Welan Gum, a Bacterial Polysaccharide, Promotes Maize Seedling Growth Through TOR and MAPK Signaling Pathways

Welan gum, a high-molecular-weight exopolysaccharide from Sphingomonas sp., exhibits exceptional rheological stability and robust hydrogel properties, emerging as a highly promising agricultural biomaterial. In this study, we show that the application of welan gum promotes maize seedling growth by enhancing nitrogen use efficiency via the coordinated transcriptional reprogramming of nitrogen regulators and transporters. Mechanistically, welan gum activates the TOR pathway and the MAPK cascade, leading to the phosphorylation of ZmS6K and the terminal MAPK ZmMPK3-1. Notably, TOR inhibition reduces ZmMPK3-1 phosphorylation, demonstrating that TOR activity is indispensable for MAPK activation. Furthermore, the TOR components ZmTOR and ZmLST8 physically interact with ZmMPK3-1, and ZmLST8 independently enhances ZmMPK3-1 kinase activity, revealing a multilayered regulatory mechanism triggered by welan gum. In addition, welan gum upregulates MPK3-targeted defense genes, effectively reconciling the plant growth–defense trade-off. Elucidating this TOR–MAPK crosstalk provides the mechanistic basis for translating welan gum into sustainable gel-based biofertilizers. Collectively, this study uncovers a cooperative signaling network that underpins welan gum-mediated growth promotion, establishing the strong potential of PGPR-derived polysaccharide hydrogels as sustainable, functional biofertilizers for modern agriculture.