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.

Transplastomic tobacco plants expressing a fatty acid desaturase gene exhibit altered fatty acid profiles and improved cold tolerance

“…Further, the somewhat inconsistent results reported in previous studies describing the overexpression of ACP D 9 desaturase genes (Zaborowska et al 2002;Kachroo et al 2007;De Palma et al 2008) suggest that differences between ours and other outcomes could be also due to the different recipient species and/or transgene used. As far as the cyanobacterial gene is concerned, in our study it was integrated and expressed within the chloroplasts, whereas in the study of Ishizaki-Nishizawa and colleagues (1996) it was integrated and transcribed in the nucleus and the resulting enzyme transferred into the chloroplast after translation in the cytoplasm.…”

“…No changes were observed in hexadecanoic fatty acids, whereas 18:1D 11 vaccenic acid was detected in both the PC (phosphatidylcholine) and PE (phosphatidylethanolamine) microsomal fractions (De Palma et al 2008). Similarly, Arabidopsis plants overexpressing either the SSI2 or the DES1 genes, both encoding a stearoyl-ACP D 9 desaturase, were morphologically indistinguishable from wild-type plants, showing comparable 18:1 levels as well as normal salicilic acid/jasmonic acid responsiveness (Kachroo et al 2007). The latter authors attributed the limited change in oleic acid content to tight regulation or rapid metabolization of 18:1-ACP, or to limitations of substrate or cofactor.…”

Choreography of Transcriptomes and Lipidomes ofNannochloropsisReveals the Mechanisms of Oil Synthesis in Microalgae

“…To synthesize glycerolipids of various chain lengths with different unsaturation levels, a series of elongation and desaturation reactions are required. In vascular plants, stearoyl-ACP desaturase (D9-desaturase) is the only soluble enzyme that introduces a double bond to the D9 position of C18:0-ACP to form C18:1-ACP, whereas other desaturases are membrane proteins that desaturate the acyl-groups of lipid molecules (Browse et al, 1989(Browse et al, , 1985Iba et al, 1993;Heilmann et al, 2004;Kachroo et al, 2007;Gao et al, 2009). In plastids, the D9-desaturasecatalyzed desaturation reaction uses reduced ferredoxin as the electron donor and molecular oxygen as one of the substrates (Schultz et al, 2000).…”

Plastidial fatty acid levels regulate resistance gene-dependent defense signaling in Arabidopsis

“…5A). The Arabidopsis genome contains seven S-ACP-DES enzymes, and four of these are expressed in leaves and are capable of synthesizing 18:1 (22). To determine whether TCV inoculation of Di-17 plants altered the expression profile of any of these seven S-ACP-DES isoforms, we analyzed their transcript levels in mock-and TCV-inoculated plants.…”

“…Because altered defense signaling in the ssi2 plants has been attributed to the reduced levels of 18:1 (15)(16)(17)22), we checked the responsiveness of HRT promoter to SA and low 18:1 conditions. Histochemical assays for transient ␤-glucuronidase (GUS) activity in cells expressing the GUS protein under control of the HRT promoter were carried out.…”