Levan-derived oligosaccharides (LOS) prime barley against fungal pathogens through asparagine metabolism.

Overuse of chemical pesticides motivates green disease control strategy. Levan-derived oligosaccharides (LOS) are immune elicitors, but its structure information and underlying regulatory mechanisms remain unclear. In this study, LOS produced from sucrose using LevB1SacB fusion enzyme, was purified by CSR-1Na resin, and characterized by HPLC, ATR-FTIR and 1D/2D NMR. Foliar LOS spraying (0.5‰) enhanced barley resistance to Rhizoctonia solani and Pyrenophora graminea, reducing necrotic area by 60.5% and 22.4%, but had little effect on the hemibiotrophic pathogen Bipolaris sorokiniana of spot blotch. LOS itself did not inhibit fungal growth in vitro. LOS priming strengthened infection-triggered antioxidant and defense responses. RNA-seq analysis indicated pathogens but not LOS enhanced transcriptional reprogramming, with differential genes enriched in amino acid metabolism. Among transcriptome-guided candidates, HvAS1, an asparagine synthetase, was verified to participate in LOS-enhanced resistance: HvAS1 overexpression in Arabidopsis accumulated more asparagine and jasmonate after infection and reduced R. solani lesions by >50%, whereas virus-induced silencing in barley compromised resistance. Besides, foliar asparagine spraying also increased barley resistance to R. solani. Together, LOS acts as an efficient elicitor and reveals the link between amino acid metabolism and plant immunity.