Identification of a stress-induced protein in stem exudates of soybean seedlings root-infected with Fusarium solani f. sp. glycines

Sudden death syndrome of soybean (Glycine max) is caused by the soilborne fungus, Fusarium solani f. sp. glycines, that infects soybean roots. Besides root necrosis, symptoms include interveinal leaf chlorosis, necrosis and premature defoliation. It is proposed that a fungal toxin is produced in soybean roots and translocated to foliage. In this study, we isolated compounds from soybean stem exudates from plants that were either inoculated or not inoculated with F. solani f. sp. glycines. A protein with an estimated molecular mass of 17 kDa and designated as FISP 17 for F. solani f. sp. glycines-induced stress protein was identified using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This protein occurred only in F. solani f. sp. glycines-infected soybean stem exudates. The N-terminal amino acid sequence of the purified protein had 100 % identity with a starvation-associated message 22 protein, and 80 and 78 % identity with purified bean pathogenesis-related proteins, PvPR1 and PvPR2, respectively. To determine if the protein was of plant or fungal origin, a synthetic peptide was designed based on the N-terminal sequence and used to raise a polyclonal antibody from rabbit. Western blot analysis showed that the antibody only reacted with a 17-kDa protein in F. solani f. sp. glycines-infected plant exudates, but no reaction occurred with healthy plant exudates or with culture filtrates of F. solani f. sp. glycines. This is the first report of the presence of a stress-induced protein in stem exudates of soybean seedlings root-infected with F. solani f. sp. glycines.