The Arabidopsis eer1 Mutant Has Enhanced Ethylene Responses in the Hypocotyl and Stem

By screening for enhanced ethylene-response (eer) mutants in Arabidopsis, we isolated a novel recessive mutant, eer1, which displays increased ethylene sensitivity in the hypocotyl and stem. Dark-grown eer1 seedlings have short and thick hypocotyls even in the absence of added ethylene. This phenotype is suppressed, however, by the ethylene biosynthesis inhibitor 1-aminoethoxyvinyl-glycine. Following ethylene treatment, the dark-grown eer1 hypocotyl response is greatly exaggerated in comparison with the wild type, indicating that the eer1 phenotype is not simply due to ethylene overproduction. eer1 seedlings have significantly elevated levels of basic-chitinase expression, suggesting that eer1 may be highly sensitive to low levels of endogenous ethylene. Adult eer1 plants display exaggerated ethylene-dependent stem thickening, which is an ethylene response previously unreported in Arabidopsis. eer1 also has enhanced responsiveness to the ethylene agonists propylene and 2,5-norbornadiene. The eer1 phenotype is completely suppressed by the ethylene-insensitive mutation etr1-1, and is additive with the constitutive ethylene-response mutation ctr1-3. Our findings suggest that the wild-type EER1 product acts to oppose ethylene responses in the hypocotyl and stem.

本研究通过在拟南芥（Arabidopsis）中筛选乙烯反应增强（enhanced ethylene-response, eer）突变体，分离得到一个全新的隐性突变体eer1，该突变体在下胚轴与茎段中表现出增强的乙烯敏感性。即使在未添加乙烯的条件下，暗培养的eer1幼苗下胚轴仍表现为短粗形态；然而，该表型可被乙烯生物合成抑制剂1-氨基乙氧基乙烯基甘氨酸（1-aminoethoxyvinyl-glycine）抑制。经乙烯处理后，暗培养的eer1幼苗下胚轴的反应相较于野生型显著增强，表明eer1的表型并非单纯由乙烯过量产生所导致。eer1幼苗的碱性几丁质酶（basic-chitinase）表达水平显著升高，提示eer1对低浓度内源乙烯（endogenous ethylene）具有高度敏感性。成熟eer1植株的乙烯依赖型茎段增粗现象显著增强，这是拟南芥中此前未曾报道过的乙烯反应。eer1对乙烯激动剂（ethylene agonists）丙烯（propylene）与2,5-降冰片二烯（2,5-norbornadiene）的响应同样增强。eer1的表型可被乙烯不敏感突变etr1-1（ethylene-insensitive mutation etr1-1）完全抑制，且与组成型乙烯反应突变ctr1-3（constitutive ethylene-response mutation ctr1-3）呈现叠加遗传效应。本研究结果表明，野生型EER1蛋白的功能是负调控下胚轴与茎段中的乙烯反应。