Rice (), the staple food for almost half of the world's population, prefers ammonium (NH ) as the major nitrogen resource, and while NH 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 NH 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 (), a key BR biosynthetic gene. NH induced the miR444-OsBRD1 signaling cascade in roots, thereby increasing the amount of BRs, whose biosynthesis and signaling were required for NH dependent root elongation inhibition. Consistently, miR444-overexpressing rice roots were hypersensitive to NH depending on BR biosynthesis, and overexpression of miR444's target, , resulted in rice hyposensitivity to NH in root elongation, which was associated with a reduction of BR content. In summary, our findings reveal a cross talk mechanism between NH and BR in which NH activates miR444-OsBRD1, an undescribed BR biosynthesis-promoting signaling cascade, to increase BR content, inhibiting root elongation in rice.

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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7054888PMC
http://dx.doi.org/10.1104/pp.19.00190DOI Listing

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