A Unique Latex Protein, MLX56, Defends Mulberry Trees from Insects

The mulberry (Morus spp.)-silkworm (Bombyx mori) relationship has been a well-known plant-herbivore interaction for thousands of years. Recently, we found that mulberry leaves defend against insect herbivory by latex ingredients. Here we report that a 56-kDa (394 amino acid) defense protein in mulberry latex designated mulatexin (MLX56) with an extensin domain, two hevein-like chitin-binding domains, and an inactive chitinase-like domain provides mulberry trees with strong insect resistance. MLX56 is toxic to lepidopteran caterpillars, including the cabbage armyworm, Mamestra brassicae and the Eri silkworm, Samia ricini, at 0.01% concentration in a wet diet, suggesting that MLX56 is applicable for plant protection. MLX56 is highly resistant to protease digestion, and has a strong chitin-binding activity. Interestingly, MLX56 showed no toxicity to B. mori, suggesting that the mulberry specialist has developed adaptation to the mulberry defense. Our results show that defensive proteins in plant latex play key roles in mulberry-insect interactions, and probably also in other plant-insect interactions. Our results further suggest that plant latexes analogous to animal venom contain a treasury of applicable defense proteins and chemicals that has evolved through inter-specific interactions.