The arabidopsis eer1 mutant has enhanced ethylene responses in the hypocotyl and stem

By screening for enhanced ethylene-response (eer) mutants in Arabidopsis, w e isolated a novel recessive mutant, eer1, which displays increased ethylen e sensitivity in the hypocotyl and stem. Dark-grown eer1 seedlings have sho rt and thick hypoeotyls even in the absence of added ethylene. This phenoty pe is suppressed, however, by the ethylene biosynthesis inhibitor 1-aminoet hoxyvinyl-glycine. Following ethylene treatment, the dark-grown eer1 hypoco tyl response is greatly exaggerated in comparison with the wild type, indic ating that the eer1 phenotype is not simply due to ethylene overproduction. eer1 seedlings have significantly elevated levels of basic-chitinase expre ssion, suggesting that eer1 may be highly sensitive to low levels of endoge nous ethylene. Adult eer1 plants display exaggerated ethylene-dependent ste m thickening, which is an ethylene response previously unreported in Arabid opsis, eer1. also has enhanced responsiveness to the ethylene agonists prop ylene and 2,5-norbornadiene. The eer1 phenotype is completely suppressed by the ethylene-insensitive mutation etr1-1, and is additive with the constit utive ethylene-response mutation ctr1-3. Our findings suggest that the wild -type EER1 product acts to oppose ethylene responses in the hypocotyl and s tem.