meliloti 2011 click here is able to increase its tolerance to a severe

acid shock when the bacteria have been previously cultivated in batch at a moderately acidic pH. In order to explore whether the adaptive ATR represents a positive trait for nodulation at low pH as well, we compared the relative ability of adapted (ATR+) and nonadapted (ATR−) rhizobia to form nodules when they were coinoculated in comparable numbers on alfalfa plants at different pH. Wild-type S. meliloti 2011 were used as control rhizobia cultivated at pH 7.0, and the isogenic GFP derivative 20MP6 (Pistorio et al., 2002) as ATR+ coinoculant competitors (Fig. 3a). The results clearly showed a marked dominance of ATR+ rhizobia within the nodules when the nodulation test was performed under acid conditions (>90% occupancy), thus strongly suggesting that the adaptive ATR operates as a significant positive trait, enabling competition for the infection of the host root at low pH. Figure 3b shows a control assay where both the S. meliloti 2011 ATR− and its isogenic derivative 20MP6 ATR+ were cultivated at the same pH (either neutral or acid) and then coinoculated onto plants growing either on neutral or acid Fåhraeus medium. The remarkable competitiveness of the acid-adapted rhizobia at low pH is most probably a consequence of better performance during the

preinfection before the bacteria penetrate the root. The increased tolerance to acidity of ATR+ rhizobia would likely make them more proficient under the acid stress in sustaining those energy-requiring cellular BYL719 solubility dmso activities that are necessary for survival and to enter into symbiosis. Nonetheless, because in other bacteria the adaptive ATR has been shown to provide cross-protection against different,

unrelated stresses, we cannot disregard the possibility that this striking competitiveness expressed tetracosactide by ATR+ rhizobia at low pH is a consequence of the enhancement of more general capabilities to face rhizospheric stresses. Note that ATR+ rhizobia were also slightly more competitive during the nodulation at pH 7.0 (Fig. 3b). In this study, we have shown that the entrance of S. meliloti into the adaptive ATR occurs under batch cultivation at moderately acid pH, but not in chemostat growth under continuous cultivation at the same acid pH, an observation that prompted us to question whether or not hydrogen ions themselves were the exclusive inducers of the transient state of acid tolerance. Although the same Evans medium was used in both experimental protocols, batch and continuous cultivation represent completely different growth systems: i.e. while a nutritional limitation must be present during the steady state in all continuous systems (N in this instance), the same limitations are not reached during the log phase of batch cultures.