Abstract
The Arabidopsis (Arabidopsis thaliana) fatty acid biosynthesis1 (fab1) mutant has increased levels of the saturated fatty acid 16:0 due to decreased activity of 3-ketoacyl-acyl carrier protein (ACP) synthase II. In fab1 leaves, phosphatidylglycerol, the major chloroplast phospholipid, contains up to 45% high-melting-point molecular species (molecules that contain only 16:0, 16:1-trans, and 18:0), a trait associated with chilling-sensitive plants, compared with less than 10% in wild-type Arabidopsis. Although they do not exhibit typical chilling sensitivity, when exposed to low temperatures (2°C-6°C) for long periods, fab1 plants do suffer collapse of photosynthesis, degradation of chloroplasts, and eventually death. A screen for suppressors of this low-temperature phenotype has identified 11 lines, some of which contain additional alterations in leaf-lipid composition relative to fab1. Here, we report the identification of two suppressor mutations, one in act1, which encodes the chloroplast acyl-ACP:glycerol-3-phosphate acyltransferase, and one in lpat1, which encodes the chloroplast acyl-ACP:lysophosphatidic acid acyltransferase. These enzymes catalyze the first two steps of the prokaryotic pathway for glycerolipid synthesis, so we investigated whether other mutations in this pathway would rescue the fab1 phenotype. Both the gly1 mutation, which reduces glycerol-3-phosphate supply to the prokaryotic pathway, and fad6, which is deficient in the chloroplast 16:1/18:1 fatty acyl desaturase, were discovered to be suppressors. Analyses of leaf-lipid compositions revealed that mutations at all four of the suppressor loci result in reductions in the proportion of high-melting-point molecular species of phosphatidylglycerol relative to fab1. We conclude that these reductions are likely the basis for the suppressor phenotypes.
Highlights
The Arabidopsis (Arabidopsis thaliana) fatty acid biosynthesis1 mutant has increased levels of the saturated fatty acid 16:0 due to decreased activity of 3-ketoacyl-acyl carrier protein (ACP) synthase II
Our screen (Barkan et al, 2006) for mutations that suppress the collapse of photosynthesis and death that are characteristic of fab1 plants exposed to 2°C (Wu and Browse, 1995; Wu et al, 1997) identified 11 suppressor lines, with five of these having additional changes in the fatty acid composition of leaf lipids
Plants of the S106 line grew normally at 22°C and, they were less robust under chilling temperatures than wild-type Arabidopsis, they maintained photosynthetic function and continued to produce new green leaf tissue long after the chilling regimen at 2°C had killed fab1 (Barkan et al, 2006)
Summary
The Arabidopsis (Arabidopsis thaliana) fatty acid biosynthesis (fab1) mutant has increased levels of the saturated fatty acid 16:0 due to decreased activity of 3-ketoacyl-acyl carrier protein (ACP) synthase II. Phosphatidylglycerol, the major chloroplast phospholipid, contains up to 45% high-melting-point molecular species (molecules that contain only 16:0, 16:1-trans, and 18:0), a trait associated with chilling-sensitive plants, compared with less than 10% in wild-type Arabidopsis. They do not exhibit typical chilling sensitivity, when exposed to low temperatures (2°C–6°C) for long periods, fab plants do suffer collapse of photosynthesis, degradation of chloroplasts, and eventually death. Analyses of leaf-lipid compositions revealed that mutations at all four of the suppressor loci result in reductions in the proportion of high-melting-point molecular species of phosphatidylglycerol relative to fab. Chilling-resistant plants contain less than 10% of their leaf PG as high-meltingpoint molecular species, whereas many chilling-sensitive plants have .30%
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