Identification of Robinia pseudoacacia target proteins responsive to Mesorhizobium amphore CCNWGS0123 effector protein NopT

Nodulation outer proteins secreted via type 3 secretion systems are involved in the process of symbiosis between legume plants and rhizobia. To study the function of NopT in symbiosis, we mutated nopT in Mesorhizobium amphore CCNWGS0123 (GS0123), which could nodulate black locust (Robinia pseudoacacia). The nopT mutant induced higher jasmonic acid, salicylic acid, and hydrogen peroxide accumulation levels in the roots of R. pseudoacacia compared to wild-type GS0123. The △nopT mutant induced higher disease-resistant gene expression 72 h post-inoculation (hpi), whereas GS0123 induced higher disease-resistant gene expression earlier, 36 hpi. Compared to the nopT mutant, GS0123 induced the up-regulation of most genes at 36 hpi and down-regulation of most genes at 72 hpi. Proteolytically active NopT_GS0123 induced hypersensitive responses when expressed transiently in the leaves of tobacco (Nicotiana benthamiana). Two NopT_GS0123 targets in R. pseudoacacia were identified, ATP-citrate synthase alpha chain protein 2 and hypersensitive-induced response protein. Their interactions with NopT_GS0123 triggered resistance by the plant immune system. In conclusion, NopT_GS0123 inhibited the host plant immune system and had minimal effect on nodulation in R. pseudoacacia. The results of the present study reveal the underlying molecular mechanism of NopT function in plant-symbiont interactions.

Keywords: Mesorhizobium amphore; Type 3 secretion system; effector; hypersensitive-induced response; nodulation; symbiosis.