Basipetal auxin transport is required for gravitropism in roots of Arabidopsis

Abstract

Auxin transport has been reported to occur in two distinct polarities, acropetally and basipetally, in two different root tissues. The goals of this study were to determine whether both polarities of indole-3-acetic acid (IAA) transport occur in roots of Arabidopsis and to determine which polarity controls the gravity response. Global application of the auxin transport inhibitor naphthylphthalamic acid (NPA) to roots blocked the gravity response, root waving, and root elongation. Immediately after the application of NPA, the root gravity response was completely blocked, as measured by an automated video digitizer. Basipetal [(3)H]IAA transport in Arabidopsis roots was inhibited by NPA, whereas the movement of [(14)C]benzoic acid was not affected. Inhibition of basipetal IAA transport by local application of NPA blocked the gravity response. Inhibition of acropetal IAA transport by application of NPA at the root-shoot junction only partially reduced the gravity response at high NPA concentrations. Excised root tips, which do not receive auxin from the shoot, exhibited a normal response to gravity. The Arabidopsis mutant eir1, which has agravitropic roots, exhibited reduced basipetal IAA transport but wild-type levels of acropetal IAA transport. These results support the hypothesis that basipetally transported IAA controls root gravitropism in Arabidopsis.

Figure 1

NPA inhibits the gravity response, root waving, and root elongation. Roots were in continuous contact with NPA for the duration of this experiment. The gravity response (□) was measured 24 h after gravity stimulation of roots. Waving (▴) and root growth (○) were measured after 7 d on 1.5% (w/v) agar plates. The average and se for 10 plants are shown. Control values were: gravity response = 87.6°; length = 47 mm; and waving = 8.5 waves per root.

Figure 2

Kinetics of the gravity response and elongation of Arabidopsis roots in the presence and absence of NPA. A, The angles of four 160-μm segments are compared with control samples having solid lines and NPA-treated samples having dotted lines. ●, 0 to 160 μm; □, 80 to 240 μm; ▴, 160 to 320 μm; ○, 240 to 400 μm. B, The length along each side of the root is shown over time for plants grown in the presence or absence of NPA. ▵, Lower control; ○, upper control; ♦, lower NPA treated; ●, upper NPA treated.

Figure 3

Basipetal [³H]IAA movement is greater at the tip of Arabidopsis roots. An agar line containing 100 nm [³H]IAA with (black bars) or without (white bars) 100 μm NPA was applied to the root tip, and after 5 h of transport the apical 1 mm was excised and discarded. The root tip was sectioned into 2-mm segments, and the amount of [³H]IAA was determined in each region. The data are the average and se of 14 plants.

Figure 4

Localized application of NPA at the root tip inhibits the gravity response. Plants were continuously treated with agar with or without NPA and grown for 24 h vertically and another 24 h after reorientation by 90°. New growth and the angle of curvature were measured and the average and se for 10 plants are shown. ●, Gravity response; □, growth.

Figure 5

The gravity response depends on the location of applied NPA. NPA was continuously applied to roots for the entire experiment at the root tip or root-shoot junction, or by addition to the agar plate on which the plant was grown. During the first 24 h, the growth was vertical, then the plants were reoriented by 90°, and the angle of curvature was determined after an additional 24 h of growth. The average and sd of 10 roots are reported. □, Global application; ○, root-shoot junction application; ▴, root tip application.

Figure 6

Excised root tips respond to gravity. The apical 5 mm of roots were excised and the plates reoriented 90°. The roots were grown for 4 additional d and then photographs were taken. A, Entire plate. B, Close-up of excised root tips.

Figure 7

NPA inhibits the gravity response in root tips (□) and in intact plants (●). Four-day-old intact seedlings and excised root tips were placed on agar plates containing the indicated concentrations of NPA for the entire experiment. Plants were grown for 24 h in continuous light and then reoriented 90° relative to the gravity vector. Growth and angle of curvature were measured 24 h after reorientation, and the average and se of 10 plants are shown.