Disease relevance of AOS

• We have designed the synthesized of a series of novel imidazole derivatives and tested them in a bioassay as AOS inhibitors using a purified recombinant AOS enzyme isolated from Arabidopsis and expressed in E. coli [1].
• Identification of Arabidopsis thaliana genes responsive to plant cell-wall-degrading enzymes of Erwinia carotovora subsp. carotovora led to the isolation of a cDNA clone with high sequence homology to the gene for allene oxide synthase, an enzyme involved in the biosynthesis of jasmonates [2].

High impact information on AOS

• In a coupled reaction with allene oxide synthase, AOC2 releases the first cyclic and biologically active metabolite, 12-oxo-phytodienoic acid (OPDA) [3].
• Purified monogalactosyl diglyceride was not converted to the sn1-(12-oxophytodienoyl) derivative by the combined action of (soybean) lipoxygenase and (A. thaliana) allene oxide synthase, an enzyme ensemble that converts free alpha-linolenic acid to free 12-oxophytodienoic acid [4].
• Northern-blot analysis showed that IAA induced the jasmonate-responsive lipoxygenase 2, AOS, and AtVSP gene transcripts and induction was strongly impaired in axr1-3 [5].
• The male-sterile phenotype was completely rescued by exogenous application of methyl jasomonate and by complementation with constitutive expression of the AOS gene [6].
• To better understand the biosynthetic mechanisms of this essential plant hormone, we isolated an Arabidopsis knock-out mutant defective in the JA biosynthetic gene CYP74A (allene oxide synthase, AOS) using reverse genetics screening methods [6].

Biological context of AOS

• The promoter region of the AOS gene exhibits, in addition to the clements of a minimal promoter and the presence of general enhancers, cis-elements that, in other promoters, are responsible for stress- and ethyleneresponsiveness [7].
• On the other hand, wound-induced AOS gene expression was independent of NO [8].
• Concurrent with the increase in JA level in senescing leaves, genes encoding the enzymes that catalyze most of the reactions of the JA biosynthetic pathway are differentially activated during leaf senescence in Arabidopsis, except for allene oxide synthase, which is constitutively and highly expressed throughout leaf development [9].
• A knock-out mutation in allene oxide synthase results in male sterility and defective wound signal transduction in Arabidopsis due to a block in jasmonic acid biosynthesis [6].
• The combination of genetic mapping and a candidate-gene approach identified a frameshift mutation in the ALLENE OXIDE SYNTHASE (AOS) gene, encoding one of the key enzymes of jasmonic acid biosynthesis [10].

Associations of AOS with chemical compounds

• The methyl esters of OPDA or JA (OPDAME, JAME) and coronatine, are strong inducers of AOS mRNA, polypeptide and enzymatic activity [11].
• Salicylic acid (SA) was an inducer of AOS activity while abscisic acid (ABA) had no effect [11].
• Our results also suggest that AOS expression is limiting JA levels in wounded plants, but that the AOS hydroperoxide substrate levels, controlled by upstream enzymes (lipoxygenase and phospholipase), determine JA levels in unwounded plants [6].
• AOS uses hydroperoxylinolenic acid as an oxygen donor as well as the substrate, thus the biochemical conversion of 13(S)-hydroperoxylinolenic acid to allene oxide can proceed in the absence of oxygen and NADPH [1].
• Among the derivatives prepared, heptyl 8-[1-(2,4-dichlorophenyl)-2-imidazolylethoxy]octanoate (k) was found to be the most potent inhibitor, with an IC(50) of 10+/-5 nM, which is 250,000-fold and 1,000,000-fold more potent than the known AOS inhibitors, acetylsalicyclic acid (2.5 mM) and ketoconazole (10 mM), respectively [1].

Other interactions of AOS

• The five genes are positional candidates for QRL and are highly sequence related to Arabidopsis genes AtSGT1b, AtPAD4, and AtAOS [12].
• CYP74A1 may be suited for structural resolution of the first plant cytochrome P450 and its 9-AOS activity and behaviour in vitro has implications for its role in planta [13].
• It is interesting that lipid hydroperoxidase activity, which is a measure of hydroperoxide lyase plus allene oxide synthase plus possibly other lipid hydroperoxide-metabolizing activities, was not changed by methyl jasmonate treatment [14].
• Molecular characterization of this gene indicates that it is expressed at high levels in floral tissue and is wound inducible but, unlike allene oxide synthase, it is not induced by treatment with methyl jasmonate [15].

References