Nitric oxide interacts with brassinosteroid signalling to regulate plant growth and development in an organ-specific manner.

Nitric oxide (NO) is a plant gasotransmitter that regulates plant growth and development by interacting with different regulatory pathways. Among these, brassinosteroids (BRs) have become candidate phytohormones that interact antagonistically and closely with NO to regulate plant growth. Seedling growth in NO-deficient mutants is enhanced and hyper-responds to BR treatments, while NO overaccumulator mutants show some developmental defects. Interestingly, this hyper-response was also observed in mutants that had both constitutively activated BR signalling and reduced NO levels. BR signalling mutants exhibited different responses to the repressive role of NO in seedlings. The phenotypes observed were attributed to the induction of BR-repressed biosynthetic genes and the repression of BR-induced growth-promoting genes detected in NO-overaccumulating plants, with opposite gene expression patterns detected in NO-deficient plants. Furthermore, the activation of BR signalling has an impact on NO accumulation, as BR-treated plants or mutants with activated BR signalling showed hyperaccumulation of NO. Finally, elevated endogenous levels of brassinolide, the most bioactive BR, were found in NO-deficient plants, which could explain the growth promotion observed in this mutant. These findings contribute to a better understanding of the growth-repressive role of NO during seedling development through its interaction with BR biosynthesis, signalling, and responses.