BottomUp Control

Bottom-up control occurs when a nematode population size is kept below a certain level by resource limitation, i.e., food availability. What would initially appear to be a simple concept potentially involves several mechanisms. Plant-parasitic nema-todes are obligate parasites which have co-evolved with their host plants; during this process, both nematodes and plants have also interacted and co-evolved with a range of other rhizosphere organisms. The outcomes of this coevolving network are still not clearly understood, and some potential processes are described below. Although the existence of partial nematode resistance in some crop varieties is common knowledge, crop plants have not been naturally coevolved with their nematode parasites and other soil biota. Therefore natural systems represent a key opportunity to investigate such ecological interactions.

In a large population study of Heterodera arenaria parasitizing marram grass in sand dunes, the nematodes were found to be early colonisers of newly-developing roots.

Heterodera

Fig. 2.2 Mechanisms of plant-parasitic nematode control in the rhizosphere of marram grass (Ammophila arenaria) in European coastal sand dunes, a natural ecosystem: A - Horizontal control through intraspecific competition, B - Horizontal control through inter-specific competition, C - Bottom-up control by an indirect effect, via Arbuscular Mycorrhizal Fungi (AMF) associations with the plant root, D - Bottom-up control through resource limitation, E - Top-down control

Fig. 2.2 Mechanisms of plant-parasitic nematode control in the rhizosphere of marram grass (Ammophila arenaria) in European coastal sand dunes, a natural ecosystem: A - Horizontal control through intraspecific competition, B - Horizontal control through inter-specific competition, C - Bottom-up control by an indirect effect, via Arbuscular Mycorrhizal Fungi (AMF) associations with the plant root, D - Bottom-up control through resource limitation, E - Top-down control

In the recently developed root layers, Heterodera arenaria populations increased to a level where they became resource-limited, (Fig. 2.2) whilst in deeper (older) root zones, when the nematode populations are established, they were affected by other parameters, such as resource quality (Van der Stoel et al. 2006). These sedentary parasites were considered mostly harmless in the coastal sand dunes under study, but sedentary endoparasites together with migratory endoparasites are the main nematode groups involved in disease complexes. They develop synergistic or additive effects on disease incidence and severity by association with plant-pathogenic bacteria or fungi (Hillocks 2001). A disease complex of such plant-parasitic nematodes (H. arenaria, Meloidogyne marítima and Pratylenchus spp.) and fungal plant-pathogens has been suggested to be involved in the decline of Ammophila arenaria (marram-grass) in coastal sand dunes (Van der Putten et al. 1993). These natural systems provide a unique opportunity for studies on the ecology and natural control of these nematodes.

Plant mutualists, such as mycorrhizal fungi and rhizobia are widespread and are thought to maintain the structure and diversity of natural communities. Many studies suggest the importance of mutualisms in improving plant nutrition and health, but there is little evidence for community-level impacts of mutualists (Christian 2001). The presence of arbuscular mycorrhizal fungi (AMF) can increase plant diversity and ecosystem productivity (van der Heijden et al. 1998). However, AMF fungi can also have a detrimental effect on plant growth: a richer community of these fungi increases plant diversity because no plant dominates with all AMF present (Klironomos 2003).

Marram grass associations with AMF might delay or even prevent its degeneration and could be critical in the nutrient-poor sand dune soils, where their numbers were shown to significantly decrease in degenerated plants (Kowalchuk et al. 2002). However, this study was observational. The role of the association between marram grass and its native AMF populations has been investigated in more detail in sequential inoculation and split-root glasshouse experiments (de la Pena et al. 2006). A local, non-systemic, competition-like interaction between AMF and migratory endoparasitic nematodes is thought to occur in the plant roots, leading to nematode population suppression by the inhibition of root colonisation, and reduced nematode multiplication (Fig. 2.2). Arbuscular mycorrhizal fungal associations with marram grass are also thought to be critical for plant establishment, because they can lead to improved plant growth, especially in younger plants (Rodriguez-Echeverria et al. 2004).

The role of the legume-rhizobia symbiotic interaction in nematode control appears to have idiosyncratic effects, being highly dependent on the interacting species identity. Some studies suggest that plant-parasitic nematodes may reduce nodule formation (Duponnois et al. 2000; Villenave and Cadet 1998). On the other hand, some rhizobia strains have been shown to elicit induced resistance in the plant against plant-parasitic nematodes (Mitra et al. 2004; Reitz et al. 2000). Plant-parasitic nematodes and rhizobia interact in the rhizosphere, and there is evidence of horizontal gene transfer between them (Scholl et al. 2003), but the outcomes of their interactions for plants are still not clear. Recent studies using the model legume Medicago truncatula have shown that rhizobial nodulation suppresses root galling by the endoparasitic nematode Meloidogyne javanica, which in turn increases nodulation (Costa et al. 2008).

Colonisation of land by vascular plants dates back an estimated 400 million years (Signor 1994). Throughout this time, plants have interacted with their herbivores, parasites and pathogens, and this has led to a coevolution process that is responsible for the development of plant chemical defence (Ehrlich and Raven 1964). Plants may not be vulnerable to herbivore attack, as is suggested by the GWH, but constantly release primary production compounds (CO2, sugars) and also secondary metabolites through root exudations and leaf volatiles, which are indicative of their physiological state. These can act as cues for their herbivores, which can be attracted or repelled, and also for natural enemies of these herbivores (Price et al. 1980; Rasmann et al. 2005).

Some plant species may produce secondary metabolites with nematotoxic effects (Gommers 1981), but such effects have, to our knowledge, not been assessed in natural systems. Tagetes plants have been studied extensively for their effects on nematode suppression and various nematicidal polythienyl compounds were isolated from them (Uhlenbroek and Bijloo 1958). Endoroot bacterial isolates of Tagetes erecta and of T. patula have a role in this effect, which could be transferred to potato

Solanum tuberosum plants, resulting in a decrease in nematode populations without affecting the potato yield (Sturz and Kimpinski 2004). Rhizosphere bacteria also have shown activity against fungal pathogens, with effects being influenced by soil type, root morphology, root exudation and plant identity (Berg et al. 2006; Lee et al. 2005).

Plant-parasitic nematode management strategies in agricultural systems should be developed taking into account and exploiting the role of the plants as an interacting organism in the food-web.

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