RESUMO
Transcription of genes encoding several enzymes and the activity of some of these enzymes of the phenylpropanoid pathway leading to synthesis of chemical and physical barriers for defense of plants against root pathogens was estimated in susceptible and resistant soybean infected with Heterodera glycines race 3 or with Meloidogyne incognita race 3. Transcription of genes encoding phenylalanine ammonia lyase (PAL) and the activity of this enzyme increased in resistant, but not susceptible, soybean cultivars after nematode infection. Likewise, transcription of the gene encoding 4-coumaryl CoA ligase and activity of this enzyme were enhanced in resistant, but not susceptible, soybean cultivars after nematode infection. Activity of PAL decreased in susceptible soybean after H. glycines or M. incognita infection. Transcription of enzymes later in the phenylpropanoid pathway leading to glyceollin synthesis increased in both resistant and susceptible soybean in response to nematode infection; the increase was greater in resistant cultivars. These results suggest possible reasons for the rapid induction of glyceollin synthesis immediately after infection of resistant soybean cultivars with H. glycines or M. incognita and the failure of this response in infected, susceptible soybean cultivars. Nematode infection had no effect on the activity of enzymes in the branch of the pathway leading to lignin synthesis.
