Abstract: Seed oil content, protein content, and yield are agronomically important, correlated traits that determine the economic value of soybean (Glycine max). However, improving seed quality and yield simultaneously is challenging because gains in one breeding target often compromise the other, and the genetic basis of this trade-off is poorly understood. Here, we performed a genome-wide association study of 429 diverse soybean accessions and identified Seed Oil and Protein 5 (SOP5), which encodes a kinesin protein, as a key locus associated with seed oil and protein content. Knockout and overexpression experiments demonstrated that GmSOP5 positively affects seed oil content and 100-seed weight and negatively influences seed protein content. GmSOP5 is located in a selective sweep region, and the domestication-related GmSOP5^H1 allele is nearly fixed in cultivated soybean, contributing to increased seed size, weight, and oil content and reduced protein content. Field trials demonstrated that neither loss-of-function GmSOP5-edited mutants, which have increased seed protein content, nor GmSOP5-overexpression lines, which have increased seed oil content, differed significantly in yield from wild-type plants, because changes in plant architecture were offset by changes in seed weight. Our results shed light on soybean domestication and suggest how pleiotropy can be harnessed in breeding to enhance seed quality without compromising yield.

Key words: domestication, GWAS, kinesin, seed oil and protein content, soybean