24
F. M. Romero et al.
festucae var lolii system displays the differential expression of 38% of the genes
expressed between E+ and E− plants (Dupont et al. 2015).
The property of some entomopathogenic fungi to control insect pests, and at the
same time survive as endophytes, may also improve ISR and confer dual protection (Jaber and Ownley 2018). In this trend, it might be possible that biological
control transcends the plant-microorganism system which can be extrapolated to
other components of the ecosystem.
2.4 The Relation Between Nutrient Management
and Biological Control
Besides the roles played by endophytes in biological control described in this chapter,
many pieces of evidence show that they are also involved in mineral nutrition. This is
important since the facilitation of nutrient uptake by their hosts contributes to plantgrowth promotion (Maheshwari and Annapurna 2017). Even though there is little
information regarding the relation of nutrient uptake/management with biological
control traits displayed by beneficial microorganisms, there is evidence of crosstalk
between iron starvation responses and ISR triggered by beneficial rhizobacteria.
The comparison of the genes induced by Pseudomonas spp. WCS417 with the
iron-deficiency root transcriptome showed that 20% of the regulated genes are activated in both conditions (Zamioudis et al. 2015). One of these genes is the rootspecific R2R3-type MYB transcription factor MYB72, that was described as a key
component of ISR triggered by beneficial microbes (Segarra et al. 2009; Van der Ent
et al. 2008) as well as an important player in the iron deficiency response (Buckhout et al. 2009; Colangelo and Guerinot 2004). Interestingly, a deeper insight into
the molecular mechanisms involved in WCS417-induced ISR showed a set of five
MYB72-dependent transcripts. Among them, three genes have been reported previously to be controlled by FIT1 (Fe-deficiency Induced Transcription Factor 1), the
central transcriptional regulator of the iron deficiency response in Arabidopsis roots.
These three FIT- and MYB72-regulated genes encode the nitrate transporter NRT1.8,
the β-glucosidase BGLU42, and the cytochrome P450 monooxygenase CYP71B5
(Zamioudis et al. 2014). Interestingly, BLU42 over-expression in Arabidopsis gives
resistance to different pathogens. On the other hand, bglu42 mutant line is unable to
trigger WCS417-mediated ISR. Moreover, the mutant lines myb72 and bglu42 are
impaired in the accumulation and secretion of fluorescent phenolic compounds that
are produced via the phenylpropanoid route and excreted in the root vicinity playing
a critical role in iron acquisition by facilitating its mobilization (Zamioudis et al.
2014).
The induction in MYB72 expression was also observed in plants treated with
VOCs produced by WCS417, and this expression was accompanied by the induction of different genes involved in iron uptake and transport such as Iron-Regulated
Transporter 1 (IRT1), Ferric Reduction Oxidase 2 (FRO2) and FIT1. Also, it has been
F. M. Romero et al.
festucae var lolii system displays the differential expression of 38% of the genes
expressed between E+ and E− plants (Dupont et al. 2015).
The property of some entomopathogenic fungi to control insect pests, and at the
same time survive as endophytes, may also improve ISR and confer dual protection (Jaber and Ownley 2018). In this trend, it might be possible that biological
control transcends the plant-microorganism system which can be extrapolated to
other components of the ecosystem.
2.4 The Relation Between Nutrient Management
and Biological Control
Besides the roles played by endophytes in biological control described in this chapter,
many pieces of evidence show that they are also involved in mineral nutrition. This is
important since the facilitation of nutrient uptake by their hosts contributes to plantgrowth promotion (Maheshwari and Annapurna 2017). Even though there is little
information regarding the relation of nutrient uptake/management with biological
control traits displayed by beneficial microorganisms, there is evidence of crosstalk
between iron starvation responses and ISR triggered by beneficial rhizobacteria.
The comparison of the genes induced by Pseudomonas spp. WCS417 with the
iron-deficiency root transcriptome showed that 20% of the regulated genes are activated in both conditions (Zamioudis et al. 2015). One of these genes is the rootspecific R2R3-type MYB transcription factor MYB72, that was described as a key
component of ISR triggered by beneficial microbes (Segarra et al. 2009; Van der Ent
et al. 2008) as well as an important player in the iron deficiency response (Buckhout et al. 2009; Colangelo and Guerinot 2004). Interestingly, a deeper insight into
the molecular mechanisms involved in WCS417-induced ISR showed a set of five
MYB72-dependent transcripts. Among them, three genes have been reported previously to be controlled by FIT1 (Fe-deficiency Induced Transcription Factor 1), the
central transcriptional regulator of the iron deficiency response in Arabidopsis roots.
These three FIT- and MYB72-regulated genes encode the nitrate transporter NRT1.8,
the β-glucosidase BGLU42, and the cytochrome P450 monooxygenase CYP71B5
(Zamioudis et al. 2014). Interestingly, BLU42 over-expression in Arabidopsis gives
resistance to different pathogens. On the other hand, bglu42 mutant line is unable to
trigger WCS417-mediated ISR. Moreover, the mutant lines myb72 and bglu42 are
impaired in the accumulation and secretion of fluorescent phenolic compounds that
are produced via the phenylpropanoid route and excreted in the root vicinity playing
a critical role in iron acquisition by facilitating its mobilization (Zamioudis et al.
2014).
The induction in MYB72 expression was also observed in plants treated with
VOCs produced by WCS417, and this expression was accompanied by the induction of different genes involved in iron uptake and transport such as Iron-Regulated
Transporter 1 (IRT1), Ferric Reduction Oxidase 2 (FRO2) and FIT1. Also, it has been