Competing for Transport

Science Signaling  22 Jun 2010:
Vol. 3, Issue 127, pp. ec185
DOI: 10.1126/scisignal.3127ec185

Plants use nitrate as a source of nitrogen and as a signaling molecule that regulates metabolism and growth. Uptake of nitrate from the soil through lateral roots (LRs) is dependent on the plasma membrane–bound nitrate transporter protein NRT1.1. The outgrowth of LRs is predominantly dependent on the phytohormone auxin. In addition to transporting nitrate ions, NRT1.1 is required for the nitrate-dependent outgrowth of lateral roots so that plants can seek out new sources of nitrate in the soil; however, as Beeckman and Friml discuss, how the transport function of NRT1.1 is coupled to auxin-dependent growth of LRs is unclear. Krouk et al. showed in experiments with wild-type and NRT1.1-deficient Arabidopsis seedlings that, at low concentrations of nitrate, NRT1.1 prevented the accumulation of auxin at the LR primordium tip and inhibited the outgrowth of LRs. Experiments in Xenopus oocytes showed that NRT1.1 could transport auxin, which was inhibitable by high concentrations of nitrate; however, auxin had no effect on the ability of NRT1.1 to transport nitrate. Histochemical analysis showed that NRT1.1 was localized along the basipetal transport route through which auxin is removed from the LR tip and distributed to epidermal cells. The authors thus suggest a model through which the transport function of NRT1.1 is coupled to nutrient sensing: When external nitrate is low in abundance, NRT1.1 transports auxin away from the LR tip, thus inhibiting LR growth; however, when external nitrate is high, it competes with auxin for binding to NRT1.1, and the accumulation of auxin at the LR triggers outgrowth so that the plant is able to take up more nitrate.

G. Krouk, B. Lacombe, A. Bielach, F. Perrine-Walker, K. Malinska, E. Mounier, K. Hoyerova, P. Tillard, S. Leon, K. Ljung, E. Zazimalova, E. Benkova, P. Nacry, A. Gojon, Nitrate-regulated auxin transport by NRT1.1 defines a mechanism for nutrient sensing in plants. Dev. Cell 18, 927–937 (2010). [Online Journal]

T. Beeckman, J. Friml, Nitrate contra auxin: Nutrient sensing by roots. Dev. Cell 18, 877–878 (2010). [Online Journal]