Phenotypic Variation in Azospirillum spp. and Other Root-Associated Bacteria

Phenotypic or phase variation (Pv) has been demonstrated in many bacteria studied so far. It is an apparently adaptive process enabling bacteria to cope with changing environmental conditions. Pv is often caused by dynamic changes in the genome which are reversible in phase switches but may be irreversible. Accordingly, different molecular mechanisms are underlying these processes. Pv has particular relevance for niche occupation, adaptive versatility, and optimized interaction with host organisms, like in beneficial microbe-plant interactions. In this chapter, we focus on phenotypic variations in bacteria of the genus Azospirillum, because this is the most studied and applied diazotrophic biofertilizer and plant growth promotion rhizobacterium (PGPR; see Chapter 108). The examples of Pv in other root-colonizing bacteria, like Pseudomonas fluorescens, Ps. brassicacearum, and Acidovorax radicis, are briefly presented to give insights in the diversity of Pv and the underlying mechanisms in different rhizosphere bacteria. In several Azospirillum spp., phenotypic variants occur under normal laboratory and environmental stress conditions but also in the rhizosphere. While some phenotypic variants like the EPS-overproducing variants of A. brasilense perform as PGPR like the wild type in pot experiments, variants of A. radicis are impaired in plant growth promotion. The awareness of the possible occurrence of phenotypic variations is important for the provision of high-quality inoculum. Phenotypic variability may even be used to select for improved strains using the natural genetic mechanisms of diversity creation and niche adaptation.