Loss of phytochromobilin synthase activity leads to larger seeds with higher protein content in soybean

Seed weight is an important agronomic trait that is related to seed size and determines yield in soybean (Glycine max). We previously identified a spontaneous soybean mutant with light green leaves called ygl2. Here, we cloned YGL2, which encodes a phytochromobilin (P Phi B) synthase involved in synthesizing the chromophore of the photoreceptor phytochrome. The lesion in ygl2 is a 10-bp deletion, causing a frameshift mutation and a premature stop codon that truncates the encoded protein. In contrast to the wild type, ygl2 lacks P Phi B synthase activity and function. This appears to promote cell expansion, thus increasing seed weight. Surprisingly, the ygl2 mutant also exhibits excellent traits including early maturity and high protein content. Moreover, under the condition of dense planting (3 cm), the yield of YGL2 mutant was significantly increased. Mutants harboring ygl2 mutations that we generated via gene editing had enlarged seeds with high protein content. Moreover, the expression levels of the photoperiod sensitive genes (E1, FT2a, FT5a) were lower in the ygl2 mutant than in the wild type. Mutating the YGL2 gene resulted in increased biliverdin content and decreased heme content. We determined that Lhcb4, a chlorophyll a/b binding protein in photosystem II, interacts with YGL2 but not with the mutant version of the protein. We thus identified a mutation in a P Phi B synthase gene that enhances seed weight in soybean, providing a promising breeding target for this important crop.