Biochemical response of soybean roots to Fusarium solani f. sp. glycines infection

The soil-borne fungus Fusarium solani (Mart.) Sacc. f. sp. glycines (FSG) infects soybean [Glycine max (L.) Merr.] roots and causes the disease sudden death syndrome (SDS). The biochemical response of soybean roots to FSG infection, which has not been studied before, was investigated by comparing FSG-inoculated and noninoculated roots of two partially resistant ('PI 520733' and 'PI 567374') and susceptible ('Spencer') genotypes. Activity of phenylalanine ammonia-lyase (PAL), the first enzyme in the phenylpropanoid biosynthetic pathway, was increased in inoculated roots of all three genotypes. The phytoalexin glyceollin increased to much higher levels in inoculated roots of the partially resistant cultivars PI 520733 and PI 567374 than in the susceptible Spencer. The changes in phenolic metabolism were localized in lesion-containing areas of roots rather than in the new portion growing under the FSG inoculum. No clear correlation was found between the glyceollin precursor daidzein (4′,7-dihydroxyisoflavone) and its conjugates and glyceollin levels in root tissues; however, isoflavone levels increased only in roots of inoculated plants of partially resistant lines, even though constitutive isoflavone levels were higher in the susceptible control. The FSG growth on potato dextrose agar medium was inhibited by increasing concentrations of glyceollin. Induction of lignin synthesis was found in the inoculated roots of all three lines, with the highest rate of lignification observed in roots of the partially resistant genotypes, especially PI 567374. These studies show for the first time that FSG inoculation of soybean roots in soil induces the phenylpropanoid pathway to synthesize isoflavones, the phytoalexin glyceollin, and lignin, indicating that these compounds may be involved in the partial resistance response.