| Osmotic stress my result from drought, high salinity, or freezing temperatures. All three stresses cause a reduction in water potential, efflux of water from cells, and a loss of turgor. Plants have evolved a variety of responses to cope with osmotic stress. The regulation of these responses in higher plants and in a cyanobacterial model system are the focus of this dissertation.; Salt-induced genes were identified in the cyanobacterium, Anabaena sp. PCC 7120, by promoter trapping with a Tn5::luxAB construct. Second-site mutagenesis was used to identify regulatory components necessary for the salt-induction of this gene. One mutant which displays reduced luciferase activity during salt stress has an insertion in an ORF with sequence similarity to response regulators from two-component regulatory systems. The mutation was reconstructed with an interposon based vector and shown to have the same phenotype.; In higher plants, the hormone abscisic acid (ABA) regulates many responses to osmotic stress. ABA is formed by the oxidative cleavage of an epoxy-carotenoid. This is the first committed reaction in ABA biosynthesis. The reaction is of general interest, because the synthesis of other apocarotenoids, such as vitamin A in animals, may occur by a similar mechanism. A new ABA-deficient mutant of maize has been identified and the corresponding gene, VP14 , has been cloned. The recombinant VP14 protein catalyzes the cleavage of 9-cis-epoxy-carotenoids to form C25 apo-aldehydes and xanthoxin, a precursor of ABA in higher plants.; Following the cleavage reaction, xanthoxin is oxidized to ABA in two steps. The aba2 and aba3 mutants of Arabidopsis, are impaired in these later steps. The aba2 mutant is blocked in the conversion of xanthoxin to ABA-aldehyde and aba3 is impaired in the conversion of ABA-aldehyde to ABA. Extracts from the aba3 mutant also lack several additional activities which require a molybdenum cofactor (Moco). Nitrate reductase utilizes a Moco but its activity is unaffected in extracts from aba3 plants. Further characterization of the aba3 mutant indicates that it is impaired in the introduction of sulfur into the Moco. |
