Ethylene-induced differential growth of petioles in Arabidopsis. Analyzing natural variation, response kinetics, and regulation

Abstract

Plants can reorient their organs in response to changes in environmental conditions. In some species, ethylene can induce resource-directed growth by stimulating a more vertical orientation of the petioles (hyponasty) and enhanced elongation. In this study on Arabidopsis (Arabidopsis thaliana), we show significant natural variation in ethylene-induced petiole elongation and hyponastic growth. This hyponastic growth was rapidly induced and also reversible because the petioles returned to normal after ethylene withdrawal. To unravel the mechanisms behind the natural variation, two contrasting accessions in ethylene-induced hyponasty were studied in detail. Columbia-0 showed a strong hyponastic response to ethylene, whereas this response was almost absent in Landsberg erecta (Ler). To test whether Ler is capable of showing hyponastic growth at all, several signals were applied. From all the signals applied, only spectrally neutral shade (20 micromol m(-2) s(-1)) could induce a strong hyponastic response in Ler. Therefore, Ler has the capacity for hyponastic growth. Furthermore, the lack of ethylene-induced hyponastic growth in Ler is not the result of already-saturating ethylene production rates or insensitivity to ethylene, as an ethylene-responsive gene was up-regulated upon ethylene treatment in the petioles. Therefore, we conclude that Ler is missing an essential component between the primary ethylene signal transduction chain and a downstream part of the hyponastic growth signal transduction pathway.

Figure 1.

Representative example of Arabidopsis accession Col-0 treated with air (A) or 5 μL L⁻¹ ethylene (B) for 24 h. The petiole-leaf blade junction of the leaf under examination is marked with drawing ink (see arrows). A calibration object is placed in the same plane as the marked petiole. A black square on the calibration object has a dimension of 5 × 5 mm. α, Petiole angle.

Figure 2.

Petioles angle (squares) and length (circles) of 9 Arabidopsis accessions treated with 5 μL L⁻¹ ethylene or air. Plants were in continuous light during the experiment; the black bars represent the time when the night period would normally have taken place. Data are means of eight replicate plants from two separately grown batches per accession. Error bars = se.

Figure 3.

Effect of ethylene on the elongation of petioles (A) and leaf blades (B) of Arabidopsis accessions after 24 h. Data are means of eight replicate plants from two separately grown batches per accession. Error bars = se. +, P < 0.1; *, P < 0.05; **, P < 0.01. Note that the order in which the accessions are presented is based on the effect of the ethylene treatment.

Figure 4.

Differential change in petiole angle (mean ethylene − mean air; Fig. 2) for nine Arabidopsis accessions. The ethylene-induced hyponastic growth is divided into three groups: large (black symbols), intermediate (white symbols), and small (gray symbols) hyponastic response.

Figure 5.

The effect of switching off the ethylene supply after 6 h of treatment on the petiole angle of Arabidopsis Col-0. Following discontinuation of the ethylene supply, the ethylene concentration in the cuvette declined to ambient levels within 40 min. Approximately 2 h later, hyponasty started to reverse and, after approximately 10 h (= 16 h after start ethylene treatment), petiole angles had returned to control values. Data are means of four replicate plants. Error bars = se.

Figure 6.

Effect of angle manipulation (A and B), submergence (C and D), high temperature (E and F), and low light (G and H) on hyponastic growth in Ler (left) and Col-0 (right). Initial petiole angle is manipulated by tilting the pot at the start of the experiment. The high-temperature treatment is a shift from 20°C to 30°C. Low light is a spectrally neutral decrease in light intensity from 200 to 20 μmol m⁻² s⁻¹. Data are means of four replicate plants. Error bars = se.

Figure 7.

Ethylene dependency of submergence-induced hyponastic growth in Arabidopsis Ler (left) and Col-0 (right). Plants are pretreated with ethylene receptor antagonist 1-MCP (triangles) before submergence. The ethylene-insensitive receptor mutant etr1-1 in a Col-0 background (diamonds) was submerged. Data are means of four replicate plants. Error bars = se.