This review examines the discovery of naturally occurring phytochemicals antagonistic toward plant-parasitic and other nematodes. Higher plants have yielded a broad spectrum of active compounds, including polythienyls, isothiocyanates, glucosinolates, cyanogenic glycosides, polyacetylenes, alkaloids, lipids, terpenoids, sesquiterpenoids, diterpenoids, quassinoids, steroids, triterpenoids, simple and complex phenolics, and several other classes. Many other antinematodal compounds have been isolated from biocontrol and other fungi. Natural products active against mammalian parasites can serve as useful sources of compounds for examination of activity against plant parasites. The agricultural utilization of phytochemicals, although currently uneconomic in many situations, offers tremendous potential.

The recent de-registration of several chemical nematicides and the impending loss of methyl bromide from the pest-control market necessitate the development of new methods for controlling nematode-induced crop damage. One approach for developing novel target-specific controls is by exploiting fundamental differences between the biological processes of nematodes and their host plants. Researchers of the Agricultural Research Service (ARS) of the US Department of Agriculture are actively exploring these differences. Research accomplishments include the discovery of heat shock protein genes possibly involved in developmental arrest of the soybean cyst nematode, the identification of neuropeptides and female-specific proteins in the soybean cyst nematode, the disruption of nematode reproduction with inhibitors of nematode sterol metabolism, the development of novel morphological and molecular (heat shock protein genes and the D3 segment of large subunit ribosomal DNA) features useful for nematode identification and classification, and the elucidation of the population genetics of potato cyst nematode pathotypes. In addition, several ARS researchers are investigating biological determinants of nematode response to management strategies utilized in agricultural fields. These collective efforts should lead to new chemical and non-chemical alternatives to conventional nematode control strategies.

Caenorhabditis elegans requires sterol, usually supplied as cholesterol, but this is enzymatically modified, and different sterols can substitute. Sterol deprivation decreased brood size and adult growth in the first generation, and completely, reversibly, arrested growth as larvae in the second. After one generation of sterol deprivation, 10 ng/ml cholesterol allowed delayed laying of a few eggs, but full growth required 300 ng/ml. C. elegans synthesizes two unusual 4 ␣ -methyl sterols (4MSs), but each 4MS supported only limited growth as the sole sterol. However, addition of only 10 ng of cholesterol to 1,000 ng of 4MS restored full growth and egg-laying, suggesting that both a 4MS and an unmethylated sterol are required for development. Filipin stained sterols in only a few specific cells: the excretory gland cell, two amphid socket cells, two phasmid socket cells and, in males, spicule socket cells. Sterols were also present in the pharynx and in the intestine of feeding animals in a proximal-to-distal gradient.This non-random sterol distribution, the low concentration requirements, and the effects of 4MSs argues that sterols are unlikely to be used for bulk structural modification of cell membranes, but may be required as hormone precursors and/or developmental effectors. Dietary sterol is required by Caenorhabditis elegans (1, 2) because, like insects, C. elegans is incapable of synthesizing the four-ring sterol nucleus, but its functions remain largely unknown. The existence of sterol-based hormones in C. elegans has recently been suggested, but no hormone has yet been identified (3, 4). Cholesterol is known to be extensively metabolized by C. elegans to form several other sterols, including two unusual 4 ␣ -methyl sterols (4MSs), which are present in substantial amounts (5-8, 9). These sterols might thus be functional, instead of or in addition to cholesterol itself.Insects resemble these nematodes in requiring sterols but being unable to synthesize them. Two functions for cholesterol are known in insects: as the metabolic precursor of the molting hormone ecdysone (10), and as the moiety required for activation by covalent attachment to the morphogen protein hedgehog (11). Insect cells, unlike vertebrate cells, grow normally under sterol-free conditions, and thus do not need cholesterol in their plasma membranes (12)(13)(14).We have now characterized the sterol requirements of C. elegans in some detail. Conditions for stringent sterol deprivation were developed, and the consequences are described. The use of these conditions allowed us to investigate minimum cholesterol requirements, and the ability of other sterols to substitute. Partial and synergistic effects were found, suggesting that different sterols have diverse effects mediated by several pathways. The accumulation of sterol in the intestinal tract and in a few specific cells in C. elegans was also demonstrated by filipin staining, which stains all 3 ␤ -hydroxy sterols.These observations provide a basis for a comprehensive study of ster...

Caenorhabditis elegans excretes a dauer pheromone or daumone composed of ascarylose and a fatty acid side chain, the perception of which enables worms to enter the dauer state for long-term survival in an adverse environment. During the course of elucidation of the daumone biosynthetic pathway in which DHS-28 and DAF-22 are involved in peroxisomal beta-oxidation of VLCFAs (very long-chain fatty acids), we sought to investigate the physiological consequences of a deficiency in daumone biosynthesis in C. elegans. Our results revealed that two mutants, dhs-28(tm2581) and daf-22(ok693), lacked daumones and thus were dauer defective; this coincided with massive accumulation of fatty acyl-CoAs (up to 100-fold) inside worm bodies compared with levels in wild-type N2 worms. Furthermore, the deficiency in daumone biosynthesis and the massive accumulation of fatty acids and their acyl-CoAs caused severe developmental defects with reduced life spans (up to 30%), suggesting that daumone biosynthesis is be an essential part of C. elegans homoeostasis, affecting survival and maintenance of optimal physiological conditions by metabolizing some of the toxic non-permissible peroxisomal VLCFAs from the worm body in the form of readily excretable daumones.

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Caenorhabditis elegans was cultured in semi-defined medium containing yeast extract, soy peptone, glucose, hemoglobin, Tween 80, and sitosterol. Monoglycosylceramides were chromatographically purified from nematode extracts. Their structures were elucidated with mass spectrometry, nuclear magnetic resonance spectroscopy, and analysis of methanolysis products of the parent cerebrosides. The glycosylceramides were unusual in that the only long-chain sphingoid base detected was an iso-branched compound with a C-4 double bond (i.e., 15-methyl-2-aminohexadec-4-en-1,3-diol). Glucose was the only sugar moiety detected. The fatty acids consisted of a series of primarily straight-chain, saturated, 2-hydroxylated C20-C26 acids; some iso-branched analogs also occurred. The sphingomyelins of C. elegans were also hydrolyzed, and the same iso-branched C17 compound was the only sphingoid base detected. This is the first structural analysis of a nematode glycosphingolipid and the first report of an organism in which the long-chain sphingoid bases are entirely iso-branched.

A low-input sustainable agricultural system for the production of staked, fresh-market field tomatoes (Lycopersicon esculentum Mill.) is described. The system uses winter annual cover crops to fix N, recycle leftover nutrients, produce biomass, and prevent soil erosion throughout the winter and spring. Yields of tomato plants grown in hairy vetch (Vicia villosa Roth), crimson clover (Trifolium incarnatum L.), and rye (Secale cereale L.) plus hairy vetch mulches were higher than those grown in the conventional black polyethylene (BP) mulch system in 2 of 3 years. Fruit were heavier with the plant mulches than with BP mulch. Eight weeks after transplanting, N levels in tomato leaves were higher with plant than with BP mulch, although the plant mulch plots received only 50% of the N applied to the BP plots. The cover crops had no effect on populations of five phytoparasitic nematode species.

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 ltrate determined that avipin, a low molecular weight compound, was the fungus metabolite responsible for most of the nematode-antagonistic activity. Synthesis of avipin 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 avipin were applied to the soil. Contrary to expectations from the in vitro studies, the number of galls per g of roots increased with avipin treatment at the 14-day harvest. No effect of avipin 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.