The biosynthesis of abscisic acid in ripening avocado fruit and ethylene biosynthesis in ferns.

摘要

Plant hormones play critical roles in growth and development. The understanding of the biosynthesis of plant hormones makes possible the manipulation of their levels, which in turn has important practical implications. The biosynthesis of two plant hormones, ethylene and abscisic acid, was the focus of this thesis.;Ethylene is a gaseous two-carbon olefin that has numerous roles such as in fruit ripening, senescence, and abscission. In higher plants, ethylene is synthesized from S-adenosyl-methionine, which in turn is converted into 1-aminocyclopropanoic acid (ACC) through the action of ACC synthase. The ACC is converted into ethylene by ACC oxidase. The pathway of ethylene biosynthesis was examined in two lower plants, the semi-aquatic ferns Regnellidum diphyllum Lindm. and Marsilea quadrifolia L. As a positive control for the ethylene biosynthetic pathway of higher plants, leaves of Arabidopsis thaliana (L.) Heynh. were included in each experiment. Treatments that either increased or inhibited ethylene production in Arabidopsis thaliana did not affect ethylene production in Marsilea and in Regnellidium. Despite the apparent differences, ACC was detected in both ferns, as was ACC synthase activity. Compared to Arabidopsis, leaflets of Regnellidium and Marsilea incorporated little [ 14C] ACC and [14C] methionine into [14C]ethylene. From these data, it appears that formation of ethylene in both ferns occurs mainly, if not only, via an ACC independent route, even though the capacity to synthesize ACC is present in both of these lower plants.;Abscisic acid (ABA) is a 15-carbon sesquiterpenoid derived from the oxidative cleavage of epoxycarotenoids. ABA levels increase dramatically when a wilting stress is imposed on leaves. The key regulatory step governing this increase is cleavage from carotenoids. The gene encoding the 9-cis-epoxycarotenoid dioxygenase (NCED) that catalyzes this cleavage reaction was first cloned from a viviparous mutant of maize, vp14.;To address whether the developmental increase in ABA level that occurs during avocado fruit ripening is governed by oxidative cleavage from carotenoids, three Vp14 homologs were cloned from ripening avocado fruit. Two of these homologs, PaNCED1 and PaNCED3, are approximately 60% identical at the amino acid level to Vp14, and increase in expression during fruit ripening. When expressed as recombinant proteins, both PaNCED1 and PaNCED3 could catalyze the cleavage of 9- cis-violaxanthin and 9-cis-neoxanthin into xanthoxin. A third Vp14 homolog cloned from avocado fruit, PaNCED2, is 30% identical at the amino acid level to Vp14 , and is constitutively expressed during both fruit ripening and during wilting of leaves.;Hormone levels are determined by both their rates of synthesis and degradation. ABA is metabolized by a cytochrome P450 monooxygenase, ABA-8' -hydroxylase, into phaseic acid. As it is been demonstrated that the induction of gene encoding ABA-8'-hydroxylase occurs at a transcriptional level, a modified differential display approach was used to isolate cytochrome P450 monooxygenase gene that may encode ABA 8 ' hydroxylase.