Ethylene Stimulates Shade Avoidance Responses

When shaded, plants receive lower doses of light, and the light actually transmitted contains proportionally more far red light than light received by unshaded plants (Fig. 2). Plants will try to move out of the shade in order to maximize photosynthesis (Vandenbussche et al. 2005). Not only is ethy-lene production enhanced under shaded conditions, optimal shade avoidance responses also require ethylene (Vandenbussche et al. 2003b; Pierik et al. 2004a,b).

In tobacco, low R:FR (ratio of red to far red light) ratios induce hyponasty independent of ethylene, but ethylene determines the rate of low R:FR-induced stem and petiole elongation (Pierik et al. 2004a). Applying ethylene to Arabidopsis plants causes an upward movement of leaves, although this response varies depending on the conditions (Vandenbussche et al. 2003b; Millenaar et al. 2005). This is caused by differential growth at the ends of competition ! y t low high low blue ethylene R/FR

ethylene signalisation T

stem elongation

Fig. 2 Shade avoidance. When plants face competition in dense canopies, different environmental signals change. The dose of transmitted light under the canopy decreases and the ratio red/far red light decreases. Under low R: FR conditions, ethylene synthesis is stimulated, leading to a higher ethylene concentration. Both a high ethylene concentration and blue light need a fully functioning ethylene and GA signaling pathway in order to affect hyponasty and stem elongation. Low R : FR ratios can stimulate hyponas-tic movements independently of ethylene and GA. In contrast, stem elongation under low R: FR conditions is dependent on a functioning GA pathway. Stem elongation is possible without ethylene signaling, although it occurs more slowly in this case both the petiole and the leaf blade. Combining ethylene treatment with low light does not yield additive effects, except for a higher petiole angle after 20 hours of exposure (Millenaar et al. 2005). When the light dose is lowered to a level comparable with that under a canopy, ethylene-resistant tobacco plants showed no stem elongation and no hyponastic response, whereas wildtype plants did show these responses (Pierik et al. 2004a). They are caused by a lower dose of blue light. Changes in leaf angle and stem elongation of competing plants occur faster in wild-type than in ethylene-resistant tobacco plants. This delay causes a competitive disadvantage, resulting in a lower biomass of ethylene-resistant plants, whereas they have the same biomass in a noncompeting setup (Pierik et al. 2004a). When GA synthesis is blocked in tobacco, ethylene-induced hyponastic movements and stem and petiole elongation are prevented, indicating the essential role of GA in these responses. However, when R:FR ratios are lowered in the presence of PAC, stem and petiole elongation are inhibited but hyponastic movement is not. In the wildtype and ethylene-resistant tobacco mutants, petiole elongation seems to be equally sensitive to GA. In contrast, stem elongation is more sensitive to GA in wild-type than in ethylene-resistant tobacco mutants. Both processes are more pronounced for low R: FR ratios (Pierik et al. 2004b). An overview of the process involvrd in shade avoidance is given in Fig. 2.

Fig. 2 Shade avoidance. When plants face competition in dense canopies, different environmental signals change. The dose of transmitted light under the canopy decreases and the ratio red/far red light decreases. Under low R: FR conditions, ethylene synthesis is stimulated, leading to a higher ethylene concentration. Both a high ethylene concentration and blue light need a fully functioning ethylene and GA signaling pathway in order to affect hyponasty and stem elongation. Low R : FR ratios can stimulate hyponas-tic movements independently of ethylene and GA. In contrast, stem elongation under low R: FR conditions is dependent on a functioning GA pathway. Stem elongation is possible without ethylene signaling, although it occurs more slowly in this case

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