(A work in progress)
Revised 01/01/2020
A taxis is directed movement of an organism towards or away from a stimulus. A kinesis is a change in the rate of activity, or frequency of turning of an organism in the presence of a stimulus. Reduced activity or more frequent turning can result in aggregation near the stimulus. Resource-locating behavior in nematodes probably consists of a combination of taxes and kineses (Lee, 2002; Rodger, 2003; Young et al, 1998).
Taxes and kineses are characterized according to the nature of the stimulus. Thus, chemotaxis is a directed response to a chemical gradient; thermokinesis is a change in movement pattern in relation to temperature.
Although it has been known for many years that nematodes are attracted to plant roots (Prot, 1977; Prot and Van Gundy, 1980) and root exudates (Riddle and Bird, 1985; Viglierchio, 1961), no uniquely plant-derived compounds that might produce these responses have been identified. Carbon dioxide, expected to be in higher concentrations in the rhizosphere than in bulk soil is a strong attractant in a certain concentration range (Klingler, 1965; Pline and Dusenbery, 1987; Robinson, 1995).
Tracks of Heterodera schachtii J2 in relation to a Plant Root
Photograph by J. Aumann, NemaPix.
Interestingly, given the choice of plant roots and insect larvae in an olfactory tube, bacteriophagous entomophilic nematodes moved to plant roots (Boff et al, 2002), however, prior to invading the host, nematodes must sense additional factors to differentiate between the sources of general signals (Ruhm et al, 2003). CO2 may provide a directional stimulus and stimulate a taxis response. However, once the nematode is near the resource, plant signature compounds, such as flavonoids or alkaloids, may precipitate kinesis responses resulting in localization of individuals around food sources.
Attraction or repellency of host plants to nematodes has been the subject of several investigations, but only a few host- or nonhost-specific compounds mediating this attraction are known (Chitwood, 2002). Maize roots exude cyclic hydroxamic acids, one of which (2,-4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) attracts Pratylenchus zeae at concentrations in host exudates.
References
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