Components of Complex Lipid Biosynthetic Pathways in Developing Castor (Ricinus communis) Seeds Identified by MudPIT Analysis of Enriched Endoplasmic Reticulum

Identification and Functional Characterization of the RcFAH12 Promoter from Castor Bean in Arabidopsis thaliana

Castor (Ricinus communis L.) seed oil is the commercial source of ricinoleate, a valuable raw material used in many industries. Oleoyl-12-hydroxylase (RcFAH12) is a key enzyme in the biosynthesis of ricinoleate, accumulating nearly 90% of the triacylglycerol in castor seeds. Little is known about the transcriptional regulation of RcFAH12. We used rapid amplification of cDNA 5′ ends (5′RACE) to locate the transcription start site (TSS) of RcFAH12, and the sequence of a 2605 bp region, −2506~+99, surrounding the TSS was cloned. We then investigated these regions to promote β-glucuronidase (GUS) expression in transgenic Arabidopsis by the progressive 5′ and 3′ deletions strategies. The GUS staining showed that the GUS accumulation varied in tissues under the control of different deleted fragments of RcFAH12. In addition, the GUS expression driven by the RcFAH12 promoter markedly accumulated in transgenic seeds, which indicated that RcFAH12 might play an important role in the biosynthesis of ricinoleic acid. This study will lay a potential foundation for developing a tissue-specific promoter in oil-seed crops.

High light exposure on seed coat increases lipid accumulation in seeds of castor bean (Ricinus communis L.), a nongreen oilseed crop

Little was known on how sunlight affects the seed metabolism in nongreen seeds. Castor bean (Ricinus communis L.) is a typical nongreen oilseed crop and its seed oil is an important feedstock in industry. In this study, photosynthetic activity of seed coat tissues of castor bean in natural conditions was evaluated in comparison to shaded conditions. Our results indicate that exposure to high light enhances photosynthetic activity in seed coats and consequently increases oil accumulation. Consistent results were also reached using cultured seeds. High-throughput RNA-Seq analyses further revealed that genes involved in photosynthesis and carbon conversion in both the Calvin-Benson cycle and malate transport were differentially expressed between seeds cultured under light and dark conditions, implying several venues potentially contributing to light-enhanced lipid accumulation such as increased reducing power and CO2 refixation which underlie the overall lipid biosynthesis. This study demonstrated the effects of light exposure on oil accumulation in nongreen oilseeds and greatly expands our understanding of the physiological roles that light may play during seed development in nongreen oilseeds. Essentially, our studies suggest that potential exists to enhance castor oil yield through increasing exposure of the inflorescences to sunlight either by genetically changing the plant architecture (smart canopy) or its growing environment.

The multigene family of lysophosphatidate acyltransferase (LPAT)-related enzymes in Ricinus communis: cloning and molecular characterization of two LPAT genes that are expressed in castor seeds

The multigene family encoding proteins related to lysophosphatidyl-acyltransferases (LPATs) has been analyzed in the castor plant Ricinus communis. Among them, two genes designated RcLPAT2 and RcLPATB, encoding proteins with LPAT activity and expressed in the developing seed, have been cloned and characterized in some detail. RcLPAT2 groups with well characterized members of the so-called A-class LPATs and it shows a generalized expression pattern in the plant and along seed development. Enzymatic assays of RcLPAT2 indicate a preference for ricinoleoyl-CoA over other fatty acid thioesters when ricinoleoyl-LPA is used as the acyl acceptor, while oleoyl-CoA is the preferred substrate when oleoyl-LPA is employed. RcLPATB groups with B-class LPAT enzymes described as seed specific and selective for unusual fatty acids. However, RcLPATB exhibit a broad specificity on the acyl-CoAs, with saturated fatty acids (12:0-16:0) being the preferred substrates. RcLPATB is upregulated coinciding with seed triacylglycerol accumulation, but its expression is not restricted to the seed. These results are discussed in the light of a possible role for LPAT isoenzymes in the channelling of ricinoleic acid into castor bean triacylglycerol.

