Chloroplast Function in Fatty Acid Mutants of Arabidopsis

Washington State University, Pullman WA

Investigators

Abstract

This project is based on the isolation of a series of Arabidopsis mutants with specific alterations in leaf fatty acid composition. These mutants have facilitated investigations of the biochemistry of chloroplast glycerolipid synthesis and the roles of membrane composition in chloroplast structure and function. The mutants exhibit a series of very striking phenotypes that result from changes in the fatty acid composition of chloroplasts. In the fab1 mutant, increased saturated fatty acids result in the collapse of photosynthesis after 2-3 weeks at 2 deg. C and an associated breakdown of chloroplasts. The fad3 fad7 fad8 mutants lack trienoic fatty acids but their growth and photosynthesis are normal at 22 deg. C under 120-150 umol quanta/m2/s of light. The phenotypes initially discovered in this mutant result from their inability to synthesize jasmonate, an oxylipin that is derived from linolenic acid. Photosynthesis is affected in fad3 fad7 fad8 plants at low temperatures and under high light. A screen for additional Arabidopsis mutants with reduced growth at 5 deg. C did not identify additional lipid mutants but did lead to the isolation and cloning of other genetic determinants of low-temperature responses. A fad2 fad6 mutant that is deficient in polyunsaturated fatty acids achieves strong vegetative growth on sucrose media but is incapable of autotrophic growth. The effects of lipid changes on short-term photosynthesis at low temperature are very modest when expressed on a chlorophyll basis. Instead, characterization of the mutant lines points to the significance of membrane composition being on assembly or long-term maintenance of the photosystems. The overall goal of the experiments described in this proposal is to understand the biochemical and biophysical bases of these phenotypes. Measurements of photosynthesis and photoinhibition in the mutants will allow detailed comparisons of the mutant phenotypes. Experiments have been carried out to generate additional mutant lines with intermediate fatty acid compositions and to screens to isolate mutations that suppress the low-temperature phenotype of fab1. The isolation of suppressors is a particularly powerful approach for identifying genes and processes contributing to a mutant phenotype and suppressor screens with the fad3 fad7 fad8 and fad2 fad6 lines will also be performed. Genomics tools available in Arabidopsis will allow cloning of the suppressor genes and characterization of the biological processes in which they are involved. Observations on the Arabidopsis mutants and results from researchers working in other systems suggest that protein import into the chloroplast or transport into and through the thylakoid membrane may be disrupted by changes in fatty acid composition. Assays of protein trafficking in wild type and each mutant will be carried out to test this hypothesis and to determine which of the several protein-targeting mechanisms are affected by the different changes in membrane composition that are represented in the mutants. The improved understanding of chloroplast membrane structure and photosynthetic function that will result from this work is an important prerequisite to modifying these processes for increased plant productivity.