Abstract

Bradyrhizobial inoculated soybean often performs poorly on acid-soil because of the acid sensitivity of their associated root nodule bacteria. Acid tolerance in rhizobia has been considered as a key phenotypic characteristic in that it enables the bacteria to perform well under the restrictive conditions of excessive acidity. Since bacteria could develop acid tolerance to a more acid condition by using adaptive acid tolerance response (ATR), it is interesting to investigate whether bradyrhizobia could have this response and what proteins are involved in ATR. Bradyrhizobium sp. DASA01007 was selected for this study based on its ATR ability and symbiosis efficiency with soybean under acid condition. To establish an ATR in bradyrhizobia, late log phase culture of cell grown in mild acid condition was subsequently used as inoculum to more acid conditions. The 2D-gel and proteomic analyses were used to investigate the proteins response during ATR compared with non-adaptive conditions. The 29 identified proteins were grouped into 8 categories based on category orthologous group (COG) and one group of unknown categories. Hypothetical protein, transport and binding proteins, and translation protein were up-regulated at pH 4.5N (non-adaptive condition). While up-regulated proteins found during growth at pH 4.5A (adaptive condition) consisted of proteins in cellular processes, translation, energy metabolism, regulatory functions, interconversions and salvage of nucleosides and nucleotides, and conserved hypothetical proteins group. However, transport and binding proteins were absent in adaptive condition. At pH 5.5A, proteins involved in cellular processes were also detected. Several proteins overproduced in adaptive condition may be involved in ATR of bradyrhizobia. An importance of ATR in root nodule bacteria would support a better chance of survival in low pH soils than those conventionally grown in neutral pH. These results suggest that the use of ATR condition could provide an improvement in the production of inoculants.

Key words: Adaptive acid tolerance, Bradyrhizobium, Soybean, 2D-gel electrophoresis.