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respectively. These data pointed out the possible roles of QS in rhizosphere colonization,
competitive ability and plant invasion, by the regulation of relevant traits such as EPS
production, root adhesion and biofilm formation [von Bodman et al. , 1998, Denny, 1999, de
Kievit et al. , 2001, Marketon et al. , 2003].
AHLs activate the QS-dependent secretion of a number of virulence factors in well-
known plant pathogenesis, such as the induction of crown gall tumors on susceptible hosts
plants by Agrobacterium tumefaciens, (regulated by the TraI/TraR system), and the secretion
of exoenzymes which macerate the plant cell walls and cause soft-rot in potato and other
plant hosts by Erwinia carotovora (regulated by the ExpI/ExpR-CarI/CarR systems) [Barnard
et al. , 2007, White and Winans, 2007]. QS regulation of virulence in Xanthomonas
campestris , in this case mediated by DFS (methyl dodecenoic acid) as autoinducer, is also well
characterized [Fouhy et al. , 2006].
8. QS in Symbiotic Plant-Bacterial Interactions
Rhizobium, Mesorhizobium, Sinorhizobium, Azorhizobium and Bradyrhizobium species are
known for their ability to establish symbiotic interactions with leguminous plants by the
formation and colonization of root nodules, where bacteria fix N 2 to ammonia and make it
available for the plant [Gage, 2004]. This symbiosis is based on the specific recognition of
signal molecules, which are produced by both the bacterial and plant partners. In response to
flavonoids secreted by the plant, bacteria express the so-called nodulation genes ( nod ), which
encode for the production of a lipochitin oligosaccharide signal for nodulation [Hirsch et al. ,
2001]. The Nod proteins are recognized by the plant, inducing de novo organogenesis leading to
nodule formation. The bacteria invade the root through root hairs by an infection thread and
differentiate into bacteroids inside the nodule, where they fix N 2 while in return they are supplied
by the plant with carbon substrates [Hirsch, 1992, Kinkema et al. , 2006].
Rhizobium leguminosarum bv. viciae strains enter into a symbiosis with legumes such as
pea and vetch [van Rhijn and Vanderleyden, 1995]. QS signals are found in several species of
legume-nodulating bacteria, being the cell-to-cell communication systems of Rhizobium
leguminosarum bv. viciae , R. etli , Sinorhizobium meliloti and Bradyrhizobium japonicum
particularly well known [González and Marketon, 2002, Wisniewski and Downie, 2002,
González and Keshavan, 2006, Sánchez-Contreras et al. , 2007]. Most of the identified QS
regulation systems in rhizobia use AHLs as autoinducers, while these organisms seem to lack
AI-2 production [Sánchez-Contreras et al. , 2007].
8.1. QS in Rhizobium and Sinorhizobium
8.1.1. R. leguminosarum
The first AHL described in rhizobia was 3-hydroxy-C 14:1 -HSL [ N -(3 R -hydroxy-7- cis -
tetradecenoyl)- L -homoserine lactone] (Fig. 2), made by Rhizobium leguminosarum bv. viciae .
Previously to its purification and identification, this compound was detected for its
bacteriostatic bacteriocin-like activity, and it was first described as a highly diffusible
antimicrobial compound, able to produce big sized zones of inhibition of growth of sensitive
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