286
can also produce mimics. Riboflavin (vitamin B 2 ) and its derived version lumichrome are secreted by a chlorophyte, Chlamydomonas. These compounds can
mimic the QS N-acyl-homoserine lactone (AHL) molecules, thereby prematurely
inducing the bacteria (e.g. Pseudomonas or Vibrio spp.) to initiate production of
antibacterials to protect the algal host, Chlamydomonas, against possible pathogens,
even when the bacteria are at low concentrations (Teplitski et al. 2004; Rajamani
et al. 2008).
Plant hormones (phytohormones) are well characterized and are known to play a
role in plant-microbe interactions. Algae have been suggested to produce a range of
plant hormones (Tarakhovskaya et al. 2007; Lu and Xu 2015), although their
presence in algae is debated (Lau et al. 2009; Ross and Reid 2010). Plant hormones
may play a role in an alga’s stress response, as suggested for the chlorophyte
Klebsormidium crenluatum (Holzinger and Becker 2015). Auxins are an important
class of plant hormones, and the most abundant form, indole-3-acetic acid (IAA),
was shown to be produced by an axenic brown macroalgae, Ectocarpus siliculosus,
with an effect on the growth of the alga (Le Bail et al. 2010). However, subsequent
research indicated that an un-culturable microbe associated with the alga might be
responsible for the IAA production (Dittami et al. 2014). As genes for the initial
steps in IAA biosynthesis have been identified in the alga’s genome and genes
involved in the final steps found in its bacterial partner’s genome, it has been hypothesized that they cooperate to produce this compound (Dittami et al. 2014). Some
bacteria, including roseobacters (Ashen et al. 1999; Fernandes et al. 2011; Amin
et al. 2015; Labeeuw et al. 2016) and other marine groups (Maruyama et al. 1989;
Gutierrez et al. 2009), are also capable of producing IAA on their own (Kudoyarova
et al. 2015). This suggests that IAA is an important bioactive in the cross talk
between algae and marine bacteria, similar to their function between plants and
their bacteria (Patten and Glick 2002; Spaepen et al. 2007). Bacterially produced
auxin has been implicated in gall formation on the (macro) rhodophyte Prionitis
lanceolata (Ashen et al. 1999) and bud induction in another (macro) rhodophyte,
Gracilaria dura (Singh et al. 2011). The question of whether IAA, which impacts
cell differentiation and growth, would affect microalgae has been investigated.
While it is unlikely to play a role in cellular differentiation, as it does in multicellular phototrophs, it has been shown to play a role in regulating growth and cell
division (Amin et al. 2015; Borowitzka 2016). Exogenous application of IAA has
been shown to impact microalgal chlorophyte growth (Jin et al. 2008; Stirk et al.
2013; Salama et al. 2014). It has also been shown to stimulate growth in the chlorophyte Chlorella vulgaris when cocultured with an IAA-producing bacterium
(De-Bashan et al. 2008). This was further demonstrated in the diatom Pseudonitzschia multiseries, which provided its associated roseobacter Sulfitobacter sp.
SA11 with tryptophan, which the bacteria converted into IAA, which in turn
impacted the growth of the diatom (Amin et al. 2015). A similar model was proposed between the haptophyte E. huxleyi and the roseobacter Phaeobacter inhibens
(Segev et al. 2016). Recently, production of IAA in response to stimulation with
tryptophan was demonstrated in the axenic coccolith-bearing strain of E. huxleyi,
while it was not produced in the bald diploid strain. Although the addition of IAA
L. Labeeuw et al.
can also produce mimics. Riboflavin (vitamin B 2 ) and its derived version lumichrome are secreted by a chlorophyte, Chlamydomonas. These compounds can
mimic the QS N-acyl-homoserine lactone (AHL) molecules, thereby prematurely
inducing the bacteria (e.g. Pseudomonas or Vibrio spp.) to initiate production of
antibacterials to protect the algal host, Chlamydomonas, against possible pathogens,
even when the bacteria are at low concentrations (Teplitski et al. 2004; Rajamani
et al. 2008).
Plant hormones (phytohormones) are well characterized and are known to play a
role in plant-microbe interactions. Algae have been suggested to produce a range of
plant hormones (Tarakhovskaya et al. 2007; Lu and Xu 2015), although their
presence in algae is debated (Lau et al. 2009; Ross and Reid 2010). Plant hormones
may play a role in an alga’s stress response, as suggested for the chlorophyte
Klebsormidium crenluatum (Holzinger and Becker 2015). Auxins are an important
class of plant hormones, and the most abundant form, indole-3-acetic acid (IAA),
was shown to be produced by an axenic brown macroalgae, Ectocarpus siliculosus,
with an effect on the growth of the alga (Le Bail et al. 2010). However, subsequent
research indicated that an un-culturable microbe associated with the alga might be
responsible for the IAA production (Dittami et al. 2014). As genes for the initial
steps in IAA biosynthesis have been identified in the alga’s genome and genes
involved in the final steps found in its bacterial partner’s genome, it has been hypothesized that they cooperate to produce this compound (Dittami et al. 2014). Some
bacteria, including roseobacters (Ashen et al. 1999; Fernandes et al. 2011; Amin
et al. 2015; Labeeuw et al. 2016) and other marine groups (Maruyama et al. 1989;
Gutierrez et al. 2009), are also capable of producing IAA on their own (Kudoyarova
et al. 2015). This suggests that IAA is an important bioactive in the cross talk
between algae and marine bacteria, similar to their function between plants and
their bacteria (Patten and Glick 2002; Spaepen et al. 2007). Bacterially produced
auxin has been implicated in gall formation on the (macro) rhodophyte Prionitis
lanceolata (Ashen et al. 1999) and bud induction in another (macro) rhodophyte,
Gracilaria dura (Singh et al. 2011). The question of whether IAA, which impacts
cell differentiation and growth, would affect microalgae has been investigated.
While it is unlikely to play a role in cellular differentiation, as it does in multicellular phototrophs, it has been shown to play a role in regulating growth and cell
division (Amin et al. 2015; Borowitzka 2016). Exogenous application of IAA has
been shown to impact microalgal chlorophyte growth (Jin et al. 2008; Stirk et al.
2013; Salama et al. 2014). It has also been shown to stimulate growth in the chlorophyte Chlorella vulgaris when cocultured with an IAA-producing bacterium
(De-Bashan et al. 2008). This was further demonstrated in the diatom Pseudonitzschia multiseries, which provided its associated roseobacter Sulfitobacter sp.
SA11 with tryptophan, which the bacteria converted into IAA, which in turn
impacted the growth of the diatom (Amin et al. 2015). A similar model was proposed between the haptophyte E. huxleyi and the roseobacter Phaeobacter inhibens
(Segev et al. 2016). Recently, production of IAA in response to stimulation with
tryptophan was demonstrated in the axenic coccolith-bearing strain of E. huxleyi,
while it was not produced in the bald diploid strain. Although the addition of IAA
L. Labeeuw et al.