Induction of plant disease resistance by mixed oligosaccharide elicitors prepared from plant cell wall and crustacean shells
Basal plant immune responses are activated by the recognition of the conserved pathogen-associated molecular patterns (PAMPs), or breakdown molecules released from plants after damage by pathogen infection, so-called danger-associated molecular patterns (DAMPs). While chitin-oligosaccharide (CHOS), a primary component of the fungal cell wall, is most known as PAMP, plant cell wall-derived oligosaccharides, cello-oligosaccharides (COS) from cellulose and xylo-oligosaccharide (XOS) from hemicellulose, are representative DAMPs, which activate signaling steps similar to PAMP-induced immunity to elicit defenses and provide protection against pathogens. In this study, elicitor activities of COS prepared from cotton linters, XOS prepared from corn cobs as well as chitin-oligosaccharide (CHOS) from crustacean shells were comparatively investigated. In Arabidopsis, treatment of COS, XOS or CHOS triggered typical defense responses such as reactive oxygen species (ROS) production, activation of MAP kinases phosphorylation, callose depositions, and activation of the promoter for defense-related transcription factor WRKY33. When COS, XOS and CHOS were used at concentrations with similar activity in inducing ROS production and callose depositions, CHOS was particularly highly potent in activating the MAPK kinases and WRKY33 promoters. Among the COS and XOS with different degrees of polymerization (DP), cellotriose (DP3) and xylotetraose (DP4) showed the highest activity for the activation of WRKY33 promoter. Simultaneous treatment of COS, XOS and CHOS leads to a strong transcriptional change for defense-related genes, and gene ontology (GO) enrichment analysis of RNAseq data revealed that a mixture of three oligosaccharide (oligo-mix) effectively activate the plants disease resistance. In practice, treatment of the oligo-mix enhanced the resistance of tomato to powdery mildew, but plant growth was not inhibited but rather tended to be promoted, providing evidence that mixed oligosaccharides have beneficial effects on improving disease resistance in plants, making them a promising class of compounds for practical application.