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.