A Suppressor of fab1 Challenges Hypotheses on the Role of Thylakoid Unsaturation in Photosynthetic Function

Lenore Barkan , Perumal Vijayan , Anders S. Carlsson , Sergei Mekhedov , and John Browse ^*

Institute of Biological chemistry, Washington State University, Pullman, Washington 99164-6340, USA
Botany Department, Michigan State University, East Lansing, MI 48824-1312, USA

^* Corresponding author; email: jab{at}wsu.edu.

Leaf membrane lipids of the Arabidopsis fab1 mutant containa 35-40% increase in the predominant saturated fatty acid, 16:0,relative to wild type. This increase in membrane saturationis associated with loss of photosynthetic function and deathof mutant plants at low temperatures. We have initiated a suppressorscreen for mutations that allow survival of fab1 plants at 2°C.Five suppressor mutants identified in this screen all rescuedthe collapse of photosynthetic function observed in fab1 plants.While fab1 plants died after 5-7 weeks at 2°C, the suppressorsremained viable after 16 weeks in the cold, as judged by theirability to resume growth following a return to 22°C andto subsequently produce viable seed. Three of the suppressorshad changes in leaf fatty acid composition when compared tofab1, indicating that one mechanism of suppression may involvecompensating changes in thylakoid lipid composition. Surprisingly,the suppressor phenotype in one line, S31, was associated witha further substantial increase in lipid saturation. The overallleaf fatty acid composition of S31 plants contained 31% 16:0compared with 23% in fab1 and 17% in wild type. Biochemicaland genetic analysis showed that S31 plants contain a new alleleof fad5, fad5-2, and are therefore partially deficient in activityof the chloroplast 16:0 ${Delta}$ 7-desaturase. A double mutant producedby crossing fab1 to the original fad5-1 allele also remainedalive at 2°C, indicating that the fad5-2 mutation is thesuppressor in the S31 (fab1 fad5-2) line. Based on the biophysicalcharacteristics of saturated and unsaturated fatty acids, theincreased 16:0 in fab1 fad5-2 plants would be expected to exacerbate,rather than ameliorate, low-temperature damage. We propose insteadthat a change in shape of the major thylakoid lipid, monogalactosyldiacylglycerol,mediated by the fad5-2 mutation, may compensate for changesin lipid structure resulting from the original fab1 mutation.Our identification of mutants that suppress the low-temperaturephenotype of fab1 provides new tools to understand the relationshipbetween thylakoid lipid structure and photosynthetic function.