Agriculture Reference
In-Depth Information
pathogenic associations with plants, has been thoroughly investigated during the last 20
years, yet leaving many questions unanswered. This chapter reviews the basic concepts of
the quorum sensing mechanism and its implications, with particular emphasis on the
current knowledge of its importance in the interaction of legume plants and nodule-
forming bacteria.
I. T HE Q UORUM -S ENSING (Q S ) M ECHANISM : A N O VERVIEW
1. Introduction
Bacteria are classically considered as strictly unicellular organisms. Even when grouped,
the contact amongst cells is minimal, and their unions labile and temporal. Exceptions to the
rule are the rudimentary multicellular organizations of fruiting bodies of myxobacteria, and of
the filamentous cyanobacteria that develop specialized cells for nitrogen fixation (heterocysts)
[Søgaard-Andersen et al. , 2003; Zhang et al. , 2006]. Nevertheless, this traditional conception
of bacterial unicellular existence has been rewritten in the last two decades, when it has
become apparent that homogeneous bacterial populations lacking cellular specialization also
show a degree of multicellular organization. Individual bacterial cells are indeed able to
differentiate simultaneously as part of a communal response, inducing changes of the
morphology, physiology or behavior of the whole population. These changes aim for a
particular common objective, and the factor that triggers the coordinate response is the cell
density of the population. A certain numbers of individuals, or minimal unit (quorum), is thus
required for the population to take a collective decision and synchronize its behavior [Fuqua
et al. , 1994]. The quorum concept also implies that before any coordinate response of a
bacterial population can take place, each individual cell requires an estimation of the size of
the full population, and for this reason, bacteria have developed censing strategies. The
mechanism is termed quorum-sensing (QS), and basically operates as a communal device of
gene regulation relying on cell density [Fuqua et al. , 1994].
In order to survive in an open environment, bacteria need to adapt to the continuous
changes of the milieu, and cooperation is required to attain these functions [Hense et al. ,
2007]. The physiological roles known to be regulated by QS are hence widely diverse, as
summarized in Table 1. However, most of these phenotypes share a common feature: they are
only expressed when the bacterial population is able to grow to a certain cell density, which
guarantees the success of its actions. In the case of bacteria pathogenic for plants and animals,
the coordinate expression of virulence factors during the infection of the host organism is a
crucial step for the success of the process. QS controls the expression of several bacterial
functions which are relevant to humans, due to their importance in the fields of medicine,
agriculture and industry. Some of these implications will be discussed in more detail later in
this Chapter.
The quorum sensing language is composed of "chemical words": molecules synthesized
by bacteria, also named cell density factors (CDF) [Loh et al. , 2002] or autoinducers [Hense
et al. , 2007]. CDF are often low-molecular weight compounds, which are synthesized
constitutively at basal levels by the individual cells. The signal molecules are mostly
diffusible through the cell membrane and are sent out to the surrounding media, where they
may accumulate if bacteria reach high cell densities or inhabit spatially-limited environments.
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