Promotion of BR biosynthesis by miR444 is required for ammonium-triggered inhibition of root growth (Plant Physiol.) ($)

Nitrogen, as one of the most important plant nutrients, profoundly affects root growth. Nitrogen is usually taken up in one of two forms, nitrate (NO₃^–) or ammonium (NH₄⁺), and the two forms affect root growth differently. Here, Jiao et al. demonstrate a connection between ammonium, a small RNA, and brassinosteroid hormones. Specifically, they show that NH₄⁺ induces miR444, which targets MADS-box transcription repressors that repress expression of a BR synthesis gene, leading to increased levels of brassinosteroids, and root growth inhibition. Similarly, adding BRZ, an inhibitor of brassinosteroid synthesis, to the growth medium alleviated the repression induced by NH₄⁺, showing that ammonium-inhibition of root growth is mediated through brassinosteroids.  Given that ammonium is a preferred nitrogen source for rice, this finding may lead to strategies to enhance root growth. (Summary by Mary Williams) Plant Physiol. 10.1104/pp.19.00190