CRISPR/Cas9 editing of three CRUCIFERIN C homoeologues alters the seed protein profile in Camelina sativa

BACKGROUND

The oilseed Camelina sativa is grown for a range of applications, including for biofuel, biolubricants, and as a source of omega-3 fatty acids for the aquaculture feed industry. The seed meal co-product is used as a source of protein for animal feed; however, the low value of the meal hinders profitability and more widespread application of camelina. The nutritional quality of the seed meal is largely determined by the abundance of specific seed storage proteins and their amino acid composition. Manipulation of seed storage proteins has been shown to be an effective means for either adjustment of nutritional content of seeds or for enhancing accumulation of high-value recombinant proteins in seeds.

RESULTS

CRISPR/Cas9 gene editing technology was used to generate deletions in the first exon of the three homoeologous genes encoding the seed storage protein CRUCIFERIN C (CsCRUC), creating an identical premature stop-codon in each and resulting in a CsCRUC knockout line. The mutant alleles were detected by applying a droplet digital PCR drop-off assay. The quantitative nature of this technique is particularly valuable when applied to polyploid species because it can accurately determine the number of mutated alleles in a gene family. Loss of CRUC protein did not alter total seed protein content; however, the abundance of other cruciferin isoforms and other seed storage proteins was altered. Consequently, seed amino acid content was significantly changed with an increase in the proportion of alanine, cysteine and proline, and decrease of isoleucine, tyrosine and valine. CsCRUC knockout seeds did not have changed total oil content, but the fatty acid profile was significantly altered with increased relative abundance of all saturated fatty acids.

CONCLUSIONS

This study demonstrates the plasticity of the camelina seed proteome and establishes a CRUC-devoid line, providing a framework for modifying camelina seed protein composition. The results also illustrate a possible link between the composition of the seed proteome and fatty acid profile.

Proteome rebalancing in transgenic Camelina occurs within the enlarged proteome induced by β-carotene accumulation and storage protein suppression

Oilseed crops are global commodities for their oil and protein seed content. We have engineered the oilseed Camelina sativa to exhibit increased protein content with a slight decrease in oil content. The introduction of a phytoene synthase gene with an RNAi cassette directed to suppress the storage protein 2S albumin resulted in seeds with an 11-24 % elevation in overall protein. The phytoene synthase cassette alone produced enhanced β-carotene content of an average 275 ± 6.10 μg/g dry seed and an overall altered seed composition of 11 % less protein and comparable nontransgenic amounts of both oil and carbohydrates. Stacking an RNAi to suppress the major 2S storage protein resulted in seeds that contain elevated protein and slight decrease in oil and carbohydrate amounts showing that Camelina rebalances its proteome within an enlarged protein content genotype. In both β-carotene enhanced seeds with/without RNAi2S suppression, the seed size was noticeably enlarged compared to nontransgenic counterpart seeds. Metabolic analysis of maturing seeds indicate that the enhanced β-carotene trait had the larger effect than the RNAi2S suppression on the seed metabolome. The use of a GRAS (generally regarded as safe) β-carotene as a visual marker in a floral dip transformation system, such as Camelina, might eliminate the need for costly regulatory and controversial antibiotic resistance markers. β-carotene enhanced RNAi2S suppressed Camelina seeds could be further developed as a rapid heterologous protein production platform in a nonfood crop leveraging its enlarged protein content and visual marker.