The continuous cropping obstacles in monoculture fields are a major production constraint for peanuts. Application of the endophytic fungus Phomopsis liquidambari has increased peanut yields, and nodulation and N fixation increases have been considered as important factors for P. liquidambari infection-improved peanut yield. However, the mechanisms involved in this process remain unknown. This work showed that compared with only Bradyrhizobium inoculation, co-inoculation with P. liquidambari significantly elevated endogenous HO and NO levels in peanut roots. Pre-treatment of seedlings with specific scavengers of HO (CAT) and NO (cPTIO) blocked P. liquidambari-induced nodulation and N fixation. CAT not only suppressed the P. liquidambari-induced nodulation and N fixation, but also suppressed the enhanced HO and NO generation. Nevertheless, the cPTIO did not significantly inhibit the induced HO biosynthesis, implying that HO acted upstream of NO production. These results were confirmed by observations that exogenous HO and sodium nitroprusside (SNP) reversed the inhibition of P. liquidambari-increased nodulation and N fixation by the specific scavengers. The transcriptional activities of the symbiosis-related genes SymRK and CCaMK of peanut-Bradyrhizobium interactions also increased significantly in response to P. liquidambari, HO and SNP treatments. The pot experiment further confirmed that the P. liquidambari infection-enhanced HO and NO signalling pathways were significantly related to the increase in peanut nodulation and N fixation. This is the first report that endophytic fungus P. liquidambari can increase peanut-Bradyrhizobium interactions via enhanced HO/NO-dependent signalling crosstalk, which is conducive to the alleviation of continuous cropping obstacles via an increase in nodulation and N fixation.

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