Survey of the total fatty acid and triacylglycerol composition and content of 30 duckweed species and cloning of a Δ6-desaturase responsible for the production of γ-linolenic and stearidonic acids in Lemna gibba

Abstract

BackgroundDuckweeds, i.e., members of the Lemnoideae family, are amongst the smallest aquatic flowering plants. Their high growth rate, aquatic habit and suitability for bio-remediation make them strong candidates for biomass production. Duckweeds have been studied for their potential as feedstocks for bioethanol production; however, less is known about their ability to accumulate reduced carbon as fatty acids (FA) and oil.ResultsTotal FA profiles of thirty duckweed species were analysed to assess the natural diversity within the Lemnoideae. Total FA content varied between 4.6% and 14.2% of dry weight whereas triacylglycerol (TAG) levels varied between 0.02% and 0.15% of dry weight. Three FA, 16:0 (palmitic), 18:2Δ9,12 (Linoleic acid, or LN) and 18:3Δ9,12,15 (α-linolenic acid, or ALA) comprise more than 80% of total duckweed FA. Seven Lemna and two Wolffiela species also accumulate polyunsaturated FA containing Δ6-double bonds, i.e., GLA and SDA. Relative to total FA, TAG is enriched in saturated FA and deficient in polyunsaturated FA, and only five Lemna species accumulate Δ6-FA in their TAG. A putative Δ6-desaturase designated LgDes, with homology to a family of front-end Δ6-FA and Δ8-spingolipid desaturases, was identified in the assembled DNA sequence of Lemna gibba. Expression of a synthetic LgDes gene in Nicotiana benthamiana resulted in the accumulation of GLA and SDA, confirming it specifies a Δ6-desaturase.ConclusionsTotal accumulation of FA varies three-fold across the 30 species of Lemnoideae surveyed. Nine species contain GLA and SDA which are synthesized by a Δ6 front-end desaturase, but FA composition is otherwise similar. TAG accumulates up to 0.15% of total dry weight, comparable to levels found in the leaves of terrestrial plants. Polyunsaturated FA is underrepresented in TAG, and the Δ6-FA GLA and SDA are found in the TAG of only five of the nine Lemna species that produce them. When present, GLA is enriched and SDA diminished relative to their abundance in the total FA pool.

Survey of duckweed diversity in Lake Chao and total fatty acid, triacylglycerol, profiles of representative strains

“…However, this value is much lower than those reported by Yan et al . (), in which the lowest oil content of the four species was 7.8% for L. punctata . This large difference might be due to the growth conditions used in their study, which contained exogenous glucose and less nitrogen, possibly as a stress to cause directed lipids accumulation (Li et al .…”

“…Interestingly, the profiles of both TFA and TAG in the present study revealed composition differences, compared to corresponding duckweed species reported in Yan et al . (). Palmitoleic acid (C16:1), arachidic acid (C20:0) and behenic acid (C22:0) were not identified in the TFA of any of the four surveyed duckweed species, but were found in the study of Yan et al .…”

“…Palmitoleic acid (C16:1), arachidic acid (C20:0) and behenic acid (C22:0) were not identified in the TFA of any of the four surveyed duckweed species, but were found in the study of Yan et al . (). In addition, pentadecylic acid (C15:0) was identified in this study, but not in the study of Yan et al .…”

“…Although the lipids or TAG in the present study appeared to be lower than reported previously (Yan et al . ), further enhancements in lipids or TAG accumulation could be achieved by optimising the growth conditions, stress stimulations ( e.g . nitrogen starvation) etc.…”

Efficient transformation and artificial miRNA gene silencing in Lemna minor

“…Lemnaceae are the smallest aquatic flowering plants, commonly known as duckweeds, and have seen a revival in research interest during the past few years as an attractive alternative to algae biofuel feedstock, due to their robust growth in marginal environments and their particular metabolic traits, such as high starch accumulation and low lignin content (Xu et al, 2011; Xu et al, 2012; Yan et al, 2013). To increase its value in this respect, it is necessary to investigate the complexity of its metabolic pathways using genetic assays.…”