Around actively growing roots there exist several gradients of volatile and nonvolatile compounds, including amino acids, ions, pH, temperature and CO2. It is evident that nematodes use their chemosensory sensilla, the amphids, to orientate towards the roots using at least some of these gradients. The ability to orientate towards stimuli from plant roots enhances the chances of host location and reduces the time without food (Perry 1997). Evaluating the reality of the attractiveness or otherwise of an individual compound is difficult. Information is usually based on in vitro behavioural studies, often using agar plate movement bioassays, which bear little if any resemblance to the situation in the soil; care must therefore be exercised in extrapolating from such assays to the field situation (Spence et al. 2009). It will be especially important in the future for nematologists to link with plant physiologists to determine the temporal and special attributes of putative attractants in the soil. However, some generalisations can be made and certain compounds are strongly implicated in orientating nematodes to the roots.
Perry (2005) separated gradients into three types: 'long distance attractants' that enable nematodes to move to the root area, 'short distance attractants' that enable the nematode to orientate to individual roots, and 'local attractants' that are used by endoparasitic nematodes to locate the preferred invasion site. There is clear experimental evidence that CO2 is a long distance attractant (Robinson and Perry 2006). With cyst nematodes, such as Globodera spp., it is apparent that the J2 responds to host root diffusate and the evidence is persuasive that diffusate contains chemicals that constitute short distance attractants (Perry 1997; Rolfe et al. 2000). Diffusates from the roots of the host plant, potato, increased the activity of the infective J2 of G. rostochiensis and also attracted them to the roots. As detailed in Sect. 1.3 potato root diffusate (PRD) is required to stimulate hatching of the majority of J2 of the potato cyst nematodes G. rostochiensis and G. pallida but work by Devine and Jones (2002) has shown that the chemicals in PRD responsible for hatching differ from those responsible for attracting the J2 to the root. Electrophysiological analysis of sensory responses (Perry 2001) demonstrated that spike activity of J2 of G. rostochiensis increased on exposure to PRD but not to root diffusate from the non-host sugar beet, thus indicating that responses to diffusates may be host specific. Pudasaini et al. (2007) found that the migration of P. penetrans towards a host depends on both the initial distance between the nematode and the host and the nature of the host. These authors considered that the attractiveness of the host to P. penetrans seems to be correlated with its efficiency as a host; the attractiveness of hosts also declines with age.
The orientation of J2 of cyst and root-knot nematodes to the preferred invasion site, the root tip, is well established but the active factors that constitute the 'local attractants' are unknown. The nematodes may orient to an electrical potential gradient at the elongation zone of the root tip but the relative importance of electrical and chemical attractants for root tip location has not been evaluated; in addition the elevated temperature at the zone of root elongation may influence nematode perception.
Blocking sensory perception so that the nematodes are unable to orientate to roots and thus exhaust their food reserves and die is an attractive control option but may be difficult to achieve. Exposure of J2 of M. javanica and G. rostochiensis to antibodies to amphidial secretions blocked the response to host root allelochemicals (Stewart et al. 1993; Perry and Maule 2004) but responses were not permanently blocked as, after a period of between 0.5 and 1.5 h, turnover of sensilla secretions presumably 'unblocked' the amphids. A similar effect occurred with the bionema-ticide, DiTera®, where disruption of sensory perception was reversible (Twomey et al. 2002).
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