Proteomic Changes in Rhizobia after Growth in Peat Extract and Their Potential Role in Desiccation Tolerance

Abstract

Survival of rhizobia on seed is mainly affected by desiccation. Inherent differences in survival exist between rhizobia strains as well as with growth conditions, with a general improvement in survival seen in cells grown in peat over cells grown on growth media. Improving survival of rhizobia under desiccation stress may enhance nitrogen fixation and crop yields. This study aimed to relate desiccation tolerance of rhizobia to physiological changes in cells grown in peat extract. Survival of four rhizobia strains (Rhizobium leguminosarum bv. viciae, 3841, Rhizobium leguminosarum bv. trifolli TA1, Bradyrhizobium japonicum, CB1809, Bradyrhizobium diazoefficiens sp. nov.) was measured after growth in peat extract. This study reported an improvement in survival of Bradyrhizobium strains USDA110 and CB1809 compared to the Rhizobium strains 3841 and TA1. The global proteomic responses of the four rhizobia strains after growth in peat extract was compared with defined medium JMM. Differentially expressed proteins were reported across the four strains with a possible role in desiccation tolerance such as ABC transporters, stress response, transcription, translation and oxidative stress proteins. The role of one protein, PspA (RL3579), was validated by creating a knock-out mutant. The RL3579 mutant grown in peat extract reported significantly lower percentage survival of 2% as compared to the wild-type (5.3%), and could be attributed to loss in membrane integrity. This study confirms that survival of rhizobia can be improved after growth in peat extract, due to stress-related proteins that may function in protecting cells from desiccation-induced damage. The roles of these stress proteins and effect of specific components of peat could be manipulated to develop desiccation-tolerant strains and inform development of new inoculant carriers for legume inoculation.