The signaling pathway via NO has been investigated in tomato cell cultures, where NO is known to induce phosphatidic acid (PA) production after pathogen attacks. NO is required for PA production. When the NO inhibitor, CPTIO, was applied together with chitosan, a decreased level of PA accumulation was found in both the phospholipase D (PLD) and phospholipase C (PLC) pathways, which act with diacylglycerol kinase (DGK) pathways (PLC/DGK; Raho et al., 2011). In addition, PLDα-1 derived PA interacts with ABI1, which is the negative regulator of ABA, promoting ABA signaling (Zhang et al., 2004).
Based on the signaling information, the proposed cellular responses to chitosan are shown in Fig. 1. Because the chitosan receptor has not been identified, chitosan appears to elicit plant cells using charge–charge interactions between the positively charged amine groups of chitosan and the negatively charged phospholipids, and not via a receptor-specific interaction ( Kauss et al., 1989). Although a chitin receptor, CEBiP, was identified in Arabidopsis thaliana, it showed no binding activity with the chitosan heptamer ( Kaku et al., 2006).