Fatty acid metabolism and intracellular lipid trafficking in plants
Plant Physiology
Final Report Abstract
Lipid trafficking within plant cells is a vital process ensuring the appropriate distribution of specific lipid molecules in the various membrane systems. Recently, we presented data suggesting the involvement of two fatty acid activating enzymes in lipid trafficking between the endoplasmic reticulum and the plastid. In the current study, we provide additional genetic evidence as well as lipidomics analyses to support this hypothesis. In the complex mixture of several dozen molecular lipid species in total lipid extracts of mutant leaves, we found a strong accumulation of those molecules, which are considered the transferred lipid precursors. Due to ineffective fatty acid activation in the double mutant, the lipid transfer is strongly compromised, the turnover of precursors is decreased, and consequently, they accumulate highly over levels detected in wild type. From this data, we conclude that at least substantial parts of the lipid moieties, which are designated for the crucial transfer of precursors from the cytoplasm into plastids, require the activation of free fatty acids. The most intuitive explanation for this connection seems to be the cleavage of the lipid precursor into lysolipid and free fatty acid prior to the transport and the reassembly at the plastid post transport. Besides the described limitations in lipid transport, the double mutant displayed visual phenotypes, which were recently connected with elevated levels of the plant hormone jasmonate (JA). The analyses of JA levels as well as expression analysis of candidate genes revealed the permanent induction of the JA biosynthetic pathway in the lacs4 lacs9 double mutant. Parts of the visual phenotype could be rescued by introducing mutant alleles JA biosynthesis. The molecular mechanism between compromised lipid trafficking and JA response remains to be elucidated in future studies. Fatty acids are not only important constituents of membrane and storage lipids but they fulfill also crucial roles in the formation of a hydrophobic barrier protecting aerial organs against various forms of stress. The extracellular layer designated as cuticle is composed of cutin and cuticular wax. Defects of the cuticle result not only in enhanced sensitivity against various stress but may also lead to fusions between different organs of the plant. The lacs2 fad2 double mutant was identified by a severe organ fusion phenotype. Different experiments revealed an enhanced leaf cuticular permeability of the double mutant and biochemical analysis showed a lower amount of cutin and a modified cutin monomer composition compared to wild type. Most organ fusion phenotypes reported so far are due to constraints in cutin biosynthesis and consequently low amounts of cutin within the cuticle. Interestingly, the lacs2 fad2 double mutant produces about double the amount of cutin compared to the lacs2 single mutant, which shows no phenotype. We conclude from this data that only the combination of lower total amount and modified composition seems to be crucial with respect to the organ fusion phenotype.
