Auxin signal transcription factor regulates expression of the brassinosteroid receptor gene in rice

Summary The phytohormones auxins and brassinosteroids are both essential regulators of physiological and developmental processes, and it has been suggested that they act inter‐dependently and synergistically. In rice ( Oryza sativa ), auxin co‐application improves the brassinosteroid response in the rice lamina inclination bioassay. Here, we showed that auxins stimulate brassinosteroid perception by regulating the level of brassinosteroid receptor. Auxin treatment increased expression of the rice brassinosteroid receptor gene Os BRI 1 . The promoter of Os BRI 1 contains an auxin‐response element (Aux RE ) that is targeted by auxin‐response factor ( ARF ) transcription factors. An Aux RE mutation abolished the induction of Os BRI 1 expression by auxins, and Os BRI 1 expression was down‐regulated in an arf mutant. The Aux RE motif in the Os BRI 1 promoter, and thus the transient up‐regulation of Os BRI 1 expression caused by treatment with indole‐3‐acetic acid, is essential for the indole‐3‐acetic acid‐induced increase in sensitivity to brassinosteroids. These findings demonstrate that some ARF s control the degree of brassinosteroid perception required for normal growth and development in rice. Although multi‐level interactions between auxins and brassinosteroids have previously been reported, our findings suggest a mechanism by which auxins control cellular sensitivity to brassinosteroids, and further support the notion that interactions between auxins and brassinosteroids are extensive and complex.