Ethylene Stimulates Hypocotyl Growth in the Light

When plants are grown on LNM (low-nutrient medium) in the light, hypocotyl elongation (up to twofold) is induced by concentrations of ACC above 1 ^M (Smalle et al. 1997). The response is saturated at 20 ^M ACC. When grown on MS/2, the addition of ACC results in an up to 30% longer hypocotyl. Adding AgNO3—a known blocker of ethylene action—to the medium inhibits this response. This result correlates with ctr1 displaying a longer hypocotyl than the wild type when grown in the light in the absence of ACC, whereas etr1-3 does not elongate when treated with ACC. Adding ethylene to the seedlings induces the same effect as ACC, and the response in ethylene is abolished by combination with 1-MCP—another inhibitor of ethy-lene perception (De Paepe and Van Der Straeten 2005). The elongation with ethylene/ACC is a result of cell elongation. Several other hormonal signals confer hypocotyl elongation on LNM. Plants grown on LNM supplied with IAA also have longer hypocotyls (Smalle et al. 1997; Saibo et al. 2003). Van-denbussche et al. (2003a) demonstrated that the ethylene-regulated hypocotyl elongation depends on a functional auxin transport system. In addition, GA3 stimulates hypocotyl elongation (Cowling and Harberd 1999). Mutants that are defective in GA synthesis (Sun et al. 1992) or auxin signaling (Timpte et al. 1992) were shown to have shorter hypocotyls, indicating the need for both hormones for hypocotyl elongation. Saibo et al. (2003) presented a model for a network of interactions between ethylene, GA3 and auxins regulating hypocotyl growth. Growth primarily occurs within the first three days after germination, but when ACC is applied to the medium, this fast growth phase is prolonged by one day. In contrast, GA3 does not prolong the growth phase, but enhances the growth rate between day 2 and day 3 after germination. After inhibiting growth during the first two days, IAA prolongs the growth period until the sixth day after germination. Nevertheless, ACC also works independently of GA since the GA biosynthesis inhibitor paclobutrazol (PAC) only reduced the effect of ACC, whereas IAA-treated plants show no elongation when PAC is added to the medium (Saibo et al. 2003).

The ethylene signal responsible for elongation is at least partially separated from other ethylene-dependent effects. The over-expression of the EIN2 carboxyl terminus is sufficient to rescue the ethylene-dependent elongation in ein2-5, but it does not rescue the triple response (Alonso et al. 1999). Furthermore, ACC treatment also causes radial expansion of the hypocotyl which is independent of GA (Saibo et al. 2003). This indicates that at least partially different pathways control the effects of ethylene upon elongation and radial expansion.

The three abovementioned hormones achieve their effects by stimulating cell elongation. This is in part accompanied by endoreduplication. GA3 is required for endoreduplication (Gendreau et al. 1999). The effect of GA3 was enhanced by adding ACC (Saibo et al. 2003).

Recent studies also implicate a role for brassinosteroids in ethylene-regulated elongation. Wild-type seedlings treated with Brz2001 (brassinos-teroid synthesis inhibitor), cbbl and det2 (mutants in BR biosynthesis) do not show an increase in hypocotyl length in the light when treated with ACC (De Grauwe et al. 2005). The hypocotyl of hlsl is ethylene-insensitive in the light. When both ethylene and EBR (epi-brassinolide, physiological active brassi-

brassinosteroids GA ethylene auxin brassinosteroids GA ethylene auxin

cell growth

hypocotyl growth

Fig. 1 Hypocotyl elongation in the light. Hypocotyl growth is determined by two major factors: growth rate and duration of the growth period. Ethylene and auxin prolong the duration of elongation growth, whereas GA and possibly brassinosteroids affect the growth rate. If the latter is disturbed, prolonging the growth period will have no effect, supporting the dominant role of GA in this process. The dotted arrow indicates the fact that there also is a gibberellin-independent mechanism by which ethylene exerts its role nosteroid) are applied to hlsl, they show a synergistic effect. This indicates that brassinosteroids function downstream or independent of the ethylene signal (De Grauwe et al. 2005). The hypocotyl of saxl (Ephritikhine et al. 1999; hyperSensitive to Abscisic acid and auXin, necessary for BR biosynthesis) is insensitive to ACC and GA. When EBR is applied, the hypocotyl regains its sensitivity to GA, but not to ACC (Ephritikhine et al. 1999). A schematic overview of these interactions is presented in Fig. 1.

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