Damage-associated molecular patterns (DAMPs) as future plant vaccines that protect crops from pests

Domestication reduced the resistance of most crops which, therefore, require an ever-increasing application of pesticides. Nevertheless, in most domesticated species, the resistance against herbivores and disease agents can be activated by so-called elicitors. Here, we review various strategies of biological pest control and particularly focus on damage-associated molecular patterns (DAMPs) as a potential novel class of plant vaccines: plant-derived molecules that trigger immunity because they indicate damage when they appear in the extracellular space or, in the case of damage-induced volatile compounds, in the aerial space around the plant. The application of plant or algal extracts, which arguably contain many DAMPs, enhanced the resistance against herbivory in cabbage, tomato and maize and to fungal or bacterial disease in all major grain crops and in banana, apple, grapevine, melon, tomato, cucumber or carrot. Similarly, purified DAMPs such as ATP, fragmented DNA, peptides, cell-wall fragments, or volatiles, elicited resistance-related responses in species such wheat, corn, soybean, grapevine, bean, eggplant, cucumber, potato or tomato. In conclusion, plant-based preparations with different degrees of purity can improve the health of multiple crops and horticultural plants. Commercial products based on plant extracts are available, and leaf extracts can even be home-made, e.g., by low-income farmers. We conclude that plant extracts and their derivatives bear an as-yet underestimated potential to serve as environmentally friendly vaccines of cultivated plants.