A land-plant-specific glycerol-3-phosphate acyltransferase family in Arabidopsis: substrate specificity, sn-2 preference, and evolution

Arabidopsis (Arabidopsis thaliana) has eight glycerol-3-phosphate acyltransferase (GPAT) genes that are members of a plant-specific family with three distinct clades. Several of these GPATs are required for the synthesis of cutin or suberin. Unlike GPATs with sn-1 regiospecificity involved in membrane or storage lipid synthesis, GPAT4 and -6 are unique bifunctional enzymes with both sn-2 acyltransferase and phosphatase activity resulting in 2-monoacylglycerol products. We present enzymology, pathway organization, and evolutionary analysis of this GPAT family. Within the cutin-associated clade, GPAT8 is demonstrated as a bifunctional sn-2 acyltransferase/phosphatase. GPAT4, -6, and -8 strongly prefer C16:0 and C18:1 ω-oxidized acyl-coenzyme As (CoAs) over unmodified or longer acyl chain substrates. In contrast, suberin-associated GPAT5 can accommodate a broad chain length range of ω-oxidized and unsubstituted acyl-CoAs. These substrate specificities (1) strongly support polyester biosynthetic pathways in which acyl transfer to glycerol occurs after oxidation of the acyl group, (2) implicate GPAT specificities as one major determinant of cutin and suberin composition, and (3) argue against a role of sn-2-GPATs (Enzyme Commission 2.3.1.198) in membrane/storage lipid synthesis. Evidence is presented that GPAT7 is induced by wounding, produces suberin-like monomers when overexpressed, and likely functions in suberin biosynthesis. Within the third clade, we demonstrate that GPAT1 possesses sn-2 acyltransferase but not phosphatase activity and can utilize dicarboxylic acyl-CoA substrates. Thus, sn-2 acyltransferase activity extends to all subbranches of the Arabidopsis GPAT family. Phylogenetic analyses of this family indicate that GPAT4/6/8 arose early in land-plant evolution (bryophytes), whereas the phosphatase-minus GPAT1 to -3 and GPAT5/7 clades diverged later with the appearance of tracheophytes.

Tissue-specific whole transcriptome sequencing in castor, directed at understanding triacylglycerol lipid biosynthetic pathways

BACKGROUND

Storage triacylglycerols in castor bean seeds are enriched in the hydroxylated fatty acid ricinoleate. Extensive tissue-specific RNA-Seq transcriptome and lipid analysis will help identify components important for its biosynthesis.

METHODOLOGY/FINDINGS

Storage triacylglycerols (TAGs) in the endosperm of developing castor (Ricinus communis) seeds are highly enriched in ricinoleic acid (18:1-OH). We have analysed neutral lipid fractions from other castor tissues using TLC, GLC and mass spectrometry. Cotyledons, like the endosperm, contain high levels of 18:1-OH in TAG. Pollen and male developing flowers accumulate TAG but do not contain 18:1-OH and leaves do not contain TAG or 18:1-OH. Analysis of acyl-CoAs in developing endosperm shows that ricinoleoyl-CoA is not the dominant acyl-CoA, indicating that either metabolic channelling or enzyme substrate selectivity are important in the synthesis of tri-ricinolein in this tissue. RNA-Seq transcriptomic analysis, using Illumina sequencing by synthesis technology, has been performed on mRNA isolated from two stages of developing seeds, germinating seeds, leaf and pollen-producing male flowers in order to identify differences in lipid-metabolic pathways and enzyme isoforms which could be important in the biosynthesis of TAG enriched in 18:1-OH. This study gives comprehensive coverage of gene expression in a variety of different castor tissues. The potential role of differentially expressed genes is discussed against a background of proteins identified in the endoplasmic reticulum, which is the site of TAG biosynthesis, and transgenic studies aimed at increasing the ricinoleic acid content of TAG.

CONCLUSIONS/SIGNIFICANCE

Several of the genes identified in this tissue-specific whole transcriptome study have been used in transgenic plant research aimed at increasing the level of ricinoleic acid in TAG. New candidate genes have been identified which might further improve the level of ricinoleic acid in transgenic crops.