In addition to serving as chemotactic attractants for bacteria, the root exudates of
plants also facilitate microbial attachment and internal colonization of the plant roots
(Pinski et al. 2019). The secretion systems of endophytes are thus pivotal for
successful colonization by endophytes (Pinski et al. 2019). Bacteria must have
adequate transporters and enzymes for root exudates that primarily consist of sugars,
polysaccharides, amino acids, aromatic acids, aliphatic acids, fatty acids, sterols,
phenolics, plant growth regulators, secondary metabolites, proteins, and enzymes
(Badri et al. 2009). In general, plant root secretion systems are also considered to
have a role in bacteria successfully avoiding elimination by the plant defense system
(Pinski et al. 2019). Oxalate is one of the root exudate compounds found in maize
and lupin. Bacteria can utilize oxalate through the activity of oxalate decarboxylase.
The gene oxc encode for oxalate decarboxylase in Burkholderia phytofirmans PsJN.
The inactivation of oxc in Burkholderia phytofirmans PsJN showed reduction in the
early colonization of maize and lupin, even if the effect was less significant in maize.
Kost et al. (2014) suggested that this might be due to the fact that maize roots
produce less oxalate than lupin (five-fold less per g of root fresh weight at three days
after inoculation).
There are eight different types of secretion systems used for Gram-negative
bacteria respectively designated as type I secretion system to type VII secretion
system, plus Sec and Tat. For gram-positive bacteria, there are six types (Sec, Tat,
SecA2, Sortase, Injectosome, and type VIII secretion system) (Liu et al. 2017). A
type III secretion system (T3SS) in Gram-negative bacteria transports effector proteins across the inner cell membrane, through the periplasmic space and the outer
membrane of the bacteria into the cytoplasm of the plant host cells (Table 20.2). The
translocated effector proteins could modulate the hosts metabolism and the defense
system response (Pinski et al. 2019). M1 mutants of H. rubrisubalbicans have
impaired T3SS and have been found not to achieve successful endophytic colonization (Baetz and Martinoia 2014). Interestingly, endophytic colonization may also
be delimited by secretion systems as suggested by the discovery that the T6SS
mutant of Azoarcus sp. BH72 had even a higher colonization capacity than the
wild-type strain did (Shidore et al. 2012). The quorum sensing molecule DSF
(diffusible signal factor) in the well-studied endophytic S. maltophilia R551–3
regulates chemotaxis, cell motility, biofilm formation, and multidrug efflux pumps.
A mutant lacking of DSF production could not form well-organized cell aggregates
properly, and also did not colonize efficiently resulting in the bacteria not promoting
host plant growth (Alavi et al. 2013). Similarly, in a mutant of P. fluorescens 2P24
that could not produce a quorum-sensing molecule of acyl-homoserine-lactone,
colonizing efficiency of the rhizosphere was significantly reduced (Wei and Zhang
2006).
Other more general transcriptional regulators are also involved in plantendophytic bacteria interactions. For example, a deficiency mutation of rpoS,
which is a general stress response regulator and stationary-phase sigma factor,
resulted in decrease in the root attachment of Salmonella enterica (Barak et al.
2005) (Table 20.2).
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S.-C. Chun
plants also facilitate microbial attachment and internal colonization of the plant roots
(Pinski et al. 2019). The secretion systems of endophytes are thus pivotal for
successful colonization by endophytes (Pinski et al. 2019). Bacteria must have
adequate transporters and enzymes for root exudates that primarily consist of sugars,
polysaccharides, amino acids, aromatic acids, aliphatic acids, fatty acids, sterols,
phenolics, plant growth regulators, secondary metabolites, proteins, and enzymes
(Badri et al. 2009). In general, plant root secretion systems are also considered to
have a role in bacteria successfully avoiding elimination by the plant defense system
(Pinski et al. 2019). Oxalate is one of the root exudate compounds found in maize
and lupin. Bacteria can utilize oxalate through the activity of oxalate decarboxylase.
The gene oxc encode for oxalate decarboxylase in Burkholderia phytofirmans PsJN.
The inactivation of oxc in Burkholderia phytofirmans PsJN showed reduction in the
early colonization of maize and lupin, even if the effect was less significant in maize.
Kost et al. (2014) suggested that this might be due to the fact that maize roots
produce less oxalate than lupin (five-fold less per g of root fresh weight at three days
after inoculation).
There are eight different types of secretion systems used for Gram-negative
bacteria respectively designated as type I secretion system to type VII secretion
system, plus Sec and Tat. For gram-positive bacteria, there are six types (Sec, Tat,
SecA2, Sortase, Injectosome, and type VIII secretion system) (Liu et al. 2017). A
type III secretion system (T3SS) in Gram-negative bacteria transports effector proteins across the inner cell membrane, through the periplasmic space and the outer
membrane of the bacteria into the cytoplasm of the plant host cells (Table 20.2). The
translocated effector proteins could modulate the hosts metabolism and the defense
system response (Pinski et al. 2019). M1 mutants of H. rubrisubalbicans have
impaired T3SS and have been found not to achieve successful endophytic colonization (Baetz and Martinoia 2014). Interestingly, endophytic colonization may also
be delimited by secretion systems as suggested by the discovery that the T6SS
mutant of Azoarcus sp. BH72 had even a higher colonization capacity than the
wild-type strain did (Shidore et al. 2012). The quorum sensing molecule DSF
(diffusible signal factor) in the well-studied endophytic S. maltophilia R551–3
regulates chemotaxis, cell motility, biofilm formation, and multidrug efflux pumps.
A mutant lacking of DSF production could not form well-organized cell aggregates
properly, and also did not colonize efficiently resulting in the bacteria not promoting
host plant growth (Alavi et al. 2013). Similarly, in a mutant of P. fluorescens 2P24
that could not produce a quorum-sensing molecule of acyl-homoserine-lactone,
colonizing efficiency of the rhizosphere was significantly reduced (Wei and Zhang
2006).
Other more general transcriptional regulators are also involved in plantendophytic bacteria interactions. For example, a deficiency mutation of rpoS,
which is a general stress response regulator and stationary-phase sigma factor,
resulted in decrease in the root attachment of Salmonella enterica (Barak et al.
2005) (Table 20.2).
404
S.-C. Chun