Flavonoids act as negative regulators of auxin transport in vivo in arabidopsis

Abstract

Polar transport of the plant hormone auxin controls many aspects of plant growth and development. A number of synthetic compounds have been shown to block the process of auxin transport by inhibition of the auxin efflux carrier complex. These synthetic auxin transport inhibitors may act by mimicking endogenous molecules. Flavonoids, a class of secondary plant metabolic compounds, have been suggested to be auxin transport inhibitors based on their in vitro activity. The hypothesis that flavonoids regulate auxin transport in vivo was tested in Arabidopsis by comparing wild-type (WT) and transparent testa (tt4) plants with a mutation in the gene encoding the first enzyme in flavonoid biosynthesis, chalcone synthase. In a comparison between tt4 and WT plants, phenotypic differences were observed, including three times as many secondary inflorescence stems, reduced plant height, decreased stem diameter, and increased secondary root development. Growth of WT Arabidopsis plants on naringenin, a biosynthetic precursor to those flavonoids with auxin transport inhibitor activity in vitro, leads to a reduction in root growth and gravitropism, similar to the effects of synthetic auxin transport inhibitors. Analyses of auxin transport in the inflorescence and hypocotyl of independent tt4 alleles indicate that auxin transport is elevated in plants with a tt4 mutation. In hypocotyls of tt4, this elevated transport is reversed when flavonoids are synthesized by growth of plants on the flavonoid precursor, naringenin. These results are consistent with a role for flavonoids as endogenous regulators of auxin transport.

Figure 1

The effect of naringenin and NPA on Arabidopsis root development. Photographs showing seedlings grown for 12 d on 100 μm naringenin (A) or 0.1% (v/v) ethanol control (B). C, Roots were grown for 12 d on naringenin and NPA. Gravity response was measured 24 h after reorientation of roots 90 degrees relative to the gravity vector. Values represent the average and se of 10 seedlings per data point.

Figure 2

Comparison of phenotype of WT and tt4(2YY6) plants. A, The aerial phenotype of representative WT (left) and tt4(2YY6) (right) plants were compared 37 d after planting. B, Secondary root development of three WT seedlings (left) and three tt4(2YY6) seedlings (right) grown under continuous light for 13 d are compared.

Figure 3

Quantification of inflorescence phenotypes of WT and tt4 (2YY6) plants. A, Primary inflorescence height was monitored from d 28 until d 55 after planting. B, The number of secondary inflorescences were measured over time.

Figure 4

Comparison of basipetal IAA transport over time in WT and tt4(2YY6) plants. Transport at 2, 6, or 18 h was measured using 28 nm [³H]IAA. Each value represents the average and se of four segments.

Figure 5

Comparison of [³H]IAA transport measurements down the inflorescence stem of WT and tt4(2YY6) plants. Transport was measured on adjacent segments down the stem after 9 h in 134 nm [³H]IAA. Data represent the average and se of five segments. IAA transport in the presence of NPA was similar between WT and tt4(2YY6), averaging 98 and 96 cpm, respectively.