Nematodes comprise a large phylum of animals that includes plant and animal parasites as well as many free-living species (Maggenti, 1981). Plant parasitic nematodes are obligate parasites, obtaining nutrition only from the cytoplasm of living plant cells. These tiny roundworms (generally 4 mm long and barely visible to the human eye) damage food and fiber crops throughout the world and cause billions of dollars in losses annually (Sasser and Freckman, 1987). Some plant parasitic nematodes are ectoparasites, living outside their host. These species cause severe root damage and can be important virus vectors (Brown et al., 1995). Other species spend much of their lives inside roots as migratory or sedentary endoparasites. Migratory parasites move through the root, causing massive cellular necrosis. However, it is the sedentary endoparasites of the family Heteroderidae that cause the most economic damage worldwide. This group is the focus of the review. The Heteroderidae can be divided into two groups: the cyst nematodes, which include the genera Heterodera and Globodera; and the root-knot nematodes (genus Meloidogyne). The soybean cyst nematode (Heterodera glycines) is the most economically important pathogen of soybean in the United States. Potato cyst nematodes (Globodera pallida and G. rosfochiensis) cause losses in potato-growing areas worldwide (Ross, 1986). Root-knot nematodes, so-called for the characteristic root galls or root knots that they form on many hosts (Figure lA), infect thousands of plant species and cause sevele losses in yield of many crops throughout the world (Mai, 1985). Symptoms of diseased plants infected by these groups of nematodes include stunted growth, wilting, and susceptibility to other pathogens. Nematodes in these three genera have complex interactions with their host plants that generally last more than a month and result in major morphological and developmental changes in both organisms. During the infection, elaborate developmental and morphological changes occur in host root cells, especially in those that become the feeding cells that provide the sole source of nutrients for the nematode. There has been a recent burst of activity in the investigation of the molecular changes that mediate the host-parasite interaction. This ac-

Nematode Pathogenesis and Resistance in Plants

The feeding sites induced by sedentary root-endoparasitic nematodes have long fascinated researchers. Nematode feeding sites are constructed from plant cells, modified by the nematode to feed itself. Powerful new techniques are allowing us to begin to elucidate the molecular mechanisms that produce the ultrastructural features in nematode feeding cells. Many plant genes that are expressed in feeding sites produced by different nematodes have been identified in several plant species. Nematode-responsive plant genes can now be grouped in categories related to plant developmental pathways and their roles in the making of a feeding site can be illuminated. The black box of how nematodes bring about such elaborate cell differentiation in the plant is also starting to open. Although the information is far from complete, the groundwork is set so that the functions of the plant and nematode genes in feeding site development can begin to be assessed.

Gene expression in nematode feeding sites

Recent breakthroughs in research on plantnematode interactions underpinned by the availability of modern molecular techniques is attracting attention to this particular area of phytopathology. In contrast with most bacterial and fungal pathogens, sedentary nematodes are true parasites and establish a long-lasting interaction with their host. They induce profound alterations in host cell morphology (Fig. 1) that are scientifically intriguing and to a large extent a black box. It is the purpose of this paper to review the latest results that allow us an opportunity to peek inside that box. Eventually, more detailed knowledge of the molecular basis of the interaction will enable us to design elegant strategies to engineer nematode resistance in a range of crop plants.

Plant-nematode interactions.

The phytoecdysteroid, 20-hydroxyecdysone (20E), is a major molting hormone of invertebrates possibly including nematodes. As 20E is inducible in spinach, the defensive role against plant-parasitic nematodes was investigated. The effects of direct application on nematodes was assessed by treating cereal cyst nematode, Heterodera avenae, juveniles with concentrations of 20E from 8.2 × 10−8 to 5.2 × 10−5 M before applying to Triticum aestivum growing in sand. H. avenae, Heterodera schachtii (sugarbeet cyst nematode), Meloidogyne javanica (root-knot nematode) and Pratylenchus neglectus (root lesion nematode) were treated with 5.2 × 10−5 20E and incubated in moist sand. To test the protective effects of 20E in plants, the latter three nematodes were applied to Spinacia oleracea in which elevated concentrations of 20E had been induced by methyl jasmonate. Abnormal molting, immobility, reduced invasion, impaired development, and death occurred in nematodes exposed to 20E either directly at concentration above 4.2 × 10−7 M or in plants. Phytoecdysteroid was found to protect spinach from plant-parasitic nematodes and may confer a mechanism for nematode resistance.

Phytoecdysteroids: a novel defense against plant-parasitic nematodes.

On a worldwide basis, parasitic weeds represent one of the most destructive and intractable problems to agricultural production in both developed and developing countries. About 20 families (3,000–5,000 species) of higher plants are parasitic on the plant kingdom and may cause production losses of 30–80% in staple food and industrial crops on every continent. Compared with the other weeds, parasitic weeds are difficult to control by conventional means because of their life style: Parasites are intimately involved with the host and have so much metabolic overlap with the host that differential treatments are very difficult to develop. In some cases, the parasites are closely associated to the host root, concealed underground, and undiagnosed until they irreversible damage the crop. Several different approaches (cultural, mechanical, chemical, use of resistant varieties, and biological) to control parasitic weeds are currently in use, but are only partially successful. Recent reviews have covered the physiology and interactions between parasitic plants and their hosts, taxonomy, and the biology and classical control of parasitic weeds. The current review will discuss why alternative methods are needed to control parasitic weeds and will summarize conventional and new biotechnology-based control measures against the major world pests Striga, Orobanche, Cuscuta, and mistletoes (Phoradendron and Viscum genera). Effectiveness, advantages and disadvantages, environment safety, and simplicity of these new biotechnological methods will be reviewed.

https://bioone.org/journals/in-vitro-cellular-and-developmental-biology---plant/volume-43/issue-4/s11627-007-9054-5/Conventional-and-biotechnological-approaches-for-control-of-parasitic-weeds/10.1007/s11627-007-9054-5.pdf

Conventional and biotechnological approaches for control of parasitic weeds

Plants have evolved a broad array of defense mechanisms involved in disease resistance. These include synthesis of phytoalexin antibiotics and proteinase inhibitors, deposition of cell wall materials, and accumulation of hydrolytic enzymes such as chitinases. Resistance appears to depend on the ability of the host to recognize the pathogen rapidly and induce these defense responses in order to limit pathogen spread. Application of molecular technologies has yielded significant new information on mechanisms involved in pathogen recognition, signal transduction, and defense-related gene activation, and is leading to novel strategies for engineering enhanced disease resistance. We are using these approaches to analyze regulation of 3-hydroxy-3-methylglutaryl CoA reductase (HMGR), a key enzyme mediating the production of terpenoid defense compounds. This enzyme is encoded by four genes in tomato; hmg2 gene expression is specifically associated with responses to pathogen or defense elicitors. Transgenic plants containing DNA constructs that fuse the hmg2 promoter to a reporter gene have been used to analyze both tissue specificity and patterns of defense-related expression. Because this gene is rapidly induced in tissues directly surrounding the site of ingress by a variety of pathogens, it may serve as a valuable tool in engineering new disease-resistance mechanisms.

X. Yu

Papers

Regulation of Defense-related Gene Expression during Plant-Pathogen Interactions