Soybean GmHY2a encodes a phytochromobilin synthase that regulates internode length and flowering time.

Plant height and flowering time are important agronomic traits that directly affect soybean [Glycine max (L.) Merr.] adaptability and yield. Here, the Glycine max long internode 1 (Gmlin1) mutant was selected from an ethyl methyl sulfonate (EMS)-mutated Williams 82 population due to its long internodes and early flowering. Using bulked segregant analysis (BSA), the Gmlin1 locus was mapped to Glyma.02G304700, a homologue of the Arabidopsis HY2 gene, which encodes a phytochromobilin (PΦB) synthase involved in phytochrome chromophore synthesis. Mutation of GmHY2a results in failure of the de-etiolation response under both red and far-red light. The Gmlin1 mutant exhibits a constitutive shade avoidance response under normal light, and the mutations influence the auxin and gibberellin pathways to promote internode elongation. The Gmlin1 mutant also exhibits decreased photoperiod sensitivity. In addition, the soybean photoperiod repressor gene E1 is down-regulated in the Gmlin1 mutant, resulting in accelerated flowering. The nuclear import of phytochrome A (GmphyA) and GmphyB following light treatment is decreased in Gmlin1 protoplasts, indicating that the weak light response of the Gmlin1 mutant is caused by a decrease in functional phytochrome. Together, these results indicate that GmHY2a plays an important role in soybean phytochrome biosynthesis and provide insights into the adaptability of the soybean plant.