SWEET sugar transporters are differentially expressed between soybeans grown in Eastern and Western Canada.

The global population is expected to approach 10 billion over the next three decades; the growing demands for crop production to provide food security are driving an urgent need to establish more strategic and efficient agriculture worldwide. Soybean [Glycine max (L.) Merr.] is an important agronomic crop in Canada with widespread uses in human consumption, animal feed, and biotechnology. The capacity to fix nitrogen gives soybean an important role in sustainable agricultural practices by reducing the need for nitrogen fertilizers. Developing efficient agriculture in the West is pertinent to overcoming foreseeable food shortages.
It has been observed over the past two decades that soybeans grown in Western Canada have persistently had lower seed protein content than those grown in Eastern Canada. In 2021, average Eastern soybean protein content was 40.3%, while the Western-grown soybeans had an average protein content of 36.0%. Seed protein and oil content are complex quantitatively inherited traits that are influenced by genotype, environment, and the interaction between the two. A strong indirect phenotypic correlation is evident in the inverse balance of these two storage molecules. Understanding the genetic basis underlying soybean’s response to changing climate conditions will help reduce variability in year-to-year crop yields, and to promote stability of the food supply and food prices.
When considering seed storage macronutrients in soybean, an important balance exists between proteins, lipids, and carbohydrates and it is important to consider expression of genes related to each of these macromolecules. Carbohydrate metabolism is the precursor to protein and oil biosynthesis, making it a key step in downstream seed storage molecule biosynthesis. The Sugars Will Eventually be Exported Transporters (SWEETs) are a family of membrane-bound sugar transporters found on tonoplast membranes and phloem parenchyma cell membranes. SWEETs regulate the loss of sugars in a concentration-dependent manner using storage vacuoles.
To better understand the genetic mechanisms underlying the discrepancy in seed protein content between Eastern- and Western-grown soybeans, RNA-seq and differential expression analysis have been investigated. Ten soybean varieties ranging from low to high seed protein content were grown in four locations across Eastern and Western growing regions of Canada. Across 88 individual differential expression analyses, 34 differentially expressed GmSWEET genes were found, including three genes not previously identified as GmSWEETs. The most consistently differentially expressed genes were two paralogous AtSWEET2 homologs, upregulated across all ten varieties in each of the Western locations over three years; these genes are likely candidates underlying the lower seed protein content of Western soybeans. A second set of paralogs, AtSWEET12 homologs, were downregulated in the Western-grown soybeans, and may also underlie the differences in seed protein accumulation. GmSWEET genes are DE between East- and West-grown soybeans, indicating that the environmental differences may attribute to differential expression of GmSWEETs, thereby affecting downstream carbon allocation. These findings are valuable for improving soybean agriculture in Western growing regions.