## 6 Host-Induced Gene Silencing (HIGS)  
Gene silencing is a natural process by which organisms manipulate expression of their own genes or the genes other organisms. The process, which involves inhibition of specific genes by hybridization with complementary inhibitory RNA (RNAi), has been elucidated at the molecular level and has been adapted for gene expression studies in plants [86; 87]. Host-induced gene silencing makes use of the natural phenomenon of RNA exchange among plants and microbes, through processes not yet fully understood. In the HIGS approach, the host plant is made to produce an RNAi molecule complementary to a pathogen target gene, such as a pathogenicity or virulence gene, which is transferred to the pathogen during infection. Phytopathogenic nematodes and fungi produce parasitism or disease-inducing proteins, also called effectors, which are introduced into host cells or tissues, and interact with host proteins to produce the susceptible response (disease condition). Genes encoding toxins, cell-wall-degrading enzymes, ROS detoxifying enzymes, or other proteins that undermine host defense processes are attractive candidate HIGS targets [7; 88; 89]. Single-gene approaches for _P. thornei_ or _H. avenue_ resistance were enabled by the identification of orthologues of known parasitism genes. Silencing of genes for two components of nematode body wall muscle structure and function, troponin C and calponin, reduced reproduction of _P. thornei_ and _P. zeae_ (maize pathogen) in a carrot disk culture system [90]. Nematodes soaked in a solution containing the troponin C and calponin RNAi molecules showed a 77%-81% reduction in numbers of juveniles, adults, and eggs. Genes from either species were effective for both species, but _P. thornei_ was more responsive to silencing than _P. zeae_, and there was a gene-by-species effect. Two other _H. avenue_ orthologuesof parasitism genes from other nematode species were silenced in wheat roots [91; 92]. The RNAi molecules were introduced using the barley striped mosaic virus HIGS system. Numbers of \(H\). _avenae_ juveniles and females were reduced by 66%-73% and 47%-73%, respectively, compared to non-silenced controls. One parasitism gene was expressed in pre-parasitic J2 [91], whereas the other was highest in J4-stage nematodes [92]. Additional experiments done in tobacco indicated that both parasitism genes suppressed specific points in the innate plant immunity response. Alternatively, transcriptomics has been used to survey nematode genes expressed during infection, with the aim of identifying nematode parasitism genes that would be effective targets for HIGS. For instance, the transcriptome of \(H\). _avenae_ during colonization of roots of resistant _Ae. variabilis_ was examined at the pre-penetration, early, and late stages of infection [23]. The study led to the identification of 56 predicted parasitism genes, 32 of which encoded cell-wall-modifying proteins, possibly for host wall penetration or breakdown, and 10 for ROS formation or detoxification. The findings suggested that resistance in _Ae. variabilis_ surmounted or circumvented the action of these parasitism genes. Three nematode targets having no orthologues in humans, insects, and plants were selected for HIGS experiments. The silencing was accomplished by soaking J2 nematodes in solutions of the RNAi molecules. The silencing of the protease calpain and neuropeptide _FLP-4_ genes promoted nematode mortality. A similar study of the global transcriptional response of _Pratylenchus penetrans_([PERSON], 1917) Chitwood and Oteifa, 1952 to soybean root colonization provided molecular leads for HIGS in a relative of _P. thornei_ and _P. neglects_(93). This _Pratylenchus_ species is not a major pathogen of wheat in the PNW, but recently has been a problem in certain locations [94; 95; 96].