title: The Arabidopsis stearoyl-acyl carrier protein-desaturase family and the contribution of leaf isoforms to oleic acid synthesis.
abstract: In plants, changes in the levels of oleic acid (18:1), a major monounsaturated fatty acid (FA), results in the alteration of salicylic acid (SA)- and jasmonic acid (JA)-mediated defense responses. This is evident in the Arabidopsis ssi2/fab2 mutant, which encodes a defective stearoyl-acyl carrier protein-desaturase (S-ACP-DES) and consequently accumulates high levels of stearic acid (18:0) and low levels of 18:1. In addition to SSI2, the Arabidopsis genome encodes six S-ACP-DES-like enzymes, the native expression levels of which are unable to compensate for a loss-of-function mutation in ssi2. The presence of low levels of 18:1 in the fab2 null mutant indicates that one or more S-ACP-DES isozymes contribute to the 18:1 pool. Biochemical assays show that in addition to SSI2, four other isozymes are capable of desaturating 18:0-ACP but with greatly reduced specific activities, which likely explains the inability of these SSI2 isozymes to substitute for a defective ssi2. Lines containing T-DNA insertions in S-ACP-DES1 and S-ACP-DES4 show that they are altered in their lipid profile but contain normal 18:1 levels. However, overexpression of the S-ACP-DES1 isoform in ssi2 plants results in restoration of 18:1 levels and thereby rescues all ssi2-associated phenotypes. Thus, high expression of a low specific activity S-ACP-DES is required to compensate for a mutation in ssi2. Transcript level of S-ACP-DES isoforms is reduced in high 18:1-containing plants. Enzyme activities of the desaturase isoforms in a 5-fold excess of 18:1-ACP show product inhibition of up to 73%. Together these data indicate that 18:1 levels are regulated at both transcriptional and post-translational levels.
authors.affiliation: Department of Plant Pathology, University of Kentucky, Lexington, KY 40546, USA. apkach2@uky.edu
authors.forename: Aardra
authors.initials: A
authors.lastname: Kachroo
authors.forename: John
authors.initials: J
authors.lastname: Shanklin
authors.forename: Edward
authors.initials: E
authors.lastname: Whittle
authors.forename: Ludmila
authors.initials: L
authors.lastname: Lapchyk
authors.forename: David
authors.initials: D
authors.lastname: Hildebrand
authors.forename: Pradeep
authors.initials: P
authors.lastname: Kachroo
mesh_terms: D017360:Arabidopsis / Q000201:enzymology* / Q000378:metabolism; D007527:Isoenzymes / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D019301:Oleic Acid / Q000096:biosynthesis*; D018515:Plant Leaves / Q000201:enzymology*
chemical_list: D007527:Isoenzymes; D019301:Oleic Acid; D006899:Mixed Function Oxygenases; C018747:acyl-(acyl-carrier-protein)desaturase