Synergistic defensive function of raphides and protease through the needle effect

Area of Science:

• Plant biology
• Chemical ecology
• Entomology

Background:

• Raphides, needle-shaped calcium oxalate crystals in plants, are hypothesized to deter herbivores.
• Previous research lacked detailed bioassays using purified raphides to confirm their defensive roles and mechanisms.

Purpose of the Study:

• To experimentally investigate the defensive roles and functional mechanisms of purified raphides.
• To assess the synergistic effects of raphides with cysteine protease on herbivore growth and survival.

Main Methods:

• Bioassays were conducted using Eri silkmoth (Samia ricini) larvae fed castor oil plant leaves treated with purified kiwifruit raphides.
• The effect of raphides was tested alone, with cysteine protease, and compared to amorphous calcium oxalate.

Main Results:

• Raphides and cysteine protease exhibited weak individual defensive activity.
• Combined raphides and cysteine protease demonstrated strong synergistic growth-reducing effects and high caterpillar mortality.
• Amorphous calcium oxalate did not show synergism, indicating the needle shape of raphides is crucial.

Conclusions:

• This study provides the first experimental evidence for synergism between raphides and other defensive factors.
• The 'needle effect' of raphides, facilitating penetration of other bioactive factors, is proposed as a key defense mechanism.
• This mechanism may also explain raphide-induced allergies and the effects of other needle-shaped materials.