Seed germination and postgerminative growth are critical developmental phases in the life cycle of a flowering plant. The phytohormone abscisic acid (ABA) acts as a crucial signal to regulate multiple physiological processes in plants. Previous studies have highlighted the close relationship between brassinosteroids (BRs) and ABA during seed germination and postgerminative growth, demonstrating that BRs act antagonistically with ABA to promote these processes. However, the exact molecular mechanism underlying the antagonistic effect of BRs on ABA remains unclear.

Dr. HU Yanru and his teacher Prof. YU Diqiu of Xishuangbanna Tropical Botanical Garden (XTBG) conducted a study to further characterize the regulatory role of BRASSINOSTEROID INSENSITIVE2 (BIN2), a critical repressor of brassinosteroid (BR) signaling, in mediating ABA response during seed germination and subsequent postgerminative growth in Arabidopsis.

The researchers evaluated germination of bin2-1 seeds on half-strength Murashige and Skoog medium supplemented with various concentrations of ABA and found that BIN2 positively regulated ABA response during seed germination. They used the yeast two-hybrid system to identify its potentially interacting proteins involved in ABA responses and demonstrated that the BIN2 kinase physically interacted with the ABI5 transcription factor in plant cell nuclei. 

To test whether the action of BIN2 during ABA responses required ABI5, the researchers generated BIN2-overexpressing transgenic plants (Ws/35S: MYC-BIN2) and introduced the overexpression into abi5-1 mutant background (abi5-1/35S: MYC-BIN2). The observations indicated that the ABA hypersensitivity of BIN2-overexpressing plants required ABI5. They purified the bacterial-expressed 63His-BIN2 and GST-ABI5 fused proteins and conducted in vitro kinase assays and found that BIN2 positively affected the ABA-induced accumulation of ABI5 protein, while BRs repressed the regulation of BIN2. 

The research findings demonstrated that BIN2 interacted with and phosphorylated ABI5 to positively regulate ABA response during seed germination, while BRs suppressed the activity of BIN2 to antagonize ABA’s inhibitory role. 

The study entitled “BRASSINOSTEROID INSENSITIVE2 Interacts with ABSCISIC ACID INSENSITIVE5 to Mediate the Antagonism of Brassinosteroids to Abscisic Acid during Seed Germination in Arabidopsis” has been published online in The Plant Cell.