ECOTYPIC DIFFERENCES IN RHYTHMICITY OF ETHYLENE PRODUCTION IN STELLARIA LONGIPES - THE POSSIBLE ROLES OF ACC, MACC, AND ACC OXIDASE

The alpine tundra ecotype of Stellaria longipes is characterized by a dwarf phenotype, whereas the prairie ecotype can be semidwarf or highl y elongated depending on its environment. Related to their ability to elongate, these ecotypes also show divergent abilities to produce and respond to ethylene. The prairie ecotype produces a strong daily rhyth m of ethylene, which is maintained even following stress events such a s wind. The alpine ecotype exhibits a much less pronounced rhythm but greatly increases ethylene production in response to stress. We invest igated what differences in ethylene synthesis might be responsible for the ability of the prairie ecotype to produce a large and regular dai ly rhythm of ethylene production, which in the alpine ecotype is weake r or sometimes absent. Levels of the immediate precursor to ethylene, 1-aminocyclopropane-1-carboxylate (ACC), and its major conjugate, malo nyl ACC (MACC) showed no rhythm across the course of a day. Moreover A CC levels remained stable during an entire growth cycle (21 days) in t he prairie ecotype, even though ethylene is known to increase especial ly during periods of rapid elongation. By contrast, assays of ACC oxid ase performed in vivo and in vitro showed rhythms of activity similar to those of ethylene production observed in the prairie ecotype. Howev er, the levels of ethylene produced in the ACC oxidase assays were con siderably higher than levels of ethylene normally produced by unstress ed plants, and the rhythm of ACC oxidase activity was observed in both ecotypes, despite the fact that alpine Stellaria longipes exhibits a less pronounced ethylene rhythm. Thus, we concluded that although ACC oxidase activity may partially account for rhythmic production of ethy lene in prairie ecotypes, other controlling factors such as spatial se paration of ACC from ACC oxidase should be investigated.