Rhizobium japonicum USDA 191 has two nodD genes that differ in primary structure and function

Area of Science:

• Microbiology
• Plant-Bacterial Symbiosis
• Molecular Genetics

Background:

• Rhizobium nod genes are essential for initiating legume nodule formation.
• The symbiotic plasmid of USDA 191 contains two distinct nodD genes, nodD1 and nodD2.
• NodD proteins regulate the transcription of other nod genes in response to plant signals.

Purpose of the Study:

• To characterize the DNA sequence and function of two nodD genes from Rhizobium strain USDA 191.
• To elucidate the specific roles of nodD1 and nodD2 in the symbiotic relationship with soybeans.
• To compare the functions of nodD1 and nodD2 with known nodD genes in other Rhizobium species.

Main Methods:

• DNA sequencing and functional analysis of nodD1 and nodD2 genes.
• Complementation of Rhizobium trifolii nodD mutants.
• Construction of nodD1 and nodD2 insertion mutants and analysis of plant phenotypes.
• Analysis of nodule development, nitrogen content, and nodulin expression.
• Assessment of exopolysaccharide synthesis in strains overexpressing nodD2.

Main Results:

• Both nodD1 and nodD2 encode functional NodD-related proteins with 69% homology.
• NodD1 complements a nodD mutant for clover nodulation; NodD2 does not.
• nodD1 is essential for normal nodule formation and plant nitrogen status, likely activating nodABC transcription.
• nodD2 mutants show delayed nodule formation, reduced nodulin-35, and altered bacteroid content.
• Overexpression of nodD2 in USDA 191 suggests a role in regulating exopolysaccharide synthesis.

Conclusions:

• NodD1 functions similarly to NodD in temperate rhizobia, activating nod gene transcription in response to plant signals.
• NodD2 plays a distinct role in symbiosis, potentially regulating exopolysaccharide synthesis and influencing nodule development.
• Both nodD1 and nodD2 are critical for effective symbiosis between Rhizobium USDA 191 and soybeans.