GunA of Sinorhizobium (Ensifer) fredii HH103 is a T3SS-secreted cellulase that differentially affects symbiosis with cowpea and soybean

Abstract

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. A 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. GunA from S. fredii HH103 shows cellulase activity and is secreted through the T3SS in response to the inducer flavonoid genistein. Interestingly, at the beggining 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. Secretion of GunA through the S. fredii HH103 T3SS clearly and differentially impacts the symbiotic performance of this strain with soybean and cowpea. GunA, or its cellulase activity, is recognised by soybean root cells very early in the symbiotic process but, curiously, its secretion can also be detected in mature nodules. This suggests different symbiotic roles at different symbiotic stages that need to be further elucidated.