Another trait of the triple response is the shortening and thickening of the hypocotyl (Guzman and Ecker 1990; Fig. 1). The hypocotyl of Arabidopsis mainly grows by longitudinal cell expansion (Saibo et al. 2003). This elongation is not the same in all cells along the hypocotyl and differs in the light and the dark (Gendreau et al. 1997). Applying ethylene to dark-grown Arabidopsis seedlings inhibits hypocotyl cell elongation (Le et al. 2005). The decrease in cell elongation is correlated with a change in microtubule orientation. In the dark, the strongest elongation is seen in the top two-thirds of the hypocotyl, from cell 8 upwards. This correlates with a transverse microtubule orientation in the apical cells, whereas in the basal cells microtubules are longitudinally oriented (Le et al. 2005). Under the influence of ACC, dark-grown seedlings had a smaller zone at the top of the hypocotyl showing transverse micro-tubule orientation (Le et al. 2005). The decrease in the hypocotyl growth rate is already visible 15 minutes after ethylene addition and is reversible (Binder et al. 2004). A partial ethylene oversensitive mutant, eer1 (enhanced ethylene response), shows a thicker and shorter basal part of the hypocotyl as compared to the wild type when grown in the dark over a broad range of ACC concentrations (Larsen and Chang 2001). Since cell elongation in etiolated seedlings requires GA (Cowling and Harberd 1999), crosstalk between ethylene and GA was suggested. The GA signaling pathway is negatively regulated by DELLA proteins. These proteins are destabilized by GA in a 26S proteasome-dependent manner (Peng et al. 1997; Fu and Harberd 2003; Itoh et al. 2003). In contrast, ethylene stabilizes DELLA proteins (Achard et al. 2003; Vriezen et al. 2004), repressing cell elongation and hence resulting in shorter hypocotyls.
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