The symbiosis between Sinorhizobium fredii HH103 and its host legumes is influenced by the type 3 secretion system (T3SS), which delivers proteins (effectors) directly into the host cells to promote infection. GunA, one of the predicted HH103 effectors, potentially codes for a cellulase. In this work we tried to characterise GunA and elucidate its role in symbosis with soybean and cowpea. MethodsA GunA::HA fusion protein was constructed to study T3SS-dependent secretion.Cellulase activity of GunA was measured and gunA::uidA-GFP and gunA::cyA fusions were constructed to monitor gunA expression in nodules and to study translocation to the host cells, respectively. Finally, the symbiotic performance of a gunA mutant was studied in soybean and cowpea. ResultsGunA from S. fredii HH103 shows cellulase activity and is secreted through the T3SS in response to the inducer flavonoid genistein. Interestingly, at the beginning of the symbiotic process, GunA was partially responsible for the induction of the expression of the soybean GmPR1 gene, a gene used as a marker for plant defense responses.However, GunA was also detected in soybean and cowpea developed nodules. Finally, nodulation assays indicate that GunA is beneficial for symbiosis with soybean but detrimental with cowpea. Conclusion

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