BES1-Mediated Brassinosteroid Signaling Negatively Regulates Biosynthesis and Signaling of Abscisic Acid to Increase Seed Size in Arabidopsis

Expression of ABA-deficient 2 (ABA2) gene involved in ABA biosynthesis was downregulated in bes1-D, but upregulated in bes1-KO. Hence, the expression of ABA2 is negatively controlled by the BES1 transcription factor of brassinosteroid signaling in Arabidopsis thaliana. BES1 is directly bound to the E-box sequences in the promoter of ABA2, which reduced the endogenous levels of ABA in the plant. The seeds of aba2-1 and bes1-KO were larger and smaller, respectively, than those of the wild type. In bes1-KO x aba2-1, the reduced seed size in bes1-KO was partly restored to the seed size in aba2-1. Hence, BES1-mediated regulation of seed size is an upstream process for the homeostasis of endogenous ABA to control the seed size. BES1 suppressed the expression of ABA Insensitive 5 (ABI5), a major transcription factor gene in ABA signaling that determines the seed size. It is directly bound to the promoter of ABI5 by BES1-induced downregulation of ABI5. The expression of SHORT HYPOCOTYL UNDER BLUE1, MINISEED3, and HAIKU2 as positively regulatory genes determining the seed size was activated, increasing the seed size in bes1-D. Conclusively, brassinosteroid signaling through BES1 downregulated both the biosynthesis and signaling of ABA, which increased the seed size in Arabidopsis.