The CsPHL11-CsPAL2 module mediates chitooligosaccharide-induced cold tolerance in cucumber seedlings

Cold stress is an important abiotic stressor that significantly affects cucumber production in protected cultivation. Although chitooligosaccharide (COS) is known to enhance plant stress tolerance, the precise molecular mechanisms underlying COS-mediated cold tolerance remain incompletely understood. In this study, we identified a novel transcriptional module, CsPHL11 (PHR1-LIKE 11)-CsPAL2, and elucidated its role in regulating cold response in cucumber seedlings. Exogenous application of COS robustly enhanced cold tolerance by suppressing the increases in relative electrolyte leakage and malondialdehyde levels, while promoting proline accumulation and antioxidant enzyme activities. Transcriptomic and physiological analyses revealed that COS acted as a potent elicitor, strongly inducing the expression of the MYB transcription factor CsPHL11 under cold stress. CsPHL11 directly bound to the promoter of CsPAL2 and activated its transcription, leading to elevated phenylalanine ammonia-lyase activity. This activation facilitated the accumulation of protective secondary metabolites, particularly total phenols and flavonoids, which efficiently scavenged excessive reactive oxygen species and preserved cell membrane integrity. Conversely, silencing of either CsPHL11 or CsPAL2 by virus-induced gene silencing significantly impaired cold tolerance and exacerbated oxidative damage. Overall, these results uncovered a previously unrecognized CsPHL11-CsPAL2 regulatory cascade, providing new mechanistic insights into COS-mediated cold tolerance and highlighting the potential of oligosaccharides for protecting cold-sensitive horticultural crops.