A comparative proteomic analysis of the early response to compatible symbiotic bacteria in the roots of a supernodulating soybean variety.

To reveal the processes involved in the early stages of symbiosis between soybean plants and root nodule bacteria, we conducted a proteomic analysis of the response to bacterial inoculation in the roots of supernodulating (En-b0-1) and non-nodulating (En1282) varieties, and their parental normal-nodulating variety (Enrei). A total of 56 proteins were identified from 48 differentially expressed protein spots in normal-nodulating variety after bacterial inoculation. Among 56 proteins, metabolism- and energy production-related proteins were upregulated in supernodulating and downregulated in non-nodulating varieties compared to normal-nodulating variety. The supernodulating and non-nodulating varieties responded oppositely to bacterial inoculation with respect to the expression of 11 proteins. Seven proteins of these proteins was downregulated in supernodulating varieties compared to non-nodulating variety, but expression of proteasome subunit alpha type 6, gamma glutamyl hydrolase, glucan endo-1,3-beta glucosidase, and nodulin 35 was upregulated. The expression of seven proteins mirrored the degree of nodule formation. At the transcript level, expression of stem 31kDa glycoprotein, leucine aminopeptidase, phosphoglucomutase, and peroxidase was downregulated in the supernodulating variety compared to the non-nodulating variety, and their expression in the normal-nodulating variety was intermediate. These results suggest that suppression of the autoregulatory mechanism in the supernodulating variety might be due to negative regulation of defense and signal transduction-related processes.