304
Subramoni et al. (2015) performed a bioinformatic survey of LuxR solo regulators
and identified such receptors in diverse host-associated bacteria and found that
some played an important function during host-bacterial interaction and are causing
diseases. The aim of the chapter is to unravel the complexity of macroalgal microbiome with a particular focus on the role of chemical interactions via QS systems.
We will further discuss the opportunities afforded by implementing next-generation
sequencing (NGS) technologies in future studies aimed at understanding these complex microbial-host symbioses.
15.2 Quorum Sensing and Cross Kingdom Communication
with Macroalgae
Many plant-associated bacteria undergo chemical signalling with the plant host via
yet unidentified low molecular weight compounds (Subramoni and Venturi 2009a).
There is now growing evidence that plant- and algal-derived compounds interact
with bacterial regulatory proteins that either inhibit or activate bacterial QS
(Manefield et al. 1999; Bodini et al. 2009; Patankar and Gonzalez 2009a, b; Lee
et al. 2011; Gopu et al. 2015), with the term bidirectional interkingdom signalling
now being used to describe such chemical interactions (Venturi and Fuqua 2013).
As described above, the most well-studied QS system in Gram-negative bacteria is
the LuxI-R or AI1 system consisting of a LuxI AHL synthase protein and the LuxR
transcription factor protein (Fig. 15.1) (Fuqua et al. 1994; Bassler 1999). To date, a
number of AHLs are known which are different with acyl chain lengths (from 4 to
18 carbons) and variation in the oxidation state of the C3 position of the chain.
Further studies on AHLs have found that plant-associated bacteria (PAB) have proteins closely related to LuxR that specifically responds to plant signals rather than
endogenously produced AHLs. Furthermore, such LuxR proteins synthesized independently from the luxR and are unpaired from cognate LuxI synthase (Fig. 15.1),
thus, have been termed orphan (Fuqua 2006) or solo (Subramoni and Venturi 2009a)
receptor.
LuxR solos have both AHL- and DNA-binding domains similar to QS LuxR;
however, in some cases they no longer bind AHL due to mutation or amino acid
substitution in the AHL-binding domain of the protein (Subramoni et al. 2015). For
instance, in solo LuxR of Sinorhizobium meliloti (NesR) (Patankar and Gonzalez
2009a), Xanthomonas campestris (XccR) (Zhang et al. 2007), Xanthomonas oryzae
(OryR) (Ferluga et al. 2007) and Pseudomonas fluorescens (PsoR) (Subramoni and
Venturi 2009b), amino acid W57 and Y61 (positions with respect to TraR) are substituted by methionine (M) and tryptophan (W), respectively, in the AHL-binding
domain. Interestingly, these specific amino acid substitutions indicated that the solo
LuxR proteins were binding to low molecular weight compounds produced by plant
rather than AHLs (Patankar and Gonzalez 2009a). Further study on the function of
solo LuxR in Xanthomonas oryzae pv. oryzae found that these LuxR regulate viruR.P. Singh et al.
Subramoni et al. (2015) performed a bioinformatic survey of LuxR solo regulators
and identified such receptors in diverse host-associated bacteria and found that
some played an important function during host-bacterial interaction and are causing
diseases. The aim of the chapter is to unravel the complexity of macroalgal microbiome with a particular focus on the role of chemical interactions via QS systems.
We will further discuss the opportunities afforded by implementing next-generation
sequencing (NGS) technologies in future studies aimed at understanding these complex microbial-host symbioses.
15.2 Quorum Sensing and Cross Kingdom Communication
with Macroalgae
Many plant-associated bacteria undergo chemical signalling with the plant host via
yet unidentified low molecular weight compounds (Subramoni and Venturi 2009a).
There is now growing evidence that plant- and algal-derived compounds interact
with bacterial regulatory proteins that either inhibit or activate bacterial QS
(Manefield et al. 1999; Bodini et al. 2009; Patankar and Gonzalez 2009a, b; Lee
et al. 2011; Gopu et al. 2015), with the term bidirectional interkingdom signalling
now being used to describe such chemical interactions (Venturi and Fuqua 2013).
As described above, the most well-studied QS system in Gram-negative bacteria is
the LuxI-R or AI1 system consisting of a LuxI AHL synthase protein and the LuxR
transcription factor protein (Fig. 15.1) (Fuqua et al. 1994; Bassler 1999). To date, a
number of AHLs are known which are different with acyl chain lengths (from 4 to
18 carbons) and variation in the oxidation state of the C3 position of the chain.
Further studies on AHLs have found that plant-associated bacteria (PAB) have proteins closely related to LuxR that specifically responds to plant signals rather than
endogenously produced AHLs. Furthermore, such LuxR proteins synthesized independently from the luxR and are unpaired from cognate LuxI synthase (Fig. 15.1),
thus, have been termed orphan (Fuqua 2006) or solo (Subramoni and Venturi 2009a)
receptor.
LuxR solos have both AHL- and DNA-binding domains similar to QS LuxR;
however, in some cases they no longer bind AHL due to mutation or amino acid
substitution in the AHL-binding domain of the protein (Subramoni et al. 2015). For
instance, in solo LuxR of Sinorhizobium meliloti (NesR) (Patankar and Gonzalez
2009a), Xanthomonas campestris (XccR) (Zhang et al. 2007), Xanthomonas oryzae
(OryR) (Ferluga et al. 2007) and Pseudomonas fluorescens (PsoR) (Subramoni and
Venturi 2009b), amino acid W57 and Y61 (positions with respect to TraR) are substituted by methionine (M) and tryptophan (W), respectively, in the AHL-binding
domain. Interestingly, these specific amino acid substitutions indicated that the solo
LuxR proteins were binding to low molecular weight compounds produced by plant
rather than AHLs (Patankar and Gonzalez 2009a). Further study on the function of
solo LuxR in Xanthomonas oryzae pv. oryzae found that these LuxR regulate viruR.P. Singh et al.