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Phytophthora sojae is an oomycete plant pathogen and is one of the most important pathogens of soybean, causing root and stem rot.Zoospore is a key stage in the life cycle of P.sojae.The chemotaxis of zoospore to isoflavones is believed to be critical for host recognition and initiating infection.G protein α subunit, PsGPA 1, plays a key role in chemotaxis of P.sojae.In this study, we used the affinity purification approach to identify proteins that physically associate with PsGPA 1.One of the PsGPA1-interacting proteins identified was a HIT-domain-containing protein orthologues to human HINT1, namely PsHintl.The PsHintl protein interacted with both GTP and GDP form of PsGPA 1.Analysis of gene silencing mutants indicated that PsHint1 plays a role in chemotropic response of zoospores to isoflavones.In addition, the PsHintl-silenced mutants were defective in cyst germination, and formed abnormal germ tubes with highly branched and apical swelling.During interaction with a susceptible soybean cultivar, the soybean cells inoculated with PsHint1-silenced mutants exhibited strong defense responses accompanied with accumulation of reactive oxygen species (ROS).The PsHint1-silenced mutants displayed a significant reduction in infectious hyphal extension and caused a decrease in pathogenicity.Interestingly, transcriptional patterns of several P.sojae RxLR effectors were altered in the PsHint1-silenced mutant.These results suggest that PsHint1 may interact with PsGPA1 to regulate chemotaxis to soybean isoflavones and pathogenicity.