristematic zone in mature nodules. We identified two lax2 mutants which show a reduction in lateral root number and auxin insensitive root growth comparable to the Arabidopsis aux1 mutants. Importantly, both these alleles of Medicago lax2 had a sustained reduction in nodule number over time suggesting that auxin influx activity in cells of the developing nodule is critical for nodule organogenesis. lax2-1 lines transformed with the auxin marker DR5-GUS further revealed that fewer infection foci are associated with auxin accumulation. Moreover, the infection induced auxin response factor arf16a failed to accumulate to WT levels in lax2 roots. Together, these results present the first genetic evidence of a role for auxin in nodulation and puts forward a mechanistic model connecting auxin transport and auxin signalling in nodule organogenesis.

Abstract Number: C1-5, P2-59
Session Type: Concurrent