Hairy roots were initiated from two soybean [Glycine max (L.) Merr.] genotypes with different susceptibility (susceptible 'Spencer' and partially resistant 'PI567.374') to the disease
sudden death syndrome (SDS) caused by the soil-borne fungal pathogen Fusarium solani f. sp. glycines (FSG) to study the role of isoflavonoids in the plant response to FSG
infection. Hairy root cultures obtained by transformation with Agrobacterium rhizogenes allows normal root growth that can be visually monitored. The principal
isoflavones (
genistin,
daidzin,
glycitin and their malonyl conjugates and aglycones) and also
isoflavonoid phytoalexins (
coumestrol and
glyceollin) were measured by HPLC in extracts of the FSG-inoculated and non-inoculated hairy roots. FSG mycelia grew more slowly on inoculated PI567.374 hairy roots than on Spencer hairy roots. The
glyceollin content was higher in FSG-inoculated PI567.374 hairy roots than in Spencer hairy roots even though the
glyceollin precursor, the
isoflavone daidzein, was higher in Spencer. The de novo synthesis of
isoflavones and
glyceollin was confirmed by [(14)C]Phe incorporation into
glyceollin, which was higher both in the FSG-inoculated roots and surrounding medium of the cv. PI567.374 than that of Spencer.
Glyceollin was the most inhibitory to FSG growth among eight isoflavonoids tested. The levels of
coumestrol, a putative phytoalexin, did not change upon FSG inoculation. The defense response was also elicited by FSG culture filtrates in hairy roots grown in liquid culture. The data obtained indicate that the ability of soybean roots to rapidly produce sufficient amounts of
glyceollin in response to FSG
infection might be important in providing partial resistance to this fungus.