IBBM   21076
INSTITUTO DE BIOTECNOLOGIA Y BIOLOGIA MOLECULAR
Unidad Ejecutora - UE
artículos
Título:
Cultural conditions required for the induction of an adaptive acid-tolerance response (ATR) in Sinorhizobium meliloti and the question as to whether or not the ATR helps rhizobia improve their symbiosis with alfalfa at low pH
Autor/es:
WALTER O. DRAGHI; MARÍA FLORENCIA DEL PAPA; MARIANO PISTORIO; MAURICIO J. LOZANO; MARÍA DE LOS ANGELES GIUSTI; GONZALO A. TORRES TEJERIZO; EDGARDO JOFRE; JOSÉ LUIS BOIARDI; ANTOMIO LAGARES
Revista:
FEMS MICROBIOLOGY LETTERS
Editorial:
Wiley-Blackwell
Referencias:
Año: 2009
ISSN:
0378-1097
Resumen:
Sinorhizobium meliloti associates with Medicago and Melilotus species to develop nitrogen fixing symbioses. The agricultural relevance of these associations, the world-wide distribution  of acid soils, and the remarkable acid sensitivity of the microsymbiont have all contributed to a stimulation of research on the responses of the symbionts to acid environments. We show here that an adaptive acid-tolerance response (ATR) can be induced in S. meliloti, as previously shown for S. medicae (Dilworth et al., 1999), when the bacteria are grown in batch cultures at the slightly acid pH 6.1. In marked contrast, no increased tolerance to hydrogen ions is obtained if rhizobia are grown in a chemostat under continuous cultivation at the same acid pH of 6.1. The adaptive ATR appears as a complex process triggered by an increased hydrogen-ion concentration, but operative only if other—as-yet-unknown—concomitant factors that depend on the culture conditions are present (though are not provided under continuous cultivation). Although the stability of the ATR and its influence on acid tolerance has been characterized in rhizobia, no data have been available on the effect of the adapted state on symbiosis. Coinoculation experiments showed that acid-adapted indicator rhizobia (ATR+) were present in more than 90% of the nodules when nodulation was performed at pH 5.6, representing a more than 30% increase in occupancy compared to a control test with non adapted indicator bacteria. We provide evidence that the ATR represents a clear advantage for the rhizobia in competing for nodulation at low pH. It is not yet clear if such an effect results from an improved performance in the acid environment during preinfection, to an enhanced ability to initiate infections, or to both conditions. The practical use of ATR+ rhizobia will depend on validation experiments with soil microcosms and on field testing, as well as on the possibility of preserving the physiology of ATR+ bacteria in inoculant formulations.