Ethylene and auxin control the Arabidopsis response to decreased light intensity

Abstract

Morphological responses of plants to shading have long been studied as a function of light quality, in particular the ratio of red to far red light that affects phytochrome activity. However, changes in light quantity are also expected to be important for the shading response because plants have to adapt to the reduction in overall energy input. Here, we present data on the involvement of auxin and ethylene in the response to low light intensities. Decreased light intensities coincided with increased ethylene production in Arabidopsis rosettes. This response was rapid because the plants reacted within minutes. In addition, ethylene- and auxin-insensitive mutants are impaired in their reaction to shading, which is reflected by a defect in leaf elevation and an aberrant leaf biomass allocation. On the molecular level, several auxin-inducible genes are up-regulated in wild-type Arabidopsis in response to a reduction in light intensity, including the primary auxin response gene IAA3 and a protein with similarity to AUX22 and the 1-aminocyclopropane-1-carboxylic acid synthase genes ACS6, ACS8, and ACS9 that are involved in ethylene biosynthesis. Taken together, the data show that ethylene and auxin signaling are required for the response to low light intensities.

Figure 1.

Ethylene production in different light intensities. A, Ethylene production from 2-week-old Arabidopsis wild-type and mutant phyB-9 rosettes. Plants were kept in gas-tight vials for 2.5 h. Accumulated ethylene was measured using photo-acoustic detection. White bars = 125 μmol m^–2 s^–1 PPFD. Black bars = 35 μmol m^–2 s^–1 of PPFD. Error bars = se. B, Changes in ethylene production in ACC-treated rosettes upon a switch in light intensity from 30 to 125 μmol m^–2 s^–1 PPFD. Measurements were performed online in 2-week-old plants on medium containing 50 μm ACC. Black triangles, CO₂ uptake; white circles, emanated ethylene. Arrowhead, Time point of light switch. FW, Fresh weight. B and C, Changes in ethylene production in ACC-treated rosettes upon a switch in light intensity from 125 to 30 μmol m^–2 s^–1 PPFD. Measurements were performed online in 2-week-old plants on medium containing 50 μm ACC. Black triangles, CO₂ uptake; white circles, emanated ethylene. Arrowhead, Time point of light switch. FW, Fresh weight.

Figure 2.

A, Alh1 has a larger leaf elevation angle. Left to right, Wild-type Col-0, alh1, and PhyB-9. Plants were grown for 33 d on soil at a PPFD of 30 μmol m^–2 s^–1. B, Treatment of wild-type Arabidopsis with ethylene keeps leaves vertically oriented and phenocopies alh1. Left, Air-treated alh1; middle, air treated wild type Col-0; right, ethylene-treated wild type. Plants were grown for 4 d on Murashige and Skoog/2 + 1% (w/v) Suc. Subsequently, they were exposed to 100 nL L^–1 ethylene for 6 d.

Figure 3.

Ethylene and auxin mutants differ from wild type in elevation angle. Plants were grown on soil at a PPFD of 125 μmol m^–2 s^–1 (white bars) or 45 μmol m^–2 s^–1 (black bars), with or without supplemented FR. Error bars = se.

Figure 4.

The auxin-insensitive mutants axr1-3 and axr2-1 are attenuated in the leaf elevation response induced by ethylene. Upper row, Untreated plants; lower row, plants treated with 100 nL L^–1 of ethylene for 6 d. Left to right, Col-0, axr1-3, and axr2-1.

Figure 5.

Ethylene and auxin mutants differ from wild type in allocation of biomass in low light intensities. Plants were grown for 1 week in 45 μmol m^–2 s^–1 PPFD. Photographs were taken 1.5 weeks after transfer to the indicated light conditions.

Figure 6.

Comparison of biomass allocation in ethylene mutants (A), auxin mutants (B), and the effect of exogenous ACC (C), using a defined parameter: degree of similarity to shade avoidance (DSA). DSA = (petiole length)² × (leaf blade surface area)^–1. Plants were grown for 1 week in 45 μmol m^–2 s^–1 PPFD. After transfer (1.5 weeks) to 125 μmol m^–2 s^–1 (white bars), 45 μmol m^–2 s^–1 (gray bars), or 15 μmol m^–2 s^–1 (black bars), leaf 4 was analyzed for leaf blade surface and petiole length. Error bars = se.

Figure 7.

A, Cluster of shade-regulated fragments derived from cDNA-AFLP transcript profiling analysis. Colored squares, Relative transcript expression values of the genes in leaf blades and petioles of rosettes that were grown either in 65 or 15 μmol m^–2 s^–1 PPFD. Red, Up-regulated; green, down-regulated; black, neutral; gray, missing value. B, Histograms of the expression pattern in leaf blades and petioles of auxin-related genes IAA3 and At4g32285 derived from the semiquantitative analysis of cDNA-AFLP fragments, represented in A. Expression of the genes at a higher light intensity (65 μmol m^–2 s^–1, white bars) and a lower light intensity (15 μmol m^–2 s^–1, gray bars) are compared. Values are normalized and express the induction or repression compared with the mean of all samples, which is set as 0. The following formula was used: normalized value = (sample value – mean value of all samples) × (sd of all samples)^–1. A value lower than 0 represents a repression and higher than 0 an induction relative to the mean value of all tested samples.

Figure 8.

RT-PCR analysis of ethylene biosynthesis enzymes in wild type and phyB-9 mutants. RNA was prepared from leave blades and petioles from plants grown in 65 μmol m^–2 s^–1 (65) or 15 μmol m^–2 s^–1 (15).