Fatty acid b-oxidation is essential for seedling establishment of oilseed plants, but little is known about its role in leaf metabolism of adult plants. Arabidopsis thaliana plants with loss-of-function mutations in the peroxisomal ABC-transporter1 (PXA1) or the core b-oxidation enzyme keto-acyl-thiolase 2 (KAT2) have impaired peroxisomal b-oxidation. pxa1 and kat2 plants developed severe leaf necrosis, bleached rapidly when returned to light, and died after extended dark treatment, whereas the wild type was unaffected. Dark-treated pxa1 plants showed a decrease in photosystem II efficiency early on and accumulation of free fatty acids, mostly a-linolenic acid [18:3(n-3)] and pheophorbide a, a phototoxic chlorophyll catabolite causing the rapid bleaching. Isolated wild-type and pxa1 chloroplasts challenged with comparable a-linolenic acid concentrations both showed an 80% reduction in photosynthetic electron transport, whereas intact pxa1 plants were more susceptible to the toxic effects of a-linolenic acid than the wild type. Furthermore, starch-free mutants with impaired PXA1 function showed the phenotype more quickly, indicating a link between energy metabolism and b-oxidation. We conclude that the accumulation of free polyunsaturated fatty acids causes membrane damage in pxa1 and kat2 plants and propose a model in which fatty acid respiration via peroxisomal b-oxidation plays a major role in dark-treated plants after depletion of starch reserves.

In the present study, acyl‐CoA synthetase mutants of Saccharomyces cerevisiae were employed to investigate the impact of this activity on certain pools of fatty acids. We identified a genotype responsible for the secretion of free fatty acids into the culture medium. The combined deletion of Faa1p and Faa4p encoding two out of five acyl‐CoA synthetases was necessary and sufficient to establish mutant cells that secreted fatty acids in a growth‐phase dependent manner. The mutants accomplished fatty acid export during exponential growth‐phase followed by fatty acid re‐import into the cells during the stationary phase. The data presented suggest that the secretion is driven by an active component. The fatty acid re‐import resulted in a severely altered ultrastructure of the mutant cells. Additional strains deficient of any cellular acyl‐CoA synthetase activity revealed an almost identical phenotype, thereby proving transfer of fatty acids across the plasma membrane independent of their activation with CoA. Further experiments identified membrane lipids as the origin of the observed free fatty acids. Therefore, we propose the recycling of endogenous fatty acids generated in the course of lipid remodelling as a major task of both acyl‐CoA synthetases Faa1p and Faa4p.

Establishment and maintenance of equilibrium in the fatty acid (FA) composition of phospholipids (PL) requires both regulation of the substrate available for PL synthesis (the acyl-CoA pool) and extensive PL turnover and acyl editing. In the present study, we utilize acyl-CoA synthetase (ACS) deficient cells, unable to recycle FA derived from lipid deacylation, to evaluate the role of several enzymatic activities in FA trafficking and PL homeostasis in Saccharomyces cerevisiae. The data presented show that phospholipases B are not contributing to constitutive PL deacylation and are therefore unlikely to be involved in PL remodeling. In contrast, the enzymes of neutral lipid (NL) synthesis and mobilization are central mediators of FA trafficking. The phospholipid:DAG acyltransferase (PDAT) Lro1p has a substantial effect on FA release and on PL equilibrium, emerging as an important mediator in PL remodeling. The acyl-CoA dependent biosynthetic activities of NL metabolism are also involved in PL homeostasis through active modulation of the substrate available for PL synthesis. In addition TAG mobilization makes an important contribution, especially in cells from stationary phase, to FA availability. Beyond its well-established role in the formation of a storage pool, NL metabolism could play a crucial role as a mechanism to uncouple the pools of PL and acyl-CoAs from each other and thereby to allow independent regulation of each one.

Addendum to: Kunz HH, Scharnewski M, Feussner K, Feussner I, Flügge UI, Fulda M, Gierth M. The ABC transporter PXA1 and peroxisomal β-oxidation are vital for metabolism in mature leaves of Arabidopsis during extended darkness.

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