β-1,4- <scp>d</scp> -Glucan-Based Oligomers Modulate PTI to Fine-Tune Lettuce Immune Responses

Damage to plant cell walls releases endogenous molecules that act as damage-associated molecular patterns (DAMPs) to activate immunity. The contribution of cellulose-derived β-1,4-glucan oligomers (cello-oligosaccharides) to this process remains unclear. Here, we show that cello-oligosaccharides elicit pattern-triggered immunity (PTI) in <i>Lactuca sativa</i> in a chain-length-dependent manner. Across DP2-DP5, cellotetraose (CEL4) elicited the strongest responses, including rapid ROS production, MPK6 phosphorylation, defense gene induction, stomatal closure, and callose/lignin deposition. Transcriptomic analyses revealed that chain length qualitatively encodes the signaling output by selectively engaging defense modules rather than simply amplifying response amplitude. CEL4 further potentiated flg22-induced immunity and reduced <i>Botrytis cinerea</i> infection, indicating DAMP-PAMP synergy. CEL4-induced immunity was transient and growth-compatible, with photosynthetic and metabolic recovery within 12 h. Thus, CEL4 represents a structure-specific, growth-compatible immunomodulator, and chain length emerges as a key informational cue for decoding carbohydrate DAMPs, with implications for sustainable, immune-centered crop protection.