The phytotoxin coronatine is a structural analog of octadecanoid signaling molecules, which are well known mediators of plant defense reactions. To isolate novel coronatine-regulated genes from Arabidopsis thaliana, differential mRNA display was performed. Transcript levels of CORI-7 (coronatine induced-7) were rapidly and transiently increased in coronatine-treated plants, and the corresponding cDNA was found to encode the sulfotransferase AtST5a. Likewise, upon wounding, an immediate and transient increase in AtST5a mRNA levels could be observed in both locally wounded and unwounded (systemic) leaves. Furthermore, application of octadecanoids and ethylene as compounds involved in plant wound defense reactions resulted in AtST5a gene activation, whereas pathogen defense-related signals (yeast elicitor and salicylic acid) were inactive. AtST5a and its close homologs AtST5b and AtST5c were purified as His 6 -tagged proteins from Escherichia coli. The three enzymes were shown to catalyze the final step in the biosynthesis of the glucosinolate (GS) core structure, the sulfation of desulfoglucosinolates (dsGSs). They accept a broad range of dsGSs as substrates. However, in a competitive situation, AtST5a clearly prefers tryptophan-and phenylalanine-derived dsGSs, whereas long chain dsGSs derived from methionine are the preferred substrates of AtST5b and AtST5c. Treatment of Arabidopsis plants with low concentrations of coronatine resulted in an increase in the amounts of specific GSs, primarily glucobrassicin and neoglucobrassicin. Hence, it is suggested that AtST5a is the sulfotransferase responsible for the biosynthesis of tryptophan-derived GSs in vivo.Compared with the animal cell, very little is known regarding the structural and regulatory roles of the sulfate group in plants. The transfer of the active sulfate group from 3Ј-phosphoadenosine 5Ј-phosphosulfate (PAPS) 1 to acceptor molecules is catalyzed by sulfotransferases. Members of the superfamily of sulfotransferases are known in prokaryotes as well as eukaryotes; however, the study of enzymes that catalyze the sulfation reaction in plants considerably lags behind that in animal systems. Cytosolic sulfotransferases from plants have been characterized in some detail (Ref. 1 and references therein), and some cDNAs have been identified. These fall into three subgroups: the flavonol sulfotransferases described for Flaveria species (1), the steroid sulfotransferases identified in Brassica napus (2, 3), and a hydroxyjasmonic acid-specific sulfotransferase from Arabidopsis thaliana (4). An additional sulfotransferase (RaR047, At2g03760) has been cloned from A. thaliana, and its mRNA level was found to be up-regulated by pathogens and salicylic acid; however, its physiological substrate is still unknown (5).In plants, sulfate groups occur in a number of secondary metabolites, notably the sulfoflavonoids (6) and the glucosinolates (7). Glucosinolates (GSs) are secondary compounds found in at least 16 different plant families, 15 of which belong to the order Cappa...

Chloroplasts synthesize an abundance of different tetrapyrrole compounds. Among them are chlorophyll and its precursor protochlorophyllide (Pchlide), which accumulate in light-and dark-grown plants, respectively. Pchlide is converted to chlorophyllide by virtue of the NADPH:Pchlide oxidoreductase (POR), which, in angiosperms, is the only known light-dependent enzyme of the chlorophyll biosynthetic pathway. In etiolated barley plants, two closely related POR proteins exist termed PORA and PORB, which are nuclear gene products. Here we identified plastid envelope proteins that interact with the cytosolic PORA precursor (pPORA) during its posttranslational chloroplast import. We demonstrate that pPORA interacts with several previously unreported components. Among them is a Pchlide a oxygenase, which provides Pchlide b as import substrate for pPORA, and a tyrosine aminotransferase thought to be involved in the synthesis of photoprotective vitamin E. Two other constituents were found to be orthologs of the GTP-binding proteins Toc33͞34 and of the outer plastid envelope protein Oep16.