The sunflower transcription factor HaHB11 increases soybean grain number and heat tolerance across multi-season field trials

Soybean is one of the primary sources of vegetable oil and protein worldwide; however, its yield improvement has lagged behind the other major crops. In this study we explored the potential of sunflower HaHB11, a member of the homeodomain-leucine zipper I subfamily of transcription factors, to enhance soybean yield and heat-stress tolerance. We generated transgenic soybean plants expressing HaHB11 and evaluated their performance in field trials across four different years. The HaHB11 plants showed a significant increase in grain number per plant compared to controls, which was related to an increased number of nodes and pods per plant. Examination of flowering dynamics revealed delayed blooming and an increased number of flowers per node, leading to a higher pod set, particularly between nodes four and six. Principal component analysis across the field trials identified temperature as a crucial factor influencing grain number, enhancing the differences exhibited by the HaHB11 plants. The pollen from transgenic plants had better germination, and tubes were longer than controls under heat stress. Carbohydrate distribution analyses indicated differences in the allocation of nutrients that could support the increased pod number and grain set in the HaHB11 plants. In addition, the HaHB11 and control plants could be distinguished by vegetation indices at several developmental stages. Our results therefore indicated that HaHB11 enhances soybean yield under heat stress, making it a promising target for crop improvement.