etli, and quorum sensing helps regulate dispersion of existing biofilms and interactions between bacteria

Nutlin-3a datasheet and higher organisms, for example, in the Rhizobium–bean symbiosis (Daniels et al., 2004). Experimental models with abiotic surfaces are useful for initial characterization of the structure of rhizobial biofilms, and of the necessary conditions for biofilm formation. Further studies in the natural habitats of rhizobia, i.e. host plant roots and the rhizosphere, are needed to elucidate the complex events affecting passage from a planktonic to a biofilm lifestyle. As with other bacteria, establishment of a biofilm in rhizobia involves several developmental stages. Based on studies of microorganisms that associate with abiotic surfaces, we are ready to extend this biofilm formation model to plant roots (Fig. 1). Environmental signals cue planktonic cells to settle and establish microcolonies on a surface. Upon attachment, the bacteria divide

and differentiate to form three-dimensional shapes that characterize a mature biofilm. This process requires the production of AHLs, exopolysaccharides, lipopolysaccharides, and Nod factors. In studies of an abiotic surface model, individual bacteria can leave the biofilm, to function as dispersal units (Russo et al., 2006). This phenomenon may also occur in biofilms formed on host plant roots. This review has focused on analysis of interactions as related to the rhizobia. If plant root factors had been taken into account, a much more Daporinad complex analysis would be necessary (Rodríguez-Navarro et al., 2007). For example, surface characteristics vary along the length of the root. Actively growing root tissues typically exhibit higher rates of exudation into the soil, and biofilms are known to be strongly influenced by nutrient release and exudation

at different sites. Lectins released from plant roots affect bacterial attachment and biofilm formation. A separate review is needed for analysis of such plant-dependent variables that affect bacterial attachment to the root surface. A fundamental question is whether the Bay 11-7085 process of biofilm formation significantly affects legume nodulation. Studies to date indicate that the biofilm lifestyle allows rhizobia to survive under unfavorable conditions (temperature and pH extremes, desiccation, UV radiation, predation, and antibiosis). A large number of viable rhizobia may indirectly ensure the success of nodulation, but there is no direct evidence so far that biofilm formation significantly promotes effective symbiosis with the legume host. A challenge for future studies is to determine how rhizobial biofilm formation is integrated with productive symbiosis. We would like to thank Dr Ann M. Hirsch and Dr Angeles Zorreguieta for stimulating discussions over years of collaborative research. We also thank Dr Simon Silver and the anonymous reviewers for their motivating comments during the preparation of this manuscript.