Nitric oxide action in the digestive fluid of Nepenthes × ventrata is linked to the modulation of ROS level

Nepenthes are carnivorous plants with photoactive leaves converted into jug-shaped containers filled with the digestive fluid. The digestion requires various enzymes and reactive oxygen species (ROS) that facilitate proteolysis. Reactive nitrogen species are present in the digestive fluid of Nepenthes × ventrata, and the increased nitric oxide (NO) formation is associated with protein degradation. The aim of the work was to verify the beneficial effect of NO application into the trap on the dynamics of protein digestion and ROS homeostasis. Measurements were done using the digestive fluid or the tissue collected from the mature pitcher plants (fed) grown in a greenhouse. Two independent methods confirmed NO formation in the digestive fluid of fed and non-fed traps. NO supplementation with food into the trap accelerated protein degradation in the digestive fluid by increasing the proteolytic activity. NO modulated free radical formation (as the result of direct impact on NADPH oxidase), stimulated ROS scavenging capacity, increased -SH groups and flavonoids content, particularly at the beginning of the digestion. In non-fed traps, the relatively high level of protein nitration in the digestive fluid may prevent self-protein proteolysis. Whereas, after initiation of the digestion decreasing level of nitrated proteins in the fluid may indicate their accelerated degradation. Therefore, it can be assumed that NO exhibits a protective effect on the fluid and the trap tissue before digestion, while during digestion, NO is an accelerator of protein decomposition and the ROS balance keeper.