## 4 Genes Associated with Disease Resistance against Soilborne Pathogens
The _tryptophan decarboxylase_ gene manipulations resulted in changes in ferulic acid, a cell wall strengthener in the grasses; caffe acid, an antioxidant and precursor to ferulic acid; serotonin, thought to affect root development and antagonize IAA [73]; and expression of the gene for tryptamine-5-hydroxylase, the enzyme that converts tryptamine to serotonin [74]. However, IAA and IAA biosynthetic genes were not affected. The findings suggest that the _Aegilops Cre_-mediated resistance involves the production of secondary metabolites that condition cell wall rigidity and root development. To examine the role of phytohormone signaling in _M. graminicola_ resistance, expression of genes for salicylic acid, jasmonate, and ethylene signaling was compared in resistant and susceptible rice cultivars [4]. The resistant cultivar showed reduced numbers of galls, females, and eggs relative to the susceptible cultivar. In this cultivar, a gene for ethylene biosynthesis, and genes for jasmonic acid biosynthesis and jasmonate signal transduction were induced throughout early infection, whereas in the susceptible cultivar, expression was attenuated or transient. Two genes for phenylpropanol biosynthesis (salicylic acid pathway), genes for callose and lignin biosynthesis, and a gene for callose metabolism were also elevated during infection in the resistant cultivar. Mechanisms of resistance appear to involve ethylene and jasmonate signaling and modification of cell wall components. Barley plants carrying the _Rha2_ gene for resistance to \(H\). _avenae_ differed from susceptible plants in nematode-related morphological changes in the roots, including faster syncytial development, rapid deterioration of the feeding site, and altered syncytial cell wall composition [22]. In resistant barley, levels of (1,3;1,4)-\(\beta\)-glucan also were higher in the cell walls of the feeding site compared to susceptible barley. The authors examined the expression of genes involved in cell-wall synthesis and found higher expression of a _cellulose synthase-like F10_ gene in resistant barley. This study indicated that resistance was accompanied by alternations in cell wall metabolism and composition. For the soilborne fungi, there are fewer recent leads about how specific genes or resistance-related QTL contribute to reduced disease symptoms. In a comparison of wheat cultivars with resistance or susceptibility to _F. culmorum_, applications of the phytohormone methyl jasmonate alleviated crown and foot rot severity in the susceptible cultivar [75]. Six defense genes were induced in the resistant wheat, but only three were induced in the susceptible cultivar, suggesting that the latter had an attenuated jasmonate signal pathway. A number of studies have linked changes in lignin to both the nematode infection process and to host-nematode interactions [4; 6; 13; 58; 76]. [PERSON] et al. [25] found that the resistant landrace IWA8608077 had higher lignin content than the susceptible Louise [77]. Lignin has roles in plant growth, long-distance water transport, and defense, so enhancing its defense function without perturbing development or secondary metabolism will require a more complete understanding of host transcriptional and metabolic pathways for lignin metabolism [78].