A Root Skewing Response as A Strategy to Avoid Ammonium-induced Acidic Stress in Rice

Plants can activate complex signaling pathways to adapt their root system to varying soil conditions. Nitrogen supply has been shown to have significant impacts on the root development; however, how nitrogen affect rice root development is not fully understood. Here, we show that high concentration of ammonium, rather than nitrate, triggers a skewing pattern of rice root under hydroponic system. Interestingly, the root skewing response correlates with a reduced of pH in the medium containing high ammonium concentration, and rice root grown in acidic medium also skewed and slowly elongated. Supplement of MES to the medium buffers the pH and suppresses the ammonium-induced skewing response, suggesting acidity caused by ammonium uptake might be the factor that triggers root skewing response. We further show that this root response is enhanced by over-expression of OsAMT1;1, an ammonium transporter. Furthermore, high ammonium or low pH treatments both trigger asymmetric distribution of auxin responsive marker DR5 in the root cap and epidermis, and the skewing responses of rice root under high ammonium and low pH can be repressed by auxin transporter inhibitors, indicating that auxin transporters mediate the ammonium-induced skewing response. Therefore, our discovery reveals a novel root response and a signaling pathway essential for rice root to avoid acidic environment during nitrogen uptake.