Plant biology

Reducing Auxin Transport

Sci. Signal.  24 Aug 2010:
Vol. 3, Issue 136, pp. ec256
DOI: 10.1126/scisignal.3136ec256

As plants grow, axillary meristems form buds that can give rise to lateral branches, but these buds are inhibited from activation by auxin produced by the apical meristem, a phenomenon known as apical dominance. Polar auxin transport (PAT) is the process by which auxin is moved from the apex down the stem toward the roots. Strigolactones (SLs) are carotenoid-derived hormones that move from the roots upward and have been proposed to play a role in regulating auxin transport. The more axillary growth (max) mutants of Arabidopsis thaliana are implicated in SL biosynthesis and signaling and show increased branching as compared with wild type (WT). Crawford et al. demonstrated that MAX1, a cytochrome P-450, is likely required for SL biosynthesis, because application of the synthetic SL GR24 rescued the max1 branching phenotype. Exogenously applied GR24 also reduced the amount of radiolabeled auxin transported in both WT and max4 mutants, in which SL biosynthesis is affected, but not in plants lacking max2, which encodes a protein implicated in SL signal transduction. By expressing max1 under the control of an inducible promoter in max1 mutants and thereby controlling the timing of SL production, the authors showed that endogenously produced SLs also decreased auxin transport in explants. The auxin efflux carrier PIN1 localizes to the basal side of xylem parenchyma cells in wild-type (WT) plants, and its accumulation increases in max mutants. GR24 treatment reduced accumulation of a fluorescently tagged PIN1 recombinant protein in WT plants and in max1 mutants, but not in max2 mutants. Overexpression of PIN1 had no effect on branching, which indicated that SL inhibited PIN1 accumulation posttranscriptionally. Finally, the ability of G24 to inhibit bud activation depended on the presence of auxin in both whole plants and explant assays. In WT explants that contained two buds, the buds grew at different rates, such that one bud often outgrew the other; in max mutants, the buds grew at about the same rate, which suggested that SLs were required for competition between buds. The authors proposed a model in which SLs inhibit bud activation by inhibiting auxin transport, which is required to maintain axillary bud quiescence. This overall decrease in auxin transport also enhances competition between buds.

S. Crawford, N. Shinohara, T. Sieberer, L. Williamson, G. George, J. Hepworth, D. Müller, M. A. Domagalska, O. Leyser, Strigolactones enhance competition between shoot branches by dampening auxin transport. Development 137, 2905–2913 (2010). [Abstract] [Full Text]