Avirulent Phytophthora sojae strain R1 induces systemic resistance against virulent R5 in soybean via H 2 O 2 signaling and lignin‐based cell wall reinforcement

Abstract BACKGROUND Induced crop resistance using attenuated strains has been employed to prevent yield losses in several cultivated crops. Phytophthora root rot of soybean (PRRS), caused by Phytophthora sojae , is a devastating disease prevalent worldwide. In pursuit of more ecofriendly disease control strategies, this study investigated the characteristics and mechanisms through which the avirulent strain R1 primes systemic resistance. RESULTS Pretreatment with the avirulent P. sojae strain R1 converted the compatible soybean–R5 interaction into an incompatible one. The optimal interval was 3 days, resistance lasted 20 days and spread 20 cm with a half‐decay distance of 16.5 cm. During 12–72 h after treatment (hat), R1 raised superoxide dismutase (SOD) but reduced catalase (CAT) activity, letting H 2 O 2 accumulate from 12 to 72 hat, and triggering GmPR1 , GmPR2 , GmPR4 and GmPR9 . Additionally, upregulation of GmPAL (12, 24, and 72 hat) increased phenylalanine ammonia‐lyase (PAL) activity from 24 and 72 hat, followed by sequential induction of lignin biosynthetic genes GmC3H (12 hat), GmCOMT and GmF5H (48 hat). Meanwhile, peroxidase (POD) activity increased steadily from 12 to 72 hat. Together, these changes led to visible lignin deposition in hypocotyl phloem fibers by 72 hat. Upon challenge with R5 that pretreated with R1, CAT rose at 1 days postchallenge‐inoculation (dpci) whereas hydrogen peroxide (H 2 O 2 ) declined from its peaks. GmPR1 , GmPR2 and GmPR4 were repressed, yet GmRP9 and POD activity remained high, and lignin increased accordingly. CONCLUSION Avirulent P. sojae R1 rapidly adjusts the antioxidant balance to promote H₂O₂ accumulation; the added H₂O₂ activates defense genes that limit later R5 attack and provides substrate for R1‐induced POD for lignin reinforcement. © 2026 Society of Chemical Industry.