Host-Parasite Relationships in Fall-Sown Sugar Beets Infected by the Stem and Bulb Nematode, Ditylenchus dipsaci

Host plant status and damage threshold of spinach (Spinacia oleracea) for the temperate root-knot nematode Meloidogyne chitwoodi

Problems with plant-parasitic nematodes remain a major challenge in most field vegetable crops worldwide. In agricultural fields in Belgium, these problems have increased in recent years. A major problem is the quarantine root-knot nematode Meloidogyne chitwoodi. The host plant status of the most commonly grown cultivars of spinach (Spinacia oleracea) and the damage threshold for M. chitwoodi were determined in a climate-controlled glasshouse and in a naturally infested field. The yield data with the final population densities were fitted to Seinhorst’s yield and population dynamic models, respectively. The damage threshold values (T) for the relative fresh weight yield were 0.14, 1.08, 11.53 J2 (100 cm3 soil)−1, minimum yield (m) of 0.84, 0.64, 0.52 and maximum yield () of 12.51, 12.44 and 9.60 for ‘Gnu’, ‘Meerkat’ and ‘Whale’ spinach cultivars, respectively. The maximum multiplication rates (a) were 3.81, 2.38 and 2.72, whilst the maximum population densities (M) were 563, 916, and 922 J2 (100 cm3 soil)−1 for ‘Gnu’, ‘Meerkat’ and ‘Whale’, respectively. Meloidogyne chitwoodi had an impact on the spinach yield. The higher the nematode density the lower the yield and vice versa. The tested cultivars were all hosts for M. chitwoodi but differences between cultivars occurred. ‘Kolibri’ and ‘Rhino’ were tested as the least sensitive cultivars for M. chitwoodi during the host screening test and bring opportunities for resistance breeding. This is the first report of spinach as a host for M. chitwoodi.

Parallel adaptations and common host cell responses enabling feeding of obligate and facultative plant parasitic nematodes

Parallel adaptations enabling the use of plant cells as the primary food source have occurred multiple times in distinct nematode clades. The hallmark of all extant obligate and facultative plant-feeding nematodes is the presence of an oral stylet, which is required for penetration of plant cell walls, delivery of pharyngeal gland secretions into host cells and selective uptake of plant assimilates. Plant parasites from different clades, and even within a single clade, display a large diversity in feeding behaviours ranging from short feeding cycles on single cells to prolonged feeding on highly sophisticated host cell complexes. Despite these differences, feeding of nematodes frequently (but certainly not always) induces common responses in host cells (e.g. endopolyploidization and cellular hypertrophy). It is thought that these host cell responses are brought about by the interplay of effectors and other biological active compounds in stylet secretions of feeding nematodes, but this has only been studied for the most advanced sedentary plant parasites. In fact, these responses are thought to be fundamental for prolonged feeding of sedentary plant parasites on host cells. However, as we discuss in this review, some of these common plant responses to independent lineages of plant parasitic nematodes might also be generic reactions to cell stress and as such their onset may not require specific inputs from plant parasitic nematodes. Sedentary plant parasitic nematodes may utilize effectors and their ability to synthesize other biologically active compounds to tailor these common responses for prolonged feeding on host cells.

Anatomical Alterations in Plant Tissues Induced by Plant-Parasitic Nematodes

Plant-parasitic nematodes (PPNs) interact with plants in different ways, for example, through subtle feeding behavior, migrating destructively through infected tissues, or acting as virus-vectors for nepoviruses. They are all obligate biotrophic parasites as they derive their nutrients from living cells which they modify using pharyngeal gland secretions prior to food ingestion. Some of them can also shield themselves against plant defenses to sustain a relatively long lasting interaction while feeding. This paper is centered on cell types or organs that are newly induced in plants during PPN parasitism, including recent approaches to their study based on molecular biology combined with cell biology-histopathology. This issue has already been reviewed extensively for major PPNs (i.e., root-knot or cyst nematodes), but not for other genera (viz. Nacobbus aberrans, Rotylenchulus spp.). PPNs have evolved with plants and this co-evolution process has allowed the induction of new types of plant cells necessary for their parasitism. There are four basic types of feeding cells: (i) non-hypertrophied nurse cells; (ii) single giant cells; (iii) syncytia; and (iv) coenocytes. Variations in the structure of these cells within each group are also present between some genera depending on the nematode species viz. Meloidogyne or Rotylenchulus. This variability of feeding sites may be related in some way to PPN life style (migratory ectoparasites, sedentary ectoparasites, migratory ecto-endoparasites, migratory endoparasites, or sedentary endoparasites). Apart from their co-evolution with plants, the response of plant cells and roots are closely related to feeding behavior, the anatomy of the nematode (mainly stylet size, which could reach different types of cells in the plant), and the secretory fluids produced in the pharyngeal glands. These secretory fluids are injected through the stylet into perforated cells where they modify plant cytoplasm prior to food removal. Some species do not produce specialized feeding sites (viz. Ditylenchus, Subanguina), but may develop a specialized modification of the root system (e.g., unspecialized root galls or a profusion of roots). This review introduces new data on cell types and plant organs stimulated by PPNs using sources varying from traditional histopathology to new holistic methodologies.