Integrated proteomic analysis reveals adaptation mechanisms of paper mulberry seedling leaves to nitrogen stress.

Nitrogen is crucial for plant growth and development. Karst areas face significant degrees of both nitrogen leaching and enrichment, yet paper mulberry (Broussonetia papyrifera) thrives in these areas. Here, the physiology, proteome, phosphoproteome, and ubiquitome of paper mulberry seedling leaves were investigated to understand the mechanisms of plant adaptation to nitrogen stress. We discovered that paper mulberry responds to nitrogen stress by regulating the expression of ammonium transporter protein (AMT1.1) and nitrate and peptide transporter proteins (NPF6.4, NPF8.1, and NPF8.3). Key pathways involved in the response of paper mulberry to different nitrogen levels include photosynthesis, carbon fixation, and nitrogen metabolism. The plant also enhances its antioxidant defences to reduce ROS stress, while protein phosphorylation serves as a signalling component. Lysine ubiquitination may play an essential role in the degradation of misfolded proteins in paper mulberry under low-nitrogen stress. Amino acid, starch, and sucrose metabolism provides the energy necessary for paper mulberry adaptation to high-nitrogen stress. This study provides insights into the possible biological adaptation mechanisms of paper mulberry under controlled nitrogen stress, which may inform future studies relevant to karst areas.