Synergistic defensive function of raphides and defensive proteins: Raphides intensify the defensive activity of protease and chitinase against insect herbivores through the needle effect

Raphides, needle-shaped calcium oxalate crystals in tissues of many plants, have been thought to play defensive roles against herbivores without detailed bioassays for their defensive roles and modes of function using purified raphides. Therefore, we performed clear bioassays giving the larvae of the Eri silkmoth, Samia ricini,(Saturniidae, Lepidoptera) leaves of their host plant, the castor oil plant, Ricinus communis, painted with the raphides purified from kiwifruits, Actinidia deliciosa, in presence or absence of cysteine protease (or other defense proteins), which often coincide with raphides in plant tissues. Raphides alone or cysteine protease alone showed only weak defensive activities around experimental concentrations. However, when raphides and cysteine protease coexisted, they synergistically showed very strong growth-reducing and insecticidal activities. In contrast, amorphous calcium oxalate did not show synergism with cysteine protease, indicating that the needle-shape of raphides is essential for the synergism. The present study provides the first clear experimental evidence for the synergism between raphides (physical defense) and other defensive factors (chemical defense). Further, the study suggests the existence of “the needle effect”, which intensifies the bioactivities of other bioactive factors by making holes to the barriers (cell membrane, cuticle, epithelium, etc.) and facilitate the factors to reach the targets. We recently found that raphides and chitinase also show a very strong synergistic defensive (insecticidic) activity against insect herbivores, which further indicated the generality of synergism between raphides and defensive proteins.

Reference

Konno K, Inoue TA, Nakamura M (2014) Synergistic defensive function of raphides and protease through the needle effect. PLOS ONE 9(3): e91341