The soybean cultivar specificity gene noIX is present, expressed in a nodD-dependent manner, and of symbiotic significance in cultivar-nonspecific strains of Rhizobium (Sinorhizobium) fredii

Abstract

Rhizobium (now Sinorhizobium) fredii is a symbiotic nitrogen-fixing bacterium that can nodulate soybean in a cultivar-specific manner. This process is governed by a set of negatively acting nodulation genes termed nolXWBTUV. These genes prevent R. fredii strain USDA257 from infecting soybean cultivars such as McCall, but they do not block nodulation of cultivar Peking. R. fredii strain USDA191 contains DNA sequences that hybridize to nolXWBTUV, yet it forms normal nitrogen-fixing nodules on both McCall and Peking soybean. These sequences were isolated and their structure and function examined in comparison to nolXWBTUV of strain USDA257. Restriction maps of the two loci are identical, as is a 2-4 kb DNA sequence that corresponds to nolX and its promoter region. Expression of nolX by strain USDAl91 is f lavonoid-dependent in culture and readily detectable in nodules. The gene is not inducible in a mutant of strain USDA191 that lacks the regulatory nodDl gene, and its expression is greatly attenuated in a nodD2 mutant. nolX is also present and flavonoid-inducible in HH103, a second R. fredii strain that nodulates McCall soybean normally. Inactivation of nolX in strain HH103, USDA191 or USDA257 leads to retardation of initial nodulation rates on soybean cultivars such as Peking and to acquisition of the capacity to form nitrogen-fixing nodules on two species of Erythrina. nolX is thus of symbiotic significance in all three strains, even though it regulates soybean cultivar specificity only in strain USDA257.1

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

“…NGR234 abolishes Nops secretion and affects the symbiosis in a hostdependent manner . A similar phenotype has been reported in mutant strains of S. fredii USDA257 and HH103 that contain a non-functional secretion machinery (Bellato et al, 1997;de Lyra et al, 2006;Meinhardt et al, 1993).…”

“…The presence of multiple copies of the nodD2 gene in S. fredii USDA257 makes Nops secretion constitutive (Bellato et al, 1997;Krishnan et al, 1995). However, in Rhizobium sp.…”

Distinct Cell Surface Appendages Produced by Sinorhizobium fredii USDA257 and S. fredii USDA191, Cultivar-Specific and Nonspecific Symbionts of Soybean

“…This region was designated the soybean cultivar specificity locus. Paradoxically, the same locus was also found in USDA191 (4).…”

NolX of Sinorhizobium fredii USDA257, a Type III-Secreted Protein Involved in Host Range Determination, Is Localized in the Infection Threads of Cowpea ( Vigna unguiculata [L.] Walp) and Soybean ( Glycine max [L.] Merr.) Nodules

“…On soybean, NolX acts as the determinant of cultivar specificity, preventing S. fredii USDA257 from forming nitrogen-fixing nodules on cultivar McCall soybeans (2,21). Transmission electron microscopy studies have shown that cultivar McCall soybean roots inoculated with S. fredii USDA257 form a few aberrant infection threads, but the bacteria are not released into the host cytosol (4).…”

“…S. fredii USDA257 is able to attach and penetrate cultivar McCall roots, but the infection process is aborted prior to the formation of infection threads (4). The inability of S. fredii USDA257 to nodulate cultivar McCall and other advanced soybean cultivars is regulated by a complex genetic locus, nolXWBTUV (2,21). Inactivation of any of the genes in this locus allows the mutant to form nitrogen-fixing nodules on soybean cultivar McCall, indicating a functional role for this locus in soybean cultivar specificity.…”