Ethylene Inhibits Leaf Expansion

The constitutive ethylene mutant ctrl has a dwarfed phenotype in the light with very limited leaf expansion. This is mainly caused by a reduction in cell size (Kieber et al. 1993; Rodrigues-Pousada et al. 1993). The opposite is seen in ethylene-insensitive mutants, with a slightly larger leaf surface than the wild type. Although the extra leaf area is thought to result from larger cells (Bleecker et al. 1988; Hua et al. 1995), no detailed measurements of cell-elongation rates in leaves of ethylene-insensitive plants are available. Therefore, it is possible that the greater leaf surface can be explained either by an extended expansion phase or by a higher expansion rate (Pierik et al. 2006). Smalle et al. (1999) showed that in wild-type plants ACS1 mRNA levels are lower during leaf surface expansion than during leaf emergence or senescence. This is also reflected in a lower ethylene production. Although it cannot be excluded that this pattern is an effect of leaf development rather than being the causal factor, it does implement an inhibiting role of ethy-lene in leaf cell expansion (Smalle et al. 1999). Lower ethylene concentrations result in a relief of repression and consequent leaf expansion.

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