Field-tested HaHB11 and HaHB4 soybean exhibit increased grain number and heat tolerance at the reproductive stage

Soybean is one of the primary sources of vegetable oil and protein worldwide. However, its yield improvement has lagged behind the other major crops. This study explored the potential of the sunflower transcription factor HaHB11 to enhance soybean yield and heat stress tolerance. We generated transgenic soybean plants expressing HaHB11 and evaluated their performance across four field trials. The HaHB11 plants showed a significant increase in grain number per plant compared to controls, which can be related to an increased number of nodes and pods per plant. Flowering dynamics analysis 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 field trials identified temperature as a crucial factor influencing grain number, enhancing the differences exhibited by HaHB11 plants. The pollen from transgenic plants germinated better, and tubes were longer than controls under heat stress. Carbohydrate distribution analyses indicated differential allocation of nutrients, supporting the increased pod and grain set in HaHB11 plants. Additionally, vegetation indices can distinguish HaHB11 plants from controls in several developmental stages. These results indicated that HaHB11 enhances soybean yield under heat stress, becoming a promising technology for soybean improvement.