The aim of this review was to undertake a survey of researchers working with plant-parasitic nematodes in order to determine a ‘top 10’ list of these pathogens based on scientific and economic importance. Any such list will not be definitive as economic importance will vary depending on the region of the world in which a researcher is based. However, care was taken to include researchers from as many parts of the world as possible when carrying out the survey. The top 10 list emerging from the survey is composed of: (1) root-knot nematodes (Meloidogyne spp.); (2) cyst nematodes (Heterodera and Globodera spp.); (3) root lesion nematodes (Pratylenchus spp.); (4) the burrowing nematode Radopholus similis; (5) Ditylenchus dipsaci; (6) the pine wilt nematode Bursaphelenchus xylophilus; (7) the reniform nematode Rotylenchulus reniformis; (8) Xiphinema index (the only virus vector nematode to make the list); (9) Nacobbus aberrans; and (10) Aphelenchoides besseyi. The biology of each nematode (or nematode group) is reviewed briefly.

/pdf/top-10-plant-parasitic-nematodes-in-molecular-plant-22vrx0hc6m.pdf

Top 10 plant-parasitic nematodes in molecular plant pathology

In the spectrum of plant-microbe interactions disease is a rare outcome. In many interactions complex, integrated defense mechanisms prevent infection and disease. These defensive systems include preformed physical and chemical barriers as well as inducible defenses such as the strengthening of cell walls or synthesis of antimicrobial compounds (i.e., phytoalexins) and proteins.1,2 In certain cases plants react to pathogen attack by developing long-lasting, broad-spectrum systemic resistance to later attacks by pathogens. This phenomenon, termed systemic acquired resistance (SAR), has been observed in many species and may be ubiquitous among higher plants. In the last five years progress has been made toward understanding the molecular basis of SAR. In this review we first provide a brief history of SAR research, then describe our present knowledge of the manifestation and induction of SAR. We discuss recent findings that indicate a central role for the SAR pathway in plant health and finally present our current working model of SAR induction.

/pdf/systemic-acquired-resistance-2qr1m6rcwh.pdf

Systemic Acquired Resistance

Top 10 plant parasitic nematodes in molecular plant pathology in (Abstracts from the 6th International Congress of Nematology, Cape Town, South Africa, 4-9 May 2014)

Plants are one of the world’s richest sources of natural medicines. The use of plants and plant extracts for healing dates back to earliest recorded history. Today, such plant-derived medicines as quinine, digitalis, opiates and morphine are widely used, while new natural chemicals such as the putative anti-cancer drug taxol from yew tree bark are being characterized and developed.

The salicylic acid signal in plants

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

Summary

We have established culture conditions for successful infection and development of several economically important cyst-forming and root-knot nematodes on Arabidopsis thaliana under monoxenic conditions. Complete life cycles were obtained with the sedentary cyst nematodes Heterodera schachtii, H. trifolii, H. cajani and the root-knot nematodes Meloidogyne incognita and M. arenariaas well as with the migratory nematode Pratylenchus penetrans. In contrast, H. goettingiana and Globodera rostochiensis were unable to develop on Arabidopsis roots. Tissue-culture quality agar and medium conditions optimized for hydroponic root culture were essential for successful infections. Detailed in-vivo observations were made inside Arabidopsis roots during the early infection stages of M. incognita and during complete development of H. schachtii. Seventy-four different ecotypes of Arabidopsis were screened for their susceptibility towards H. schachtii resulting in a range of infection rates. None of the ecotypes tested showed complete resistance in vitro. The use of Arabidopsis as a host for plant-parasitic nematodes will provide a new model system for the molecular genetic analysis of this interaction.

Arabidopsis thaliana as a new model host for plant‐parasitic nematodes

Research on the invasion and migration behaviour of sedentary nematodes is reviewed, with emphasis on the root-knot nematodes, Meloidogyne spp., and with particular reference to the orientation in the host, changes that occur in the nematode, the importance of nematode secretory products and the response of the plant. A comparison with cyst-forming nematodes highlights two important features during Meloidogyne migration which are discussed: the intercellular movement of the juveniles through plant tissues and the ability of orientation within the root tip. As the second-stage juveniles of all Meloidogyne species show this behaviour independent of the host, future research on these two topics may provide information leading to the development of plants with resistance to root-knot nematodes that operates during the early stages of infection.

Invasion and Migration Behaviour of Sedentary Nematodes

Plant-parasitic nematodes are a major problem in modern agriculture and cause diseases in all crops of economic importance. Worldwide, yield losses are estimated to run into tens of billions of dollars per year. In general, symptoms of nematode infestation in the field are patches of poorly growing plants, often accompanied by wilting. In roots, symptoms depend on the nematode species and may involve galling, stunted root growth, and necrosis. To prevent the buildup of unacceptably high population densities, farmers tend to use integrated pest management systems, which combine the use of nematicides, crop rotation, and, if available, resistant cultivars. Nematicides are the most expensive and toxic pesticides that are in widespread use in modern agriculture and evoke increasing opposition for environmental reasons. This is illustrated by the drastic reductions in the use of nematicides that are proposed by the European Community for the next decade. A large-scale ban on nematicides will further increase the agronomic problems that result from these pests. Since only a limited number of resistance traits are available for use in breeding programs, genetic engineering of nematode resistance may provide crucial alternatives for germ plasm improvement. Compared to fungal and bacterial phytopathology, our knowledge of plant/nematode interactions is limited and progressing slowly (Sijmons et al. 1994). The long and complex life cycle of these obligate parasites poses a major limitation for experimental approaches under laboratory conditions. There are no protocols available for axenic growth, and only a few species have been reported to grow on undifferentiated callus...

Nicola von Mende

Papers

Arabidopsis thaliana as a new model host for plant‐parasitic nematodes

Invasion and Migration Behaviour of Sedentary Nematodes

28 Plant-parasitic Nematodes