A comparative analysis of the Arabidopsis mutant amp1-1 and a novel weak amp1 allele reveals new functions of the AMP1 protein

Ethylene and gibberellins have a synergistic stimulatory effect on hypocotyl elongation of light-grown Arabidopsis thaliana (L.) Heynh. seedlings. A screen for mutants with decreased response to these hormones led to the isolation of a novel allele (amp1-7) of the ALTERED MERISTEM PROGRAM (AMP) 1 locus. The amp1-7 allele contains a missense mutation causing a phenotype, which is weaker than that of the amp1-1 mutant that carries a nonsense mutation. The mutant phenotype prompted the hypothesis that AMP1 is involved in ethylene and GA signalling pathways or in a parallel pathway-controlling cell and hypocotyl elongation and cellular organization. Amp1 mutants contain higher zeatin concentrations causing enlargement of the apical meristem, which was confirmed by cytokinin application to wild type seedlings. Light grown amp1 seedlings have shorter hypocotyls than wild type; however, application of cytokinins promotes hypocotyl elongation of both Col-0 and amp1. We suggest that in amp1 mutants either zeatin overproduction or its action is strictly localized.