, 2008). The glucomannan and cellulose mutants were defective in root colonization when incubated with host plant Vicia hirsuta (vetch), suggesting that interactions between the rhizobia and glass surface are different from those occurring during root cap formation (Williams et al., 2008). Unlike what has been described PTC124 concentration in other rhizobial

species, disruption of the CinIR quorum-sensing system in R. leguminosarum led to an increase in biofilm formation (Edwards et al., 2009). This effect seemed to be mediated by the transcriptional regulator ExpR as well as the small protein CinS, coexpressed with the autoinducer synthase CinI (Edwards et al., 2009). The introduction of a mutation in the expR or cinS genes caused an enhanced attachment to glass; however, biofilm rings formed by the expR mutant strain were less stable than those of the cinR and cinI quorum-sensing mutants or the cinS-disrupted strain (Edwards et al., 2009). ExpR and CinS regulate expression of the exopolysaccharide glycanase PlyB, responsible for the cleavage of the acidic exopolysaccharide

(Zorreguieta et al., 2000). This suggests again that the proper size of the acidic exopolysaccharide is essential for the formation of biofilms in R. leguminosarum. Trichostatin A Although most reports indicate that exopolysaccharides play an important role during biofilm formation, this cannot be considered as a rule. Rhizobium sp. YAS34 was used to study the function of exopolysaccharides in colonization and biofilm formation on roots of two nonlegume plants: Arabidopsis thaliana and Brassica napus (Santaella et al., 2008). In this case, exopolysaccharide production by this strain was not essential for biofilm formation, either on inert surfaces (polypropylene) or on roots of the above normal plants. This bacterial

exopolysaccharide did contribute to colonization of specific zones in relation to nutrient availability (Santaella et al., 2008). Thus, in the absence of the legume host, rhizobia are able to attach and colonize roots of other plants, allowing them to take up nutrients and survive in this protected niche until optimal conditions arise for establishment of symbiosis with the host. As mentioned previously, bacterial motility mechanisms (swimming, swarming, and twitching) are known to play Cyclic nucleotide phosphodiesterase important roles in biofilm formation, including colonization and subsequent expansion into mature structured surface communities. Specifically, swarming motility enables groups of bacteria to move in a coordinated fashion on a solid surface, spreading as a biofilm (Verstraeten et al., 2008). Sequence analysis of various Rhizobium etli mutants defective in swarming showed effects on quorum sensing, polysaccharide composition or export, motility, and metabolism of amino acids and polyamines. Several such mutants showed reduced symbiotic nitrogen-fixing activity (Braeken et al., 2008).