| The labial salivary enzyme, glucose oxidase (GOX) in the caterpillar, Helicoverpa zea, and ribonuclease A (RNase A) found in the regurgitant of the Mexican bean beetle, Epilachna varivestis Mulsant, were determined to influence induced plant defenses. Caterpillar salivary gland ablation and spinneret cauterization techniques were developed that could suppress the secretion of labial saliva. Tobacco, Nicotiana tabacum, leaves fed upon by caterpillars with suppressed labial salivary secretions had significantly higher levels of nicotine, an inducible anti-herbivore defense compound, than leaves fed upon by caterpillars with unaltered labial saliva. Tobacco leaves were wounded with a cork borer to simulate insect feeding and treated with water, salivary gland extracts with active GOX, extracts with inactive GOX (autoclaved), purified GOX, or purified inactive GOX. Leaves treated with salivary gland extracts with active GOX or purified active GOX had significantly less nicotine compared to the other wounded treatments. In addition, neonates that fed upon tobacco plants treated with the GOX had higher levels of survival. Glucose oxidase, is the first characterized suppressor of a plant's herbivore defenses.; The Mexican bean beetle, Epilachna varivestis Mulsant, is a vector of Southern bean mosaic virus (SBMV) on Phaseolus vulgaris L. cv. ‘Black Valentine’ and cv. ‘Pinto’ bean. Pinto bean plants fed upon by beetles had increased resistance to SBMV compared to non-wounded or artificially wounded plants treated with phosphate buffer. Plants that were artificially wounded and treated with RNase A, a constituent of beetle regurgitant, had increased resistance to the plant virus that was equal to plants fed upon by adult beetles. Beetle larvae that fed on SBMV-infected plants weighed more than those that fed on healthy plants. This study supports the finding that RNase A is one of a few known insect-derived elicitors of plant pathogen defenses and the first one detected in beetles. In addition, our experiments also suggest that there may be a mutualistic relationship between the beetle and SBMV. The virus benefits by being transmitted and the beetle benefits with increased fitness from feeding on virus-infected leaf tissue. |
