The Arabidopsis mutant alh1 illustrates a cross talk between ethylene and auxin

Abstract

Ethylene or its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) can stimulate hypocotyl elongation in light-grown Arabidopsis seedlings. A mutant, designated ACC-related long hypocotyl 1 (alh1), that displayed a long hypocotyl in the light in the absence of the hormone was characterized. Etiolated alh1 seedlings overproduced ethylene and had an exaggerated apical hook and a thicker hypocotyl, although no difference in hypocotyl length was observed when compared with wild type. Alh1 plants were less sensitive to ethylene, as reflected by reduction of ACC-mediated inhibition of hypocotyl growth in the dark and delay in flowering and leaf senescence. Alh1 also had an altered response to auxin, whereas auxin levels in whole alh1 seedlings remained unaffected. In contrast to wild type, alh1 seedlings showed a limited hypocotyl elongation when treated with indole-3-acetic acid. Alh1 roots had a faster response to gravity. Furthermore, the hypocotyl elongation of alh1 and of ACC-treated wild type was reverted by auxin transport inhibitors. In addition, auxin up-regulated genes were ectopically expressed in hypocotyls upon ACC treatment, suggesting that the ethylene response is mediated by auxins. Together, these data indicate that alh1 is altered in the cross talk between ethylene and auxins, probably at the level of auxin transport.

Figure 1

Effect of ACC on hypocotyl elongation in alh1 in the light. Seedlings of wild type (white bars), alh1 (black bars), ein2-1 (gray bars), and eir1-1/pin2 (striped bars) grown for 10 d on LNM medium supplemented with ACC in a range of concentrations. Data are mean ± sd (n > 20).

Figure 2

Morphology of alh1 at a stage of development relative to the wild type and the mutant ctr1-1. Plants germinated and grown in the light for 7 d on MS/2 medium at the cotyledon stage.

Figure 3

Constitutive and insensitive responses to ACC of etiolated alh1 seedlings. A, Apical hook region of seedlings germinated and grown on MS/2 in the dark for 4 d. B, Hypocotyl length of seedlings germinated and grown in the dark for 7 d on MS/2 medium supplemented with the denoted concentrations of ACC. White bars, Wild type; black bars, alh1; gray bars, ein2-1.

Figure 4

Chlorophyll levels during rosette development of wild-type and ethylene mutants alh1, etr1-3, and ein2-1. Leaf discs of full-grown rosette leaves 7 and 8 were harvested immediately after the leaf expansion phase (d 1) and subsequently 4, 8, 12, 16, and 20 d later. Total chlorophyll levels were determined for the wild type (Col), alh1, etr1-3, and ein2-1. Error bars represent se.

Figure 5

ACC sensitivity in light-grown alh1 plants. Wild-type Col-0 and mutant plants were germinated and grown for 3 weeks in the light on MS/2 medium in the absence or presence of 50 μm ACC.

Figure 6

a, Kinetic analysis of root gravitropism in alh1 and wild type. Seedlings were germinated and grown for 6 d in the light on MS/2 medium and turned for 90° at time 0. Each point represents the mean of at least 15 measurements. The error bars indicate se. b through d, Organization of columella cells in propidium iodide stained root tips from wild-type (b) and alh1 (c and d) plants. Arrows indicate extra cells.

Figure 7

Effect of IAA on hypocotyl elongation in Col-0, ctr1-1, alh1, and ein2-1. Black triangles, alh1; white triangles, wild-type Col-0; black circles, ein2-1; white circles, ctr1-1. Seedlings were grown on LNM supplemented with the indicated concentration of IAA for 10 d in a long-day photoperiod. Error bars represent se.

Figure 8

Relation of the long hypocotyl and higher auxin transport in alh1. Seedlings were grown on LNM supplemented with the indicated concentration of NPA, tri-iodo-benzoic acid (TIBA), or 2-naphthaleneacetic acid (2-NAA) for 10 d in a long-day photoperiod. White bars, LNM; striped bars, LNM + 1 μm NPA; gray bars, LNM + 30 μm 2-NAA; black bars, LNM + 50 μm TIBA. Error bars represent se.

Figure 9

Ectopic expression of SAUR AC1-GUS and DR5-GUS reporter genes in developing ACC-treated seedlings. A, Kinetic study of the effect of ACC on hypocotyl elongation in Col-0 seedlings. Black triangles, Col-0 seedlings grown on LNM + 50 μm ACC; white circles, Col-0 seedlings on LNM. B, Seedlings grown on LNM (top row) and on LNM + 50 μm ACC (bottom row). From left to right: 3.5-d-old SAUR AC1-GUS, 8-d-old SAUR AC1-GUS, 3.5-d-old DR5-GUS, and 8-d-old DR5-GUS.