Folic Acid Plays a Critical Role in Nonhost Resistance

The soybean root and stem rot pathogen, Phytophthora sojae, can penetrate single cells of the Arabidopsis penetration deficient mutant, pen1-1. A mutant screen was undertaken in pen1-1 to identify mutants that are infected by P. sojae. At least 70 progenies of each of the 3,556 M₂ families generated by treating pen1-1 seeds with ethyl methane sulfonate were inoculated with P. sojae zoospores in 24-well microtiter plates to identify possible P. sojae-susceptible (pss) mutants by microscopy. Thirty putative pss mutants showing visible leaf necrosis following inoculation with P. sojae zoospores were identified. Twenty-three of the 30 mutants were inoculated with Fusarium virguliforme which causes sudden death syndrome in soybean. Fourteen of the 23 pss mutants are also infected by F. virguliforme. In pss30 mutant, P. sojae can complete its life cycle suggesting that pss30 is a new adaptive host for this oomycete pathogen. PSS30 encodes a folate transporter localized to chloroplast. Susceptibility of two T-DNA mutants (PEN1PEN1pss30pss30) for PSS30, and 3:1 segregation of PEN1 alleles among F_2:3 families suggested that PEN1 is not epistatic to PSS30. pss30 gained resistance to P. sojae following feeding of the mutant plants with folic acid. PSS30 confers resistance to both P. sojae and F. virguliforme in transgenic soybean plants. We conclude that folic acid is essential for nonhost disease resistance.