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Levels of the enzymes that produce wound response mediators have to be controlled tightly in unwounded tissues. The Arabidopsis (Arabidopsis thaliana) fatty acid oxygenation up-regulated8 (fou8) mutant catalyzes high rates of α-linolenic acid oxygenation and has higher than wild-type levels of the α-linolenic acid-derived wound response mediator jasmonic acid QA) in undamaged leaves. fou8 produces a null allele in the gene SAL1 (also known as FIERY1 or FRY1). Overexpression of the wild-type gene product had the opposite effect of the null allele, suggesting a regulatory role of SAL1 acting in JA synthesis. The biochemical phenotypes in fous were complemented when the yeast (Saccharomyces cerevisiae) sulfur metabolism 3' (2'), 5' -bisphosphate nucleotidase MET22 was targeted to chloroplasts in fou8. The data are consistent with a role of SAL1 in the chioroplast-localized dephosphorylation of 3' -phospho-5' -adenosine phosphosulfate to 5' -adenosine phosphosulfate or in a closely related reaction (e.g. 3' ,5' -bisphosphate dephosphorylation). Furthermore, the fou8 phenotype was genetically suppressed in a triple mutant (fou8 apk1 apk2) affecting chloroplastic 3'-phospho-5'-adenosine phosphosulfate synthesis. These results show that a nucleotide component of the sulfur futile cycle regulates early steps of JA production and basal JA levels. [ABSTRACT FROM AUTHOR]