Once synthesized, nitrogenase activity of A. brasilense, as well as of other Rhodospirillales, is reversibly inactivated in vivo by or anaerobiosis. This inactivation involves ADP-ribosylation of the Fe-protein (dinitrogenase reductase) catalyzed by dinitrogenase

reductase ADP-ribosyltransferase (DraT) and is reversed, upon exhaustion, by dinitrogenase learn more reductase activating glycohydrolase (DraG) (Cassan & Garcia de Salamone, 2008). The activities of both DraT and DraG enzymes are regulated according to the levels of ammonium through direct interactions with the PII proteins GlnB and GlnZ. DraG interacts with GlnZ both in vivo and in vitro, and DraT interacts with GlnB in vivo (Huergo et al., 2009). Bacteria have developed mechanisms to maintain cell viability during starvation and resume growth when nutrients become available. These include among others phase variation (Kussell et al., 2005). Phase variation has been proposed as an important mechanism by which microorganisms adapt to environmental changes, such as those existing in the soil rhizosphere (Van den Broek et al., 2005). In phase variation, the expression of a given

gene is either in an ‘ON’ or an ‘OFF’ mode, with these changes usually being reversible. Phase variation has been defined as a random event that occurs at high frequency, involves changes in the DNA, and leads to a phenotypically heterogeneous population (Van der Woude & Baumler, 2004; Wisniewski-Dye & Vial, 2008). Several studies with Azospirillum have identified and characterized phenotypic variants. In A. lipoferum 4B, phenotypic ATM/ATR tumor variation was associated with loss of a 750-kb plasmid (Vial et al., 2006). In A. brasilense Sp245, a spontaneous variant was shown to lose plasmids p40, p85, and p120; however, it gained a new plasmid of more than 300 MDa (Katsy et al., 2002). Phenotypic variants of A. brasilense Sp7 also showed altered plasmid composition, as well as changes

in LPS structure (Petrova et al., 2005). New phenotypic variants of A. brasilense Sp7 were retrieved recently, after exposure of the parental strain mainly to starvation, but also after colonization of maize roots (Lerner et al., 2010). Two isometheptene variants, Sp7E and Sp7EPS, were found to produce significantly higher EPS concentrations relative to the Sp7 parental strain and were LPS-defective. The variants were also shown to carry alterations in DNA rearrangement, EPS monosaccharide composition, and OMP profile as compared to the parental strain (Lerner et al., 2010). Importantly, the variants differed from the parental strain in cell pigmentation (Fig. 3), susceptibility to stresses, antibiotics, and capability of biofilm formation (Lerner et al., 2010). Future studies may determine how phenotypic variation is associated with survival in bacterial inoculants, root colonization, and plant growth promotion.