Promotion of BR biosynthesis by miR444 is required for ammonium-triggered inhibition of root growth

Abstract

Rice (Oryza sativa), the staple food for almost half of the world's population, prefers ammonium (NH41) as the major nitrogen resource, and while NH41 has profound effects on rice growth and yields, the underlying regulatory mechanisms remain largely unknown. Brassinosteroids (BRs) are a class of steroidal hormones playing key roles in plant growth and development. In this study, we show that NH41 promotes BR biosynthesis through miR444 to regulate rice root growth. miR444 targeted five homologous MADS-box transcription repressors potentially forming homologous or heterogeneous complexes in rice. miR444 positively regulated BR biosynthesis through its MADS-box targets, which directly repress the transcription of BR-deficient dwarf 1 (OsBRD1), a key BR biosynthetic gene. NH41 induced the miR444-OsBRD1 signaling cascade in roots, thereby increasing the amount of BRs, whose biosynthesis and signaling were required for NH41-dependent root elongation inhibition. Consistently, miR444-overexpressing rice roots were hypersensitive to NH41 depending on BR biosynthesis, and overexpression of miR444's target, OsMADS57, resulted in rice hyposensitivity to NH41 in root elongation, which was associated with a reduction of BR content. In summary, our findings reveal a cross talk mechanism between NH41 and BR in which NH41 activates miR444-OsBRD1, an undescribed BR biosynthesis-promoting signaling cascade, to increase BR content, inhibiting root elongation in rice.