Susan L. F. Meyer

Post-transcriptional gene silencing of root-knot nematode in transformed soybean roots.

RNAi constructs targeted to four different genes were examined to determine their efficacy to reduce galls formed by Meloidogyne incognita in soybean roots. These genes have high similarity with essential soybean cyst nematode (Heterodera glycines) and Caenorhabditis elegans genes. Transformed roots were challenged with M. incognita. Two constructs, targeted to genes encoding tyrosine phosphatase (TP) and mitochondrial stress-70 protein precursor (MSP), respectively, strongly interfered with M. incognita gall formation. The number of galls formed on roots transformed with constructs targeting the M. incognita TP and MSP genes was reduced by 92% and 94.7%, respectively. The diameter of M. incognita inside these transformed roots was 5.4 and 6.5 times less than the diameter of M. incognita found inside control plants transformed with the empty vector. These results indicate that silencing the genes encoding TP and MSP can greatly decrease gall formation and shows a promising solution for broadening resistance of plants against this plant-parasitic nematode.

Isolation of flavipin, a fungus compound antagonistic to plant-parasitic nematodes

An isolate of the fungus Chaetomium globosum produced culture broths that inhibited in vitro egg hatch and juvenile mobility of root-knot nematode (Meloidogyne incognita) and hatch of soybean cyst nematode (Heterodera glycines). Extraction and bioassay-directed fractionation of the culture broth filtrate determined that flavipin, a low molecular weight compound, was the fungus metabolite responsible for most of the nematode-antagonistic activity. Synthesis of flavipin permitted evaluation of the compound as a suppressor of nematode populations on plants in glasshouse studies. Muskmelon (Cucumis melo) plants in steamed and unsteamed soil were inoculated with root-knot nematodes and various concentrations of flavipin were applied to the soil. Contrary to expectations from the in vitro studies, the number of galls per g of roots increased with flavipin treatment at the 14-day harvest. No effect of flavipin on nematode populations was found at the 55-day harvest. In general, plant growth and nematode populations were greater in plants grown in steamed soil.

Novel Management Strategies for Plant Parasitic Nematodes

At present, the most effective control of plant parasitic nematodes is through the use of chemical nematicides. These nematicides are relatively stable, broad spectrum pesticides that are applied in large dosages to the field. Difficulties with pesticide applications have arisen, however, as nematicides have been found to be a major source of ground water contamination in many locations worldwide. The problems of groundwater contamination, food safety, and worker protection from pesticides make the future use of these products uncertain.

Synthesis and Biological Evaluation of 3,5‐Dimethoxystilbene Analogs

In our continuing effort to discover natural product‐based pest management agents, derivatives of 3,5‐dimethoxystilbene were synthesized yielding 27 new and six known compounds. Compounds 11 and 12 showed strong Aedes aegypti larvicidal activity (LC50 45.31 and 49.93 μm, respectively). Furthermore, 11 and 12 exhibited high effectiveness against larvae of pesticide‐susceptible and pyrethroid‐resistant strains of Ae. aegypti; activity against the adult mosquitoes was low. Compounds 6f, 6g, and 6i at either 83.3 or 166.7 μg/ml reduced the mobility of second‐stage juveniles (J2) of the root‐knot nematode (Meloidogyne incognita) that hatched from eggs immersed in the test compounds for 7 days. However, there was little or no effect on J2 placed directly into these compounds, and none of the analogs suppressed M. incognita egg hatch. The compounds were tested for inhibition of some agriculturally important fungi; 6a, 7a, and 7e demonstrated strong inhibition of Colletotrichum species. Activity of the stilbenes against some human pathogens was also explored; 11, 12, and 16 showed moderate inhibitory activity against Cryptococcus neoformans, Staphylococcus aureus, methicillin‐resistant S. aureus, and Mycobacterium intracellulare. Except for 11 and 12, which were active against mosquito larvae and some human pathogens, no single analog demonstrated activity in all the tests, indicating specific activities. Synthesis of the analogs and structure–activity relationships are discussed.