Chemotactic and electrotactic localisation of plant roots by parasitic nematodes
The rhizosphere and rhizoplane environments of higher plant roots are specialised microhabitats for soil organisms. These organisms include nematodes which are capable of responding to attractants from roots over several centimetres (Rode, 1962, 1969). The aim of this project was to investigate the various aspects of chemotaxis in host finding by nematodes, and the relative importance of electrotaxis in this process. This involved experimentation on root diffusate as a whole, on various ions etc which may be attractive components of root diffusate, on interference by lectins on the host finding process and finally into nematode response to applied electrical fields with comparison to field strengths measured at root surfaces. The working hypothesis used throughout this project was the occurrence of long distance attraction via a non-specific factor which is replaced by a more specific factor as the distance between host and parasite is reduced. Examination of the diffusate as a whole clearly demonstrated that nematode attraction is a directed response. It also gave some support to the theory of successive attractive factors, with each successive factor being of higher molecular weight and lower diffusibility. The response to H+, OH-, Na+, five amino acids and cAMP was tested. The two pH extremes were equally attractive, the Na+ and amino acids were neither attractive nor repulsive and the cAMP was repulsive at the higher concentration used. These results suggested that the ions might have a secondary involvement in attraction by stimulating the initial movement of the nematodes but would be non-specific. Any attraction to amino acids noted by other workers e.g. Bird (1959) might be due to the acidic nature of the amino acids tested. Experiments were carried out using concanavalin A to interfere with host finding as suggested by Marban-Mendoza et al. (1987) through its effects on surface carbohydrates. The experiments produced some evidence to support this theory, and further experiments were made to try to elucidate the mechanism. The results from this further work indicate the effect in pH dependent via changes in aggregate size, but also that there may be a dilution involvement as well. Application of electrical fields to the nematodes produced directional movement, but at voltages approximately 1,000 times that measured at the root surfaces with the vibrating probe. It is therfore concluded that chemotaxis is the primary means by which nematodes locate their host plants, but the possibility of electrotaxis being used to locate specific feeding sites is discussed.