Biochemical mechanisms of plant resistance to nematodes: a review.

In general, a plant resistant to nematodes resists attack or exhibits little damage and reduces nematode populations. The following four types of resistance may be distinguished: (i) The plant may produce toxins that kill the nematodes; e.g., Asparagus officinalis contains in its stems, leaves and roots a glycoside toxic to Trichodorus christiei, and populations of this nematode rapidly decline around the roots of these plants (57). The roots of Tagetes patula and T. erecta contain a-terthienyl and derivatives of bithienyl (68) which limit Meloidogyne and Pratylenchus populations (50, 77). Eragrostis curvula is resistant to four species of Meloidogyne (61) due to high concentration of pyrocatechol in its roots. (ii) The resistance depends upon the fact that not every plant contains substances necessary for the development and reproduction of a certain nematode species, or contains them in unsufficient amounts; e.g., Webster (73) stated that the indoleacetic acid and its precursor, tryptophan, are necessary for the reproduction of Aphelenchoides ritzemabosi. Szczygiel and Giebel (64) found systems of IAA-oxidase in strawberry varieties resistant to A. fragariae that were more active than those in susceptible varieties. Conceivably, indoleacetic acid or its derivatives represent some of the compounds that stimulate nematode development and reproduction. (iii) The resistance can be based on lack of attraction. Here, different chemical compounds secreted by plants may be neutral or act as attractants on different nematode species. (iv) Plant resistance is based on plant tissue hypersensitivity to nematode infection. The host-parasite interaction stimulates in the host definite biochemical reactions that cause histological changes; i.e., host cell necroses. These necroses form around the nematodes walling them off and either delaying development or causing them to die (8, 72). With respect to susceptible host response to nematode invasion, Viglierchio (71) discerns