Characterization of NopP, a Type III Secreted Effector of Rhizobium sp. Strain NGR234

Abstract

The type three secretion system (TTSS) encoded by pNGR234a, the symbiotic plasmid of Rhizobium sp. strain NGR234, is responsible for the flavonoid-and NodD1-dependent secretion of nodulation outer proteins (Nops). Abolition of secretion of all or specific Nops significantly alters the nodulation ability of NGR234 on many of its hosts. In the closely related strain Rhizobium fredii USDA257, inactivation of the TTSS modifies the host range of the mutant so that it includes the improved Glycine max variety McCall. To assess the impact of individual TTSS-secreted proteins on symbioses with legumes, various attempts were made to identify nop genes. Amino-terminal sequencing of peptides purified from gels was used to characterize NopA, NopL, and NopX, but it failed to identify SR3, a TTSS-dependent product of USDA257. By using phage display and antibodies that recognize SR3, the corresponding protein of NGR234 was identified as NopP. NopP, like NopL, is an effector secreted by the TTSS of NGR234, and depending on the legume host, it may have a deleterious or beneficial effect on nodulation or it may have little effect.A variety of prokaryotic organisms are capable of enzymatically reducing atmospheric nitrogen to ammonia. This process, known as biological nitrogen fixation, can be performed either by free-living bacteria (e.g., Klebsiella pneumoniae) or by symbiotic bacteria. Nitrogen-fixing symbioses between plants belonging to the family Leguminosae and soil bacteria collectively called rhizobia contribute substantially to plant productivity. Ultimately, these associations lead to the formation of specialized structures called nodules on the stems or roots of host plants, where infecting rhizobia differentiate into bacteroids that reduce atmospheric nitrogen to ammonia. The ammonia is incorporated into amino acids that are supplied to the host, which return the favor by supplying the microsymbionts with photosynthates. Nodulation, the process that leads to bacterial colonization of root or stem nodules, is highly selective; a continuous exchange of molecular signals between the two symbionts enables the host to distinguish compatible rhizobia from potential pathogens. Initially, lipochitooligosaccharidic nodulation factors (Nod factors) that are secreted by rhizobia in response to plant flavonoids are essential for infection. Later, additional bacterial signals, such as surface polysaccharides or secreted proteins, are also required for efficient nodulation of specific hosts (7,35).Among the known microsymbionts, Rhizobium sp. strain NGR234 has the rare ability to nodulate more than 112 genera of legumes (37). The closely related strain Rhizobium fredii USDA257 forms nodules on a smaller subset of plants (Ͼ79 genera), but it fixes nitrogen with Glycine max and Glycine soja, two hosts that fail to establish effective symbioses with NGR234. Early work showed that mutations in the cultivar specificity locus nolXWBTUV modified the host range of USDA257 so that it included the improved soybean variety McCall (14,28). Fla...

Regulation and symbiotic significance of nodulation outer proteins secretion in Sinorhizobium fredii HH103

“…Genes responsible for the biosynthesis of the rhizobial T3SS are organized in the tts region, which also contains genes that encode secreted proteins collectively known as nodulation outer proteins (Nops). To date, eight secreted proteins have been identified: NopA, NopB, NopC, NopD, NopL, NopM, NopP and NopX (Ausmees et al, 2004;Bartsev et al, 2003;Deakin et al, 2005;Krishnan, 2002;Lorio et al, 2004;Rodrigues et al, 2007;Saad et al, 2005;Skorpil et al, 2005). Recently, eight proteins have been found to be type III-secreted in B. japonicum; one of them, GunA2, has been identified as an endoglucanase (Süss et al, 2006).…”

Functional Analysis of the Type 3 Effector Nodulation Outer Protein L (NopL) from Rhizobium sp. NGR234

“…strain NGR234 (20) possesses a functional type 3 secretion system (21), which can affect nodulation of host plants either positively or negatively. Up to now, four nodulation outer proteins (Nops) of NGR234 have been identified as type 3 effectors, namely NopL (21-24), NopP (25,26), NopT (27,28), and NopM (28). Their functions remain unclear, however.…”

Suppression of soil-borne root pathogens of arid legumes by Sinorhizobium saheli

“…A large number of plants were associated with nodules because it is inherent characteristics of leguminous plants to form nodules under natural growing conditions Garcia-Fraile et al, 2007). Earlier researchers also did not find host specificity at species or genus level in different rhizobial species and their leguminous host (Ausmees et al, 2004;Singh et al, 2011).…”

“…Sinorhizobium species strain NGR 234 formed nodules on more than 100 plant genera (Ausmees et al, 2004). Singh et al (2011) found that S. saheli isolate (PC-6) from Prosopis cineraria was cross infective to Acacia senegal.…”