expression that leads to the degradation of the TCP TF genes that in turn affect leaf
growth and senescence (Mendoza-Soto et al. 2012) (Fig. 8.1).
8.6
TFs-miRNA: Regulating Phosphate Homeostasis
Phosphate is a very crucial macroelement for all life forms. It is a major structural
component of DNA/RNA and phospholipids that is also involved in energy transfer,
protein activation, and metabolic regulation. Inorganic phosphate (Pi) is required for
the growth and development of plants (Liu et al. 2014; Kuo and Chiou 2011; Nilsson
et al. 2010). Plants acquire Pi from the soil by roots. But the primary challenge is low
availability of Pi in soil as the majority of Pi gets converted into organic matter
(Marschner 1995). Consequently, plants have evolved counter strategies to optimize
Pi acquisition and assimilation. Phosphate starvation response 1 (PHR1) and PHL1
(PHR1-like 1) are MYB TFs that play a critical role in Pi deficiency signaling by
transcriptionally regulating a wide range of phosphate starvation genes (Rubio et al.
2001; Bustos et al. 2010). PHR1 acts upstream to the miR399 by controlling the
transcription of all six forms of miR399 (Bari et al. 2006). Pi deficiency induced
miR399 reduces the expression of its target gene PHO2 (phosphate overaccumulator 2), an E2 conjugase gene (Fujii et al. 2005; Chiou et al. 2006; Zhu
et al. 2020). Under Pi-sufficient condition, the overexpression of miR399 increases
the uptake of Pi and allocation to shoots which eventually causes Pi toxicity in
Arabidopsis (Fujii et al. 2005; Aung et al. 2006; Bari et al. 2006; Chiou et al. 2006).
In constitutively overexpressing osa-miR399f or osa-miR399j transgenic rice, a
similar phenotype is observed (Hu et al. 2011). The Pi content in shoot and root
increases in heterologous overexpression of Arabidopsis miR399d in tomato due to
enhanced expression of Pi transporter genes and improves proton exudation from
roots (Gao et al. 2015a). Moreover, the expression of osa-miR399a and osa-miR399j
is also regulated by an R2R3 MYB TF, OsMYB2P-1 in response to Pi deficiency
(Dai et al. 2012). The transcriptional activity of osa-miR399a, osa-miR399f, and
osa-miR399j is regulated by OsWRKY74 TF in Pi deficient conditions. Plants
overexpressing OsWRKY74 have enhanced expression of osa-miR399a,
osa-miR399f, and osa-miR399j (Dai et al. 2016). The activity of miR399 is also
controlled by a long non-coding RNA IPS1 that contains a near-perfect
non-cleavable binding site for miR399 that efficiently sequesters miR399 (FrancoZorrilla et al. 2007) (Fig. 8.2). This results in reduction of miR399 levels leading to
protection of PHO2 transcripts from cleavage (Franco-Zorrilla et al. 2007). Notably,
the regulatory module IPS1:miR399:PHO2 is conserved within different plant
species including Arabidopsis, soybean, common bean, Medicago, and tomato
(Valdés-López et al. 2008; Branscheid et al. 2010; Liu et al. 2010; Hu et al. 2011;
Huang et al. 2011; Wang et al. 2020). In rice, Pi deficiency response leads to the
activation of OsPHR2 TF-regulated miRNA827:OsSPX-MFS1,2 (SYG1/Pho81/
XPR1-MAJOR FACILITATOR SUPERFAMILY) module that in turn maintains
phosphate homeostasis (Wang et al. 2012a).
262
S. Rao et al.
growth and senescence (Mendoza-Soto et al. 2012) (Fig. 8.1).
8.6
TFs-miRNA: Regulating Phosphate Homeostasis
Phosphate is a very crucial macroelement for all life forms. It is a major structural
component of DNA/RNA and phospholipids that is also involved in energy transfer,
protein activation, and metabolic regulation. Inorganic phosphate (Pi) is required for
the growth and development of plants (Liu et al. 2014; Kuo and Chiou 2011; Nilsson
et al. 2010). Plants acquire Pi from the soil by roots. But the primary challenge is low
availability of Pi in soil as the majority of Pi gets converted into organic matter
(Marschner 1995). Consequently, plants have evolved counter strategies to optimize
Pi acquisition and assimilation. Phosphate starvation response 1 (PHR1) and PHL1
(PHR1-like 1) are MYB TFs that play a critical role in Pi deficiency signaling by
transcriptionally regulating a wide range of phosphate starvation genes (Rubio et al.
2001; Bustos et al. 2010). PHR1 acts upstream to the miR399 by controlling the
transcription of all six forms of miR399 (Bari et al. 2006). Pi deficiency induced
miR399 reduces the expression of its target gene PHO2 (phosphate overaccumulator 2), an E2 conjugase gene (Fujii et al. 2005; Chiou et al. 2006; Zhu
et al. 2020). Under Pi-sufficient condition, the overexpression of miR399 increases
the uptake of Pi and allocation to shoots which eventually causes Pi toxicity in
Arabidopsis (Fujii et al. 2005; Aung et al. 2006; Bari et al. 2006; Chiou et al. 2006).
In constitutively overexpressing osa-miR399f or osa-miR399j transgenic rice, a
similar phenotype is observed (Hu et al. 2011). The Pi content in shoot and root
increases in heterologous overexpression of Arabidopsis miR399d in tomato due to
enhanced expression of Pi transporter genes and improves proton exudation from
roots (Gao et al. 2015a). Moreover, the expression of osa-miR399a and osa-miR399j
is also regulated by an R2R3 MYB TF, OsMYB2P-1 in response to Pi deficiency
(Dai et al. 2012). The transcriptional activity of osa-miR399a, osa-miR399f, and
osa-miR399j is regulated by OsWRKY74 TF in Pi deficient conditions. Plants
overexpressing OsWRKY74 have enhanced expression of osa-miR399a,
osa-miR399f, and osa-miR399j (Dai et al. 2016). The activity of miR399 is also
controlled by a long non-coding RNA IPS1 that contains a near-perfect
non-cleavable binding site for miR399 that efficiently sequesters miR399 (FrancoZorrilla et al. 2007) (Fig. 8.2). This results in reduction of miR399 levels leading to
protection of PHO2 transcripts from cleavage (Franco-Zorrilla et al. 2007). Notably,
the regulatory module IPS1:miR399:PHO2 is conserved within different plant
species including Arabidopsis, soybean, common bean, Medicago, and tomato
(Valdés-López et al. 2008; Branscheid et al. 2010; Liu et al. 2010; Hu et al. 2011;
Huang et al. 2011; Wang et al. 2020). In rice, Pi deficiency response leads to the
activation of OsPHR2 TF-regulated miRNA827:OsSPX-MFS1,2 (SYG1/Pho81/
XPR1-MAJOR FACILITATOR SUPERFAMILY) module that in turn maintains
phosphate homeostasis (Wang et al. 2012a).
262
S. Rao et al.