Phytochrome B promotes branching in Arabidopsis by suppressing auxin signaling

Abstract

Many plants respond to competition signals generated by neighbors by evoking the shade avoidance syndrome, including increased main stem elongation and reduced branching. Vegetation-induced reduction in the red light:far-red light ratio provides a competition signal sensed by phytochromes. Plants deficient in phytochrome B (phyB) exhibit a constitutive shade avoidance syndrome including reduced branching. Because auxin in the polar auxin transport stream (PATS) inhibits axillary bud outgrowth, its role in regulating the phyB branching phenotype was tested. Removing the main shoot PATS auxin source by decapitation or chemically inhibiting the PATS strongly stimulated branching in Arabidopsis (Arabidopsis thaliana) deficient in phyB, but had a modest effect in the wild type. Whereas indole-3-acetic acid (IAA) levels were elevated in young phyB seedlings, there was less IAA in mature stems compared with the wild type. A split plate assay of bud outgrowth kinetics indicated that low auxin levels inhibited phyB buds more than the wild type. Because the auxin response could be a result of either the auxin signaling status or the bud's ability to export auxin into the main shoot PATS, both parameters were assessed. Main shoots of phyB had less absolute auxin transport capacity compared with the wild type, but equal or greater capacity when based on the relative amounts of native IAA in the stems. Thus, auxin transport capacity was unlikely to restrict branching. Both shoots of young phyB seedlings and mature stem segments showed elevated expression of auxin-responsive genes and expression was further increased by auxin treatment, suggesting that phyB suppresses auxin signaling to promote branching.

Figure 1.

The number of rosette branches (A and B), CIIs (C and D), and branch lengths (E and F) of intact and decapitated (A, C, and E) or control and TIBA-treated (B, D, and F) wild type and phyB at 10 DPA. Data are means ± se with n = 72 (decapitation) or n = 50 (TIBA). Asterisks indicate a significant difference between intact and decapitated values or control and TIBA-treated values within a genotype at α = 0.05. WT, Wild type.

Figure 2.

IAA abundance in the wild-type and phyB shoots of 14-d-old seedlings (A) and basal stem segments of mature plants, expressed on a per-weight basis (B) and per 15-mm stem segment (C). Data are means ± se with n = 4. Asterisks indicate a significant difference between the wild type and phyB at α = 0.05. WT, Wild type.

Figure 3.

Elongation of the wild-type and phyB axillary buds with and without apically supplied auxin (200 nm NAA), using a split plate in vitro assay. Data are means ± se with n = 18 to 20. Number signs and asterisks indicate significant differences between control and treated buds at α = 0.05 for the wild type and phyB, respectively. WT, Wild type.

Figure 4.

Auxin transport in main shoots of the wild type and phyB supplied with 2.5 pmol (A), 25 pmol (B), and 250 pmol (C) supplemental IAA. Data are means ± se with n = 5. Asterisks indicate a significant difference between the wild type and phyB at α = 0.05. WT, Wild type.

Figure 5.

Expression of auxin-responsive genes in shoots of 14-d-old seedlings (A–H) and in basal stem segments of mature plants (at anthesis, I–P) of the wild type and phyB with and without auxin (50 μm NAA) treatment. Data are means ± se with n = 4. Asterisks indicate a significant difference between corresponding treatments of the wild type and phyB at α = 0.05. WT, Wild type.