The mulberry-silkworm interactions: A plant-herbivore coevolutionary arms race mediated by defense chemicals and proteins in mulberry latex and adaptive enzymes in the silkworm

We found that mulberry leaves, the hostplant of the silkworm Bombyx mori, are toxic to generalist caterpillars (Samia ricini, Mamestra brassicae) due to the ingredients of latex that exudes from damaged leaf veins[1]. Mulberry (Morus australis) latex contained high concentrations (1.5-2.5%fresh latex, 8-18%dried latex) of toxic alkaloidal sugar-mimic glycosidase inhibitors such as 1,4-dideoxy-1,4-imino-D-arabinitol (D-AB1) and 1-deoxy nojirimycin (DNJ)[1]. Further we identified a novel 56kD defense protein designated mulatexin (MLX56) with chitin-binding and extensin domains from mulberry latex, which inhibited the growth of lepidopteran larvae at low concentrations (0.02 % to wet diet)[2]. MLX56 bound to chitin, and was protease-resistant, which may guaranty its defensive function in insect midgut[2]. Both sugar-mimic alkaloids and MLX56 showed toxicities to generalists, but not at all to B. mori, a mulberry specialist[1-3]. In S. ricini, sucrose absorption, and trehalose metabolism, are inhibited or disturbed by sugar-mimic alkaloids, but not in B. mori[3]. Sucrase and trehalase activities were inhibited by low concentrations of D-AB1 and DNJ (IC50s of D-AB1: 0.9 μM and 5.5 μM, respectively for midgut sucrase and muscle trehalase) in S. ricini, but not in B. mori even at high concentrations (IC50s: >1000 μM and 160 μM, respectively)[3], which shows that B. mori, has developed enzymatic adaptations to mulberry defense. The results suggest the existence of coevolutionary arms race between mulberry trees and the silkworm. 1. Konno K. and Ono H. et al. (2006) Proc. Natl. Acad. Sci. USA., 103:1337-1341. 2. Wasano N. and Konno K. et al. (2009) Phytochemistry, 70:880-888. 3. Hirayama C. and Konno K. et al. (2007) Insect Biochem. Mol. Biol., 37:1348-1358.