subunit of mitochondrial cytochrome oxidase, COX5b-1 mRNAs in Arabidopsis
(Sunkar et al. 2006; Yamasaki et al. 2007; Beauclair et al. 2010; Brousse et al. 2014).
miR408 targets plantacyanin gene family members in rice (Li et al. 2010; Zhou et al.
2010; Mutum et al. 2016). miR397 is involved in the downregulation of the laccase
gene family members (Abdel-Ghany and Pilon 2008; Lu et al. 2013; Wang et al.
2014). The above described copper Cu-miRNA:targets are organized into
intertwined feedback loops that regulate the cellular Cu levels (Pilon 2017). The
SPL7 TF governs the abundance of Cu-miRNA:target modules, by sensing any
change in the available pool of Cu. Lower levels of Cu lead to activation of SPL7
which enhances the transcription of Cu-miRNAs, which, in turn, lead to the reduction in target gene expression causing a re-adjustment of the system by diminishing
the Cu utilization by target genes (Pilon 2017). The above regulatory module helps
plants to regulate the local and systemic distribution of Cu making the plant to
modulate the Cu protein expression and development (Pilon 2017). The Cu-miRNA
shows a wide range of stress response and developmental roles suggesting the
critical involvement of their regulatory functions across diverse plant species
(Pilon 2017).
8.9
TFs-miR395: Regulating Sulfate Homeostasis
Sulfur is another essential macronutrient available in soil in the form of sulfate. On
absorption, by plants, sulfate is initially assimilated into the amino acid cysteine and
subsequently into glutathione, phytoalexins, and glucosinolates required by plant for
optimum growth under normal and stress conditions (Rausch and Wachter 2005).
During sulfate deprivation, plants tightly regulate sulfur uptake and metabolism at
different levels and miRNAs are also involved in this regulatory network
(Lewandowska and Sirko 2008). In sulfate deprived conditions, the SLIM1 (SULFUR LIMITATION 1) TF gets accumulated which induces the expression of
miR395 (Liang et al. 2010). Enhanced miR395 levels further target the APS (ATP
SULFURYLASE) genes and thus regulate the accumulation of sulfate in shoot.
miR395 also cleaves the transporter SULTR2;1 (SULFATE TRANSPORTER 2;1)
gene which finally affects the translocation of sulfate between the leaves. Besides
miR395, levels of miR160, miR164, miR167, miR168, miR156, and miR394 are
also altered during sulfate deprivation in Brassica rapa (Huang et al. 2010),
suggesting possible roles of these miRNAs in modulating necessary growth and
developmental adjustments during sulfate deprived conditions (Fig. 8.2 and
Table 8.1).
8.10 Conclusion and Future Perspective
Plant stress response is a complex trait involving some mechanisms that are
conserved across plant species while other processes that are specific to only a few
plants. The role of TFs in various stress responses is well worked out in many plant
266
S. Rao et al.
(Sunkar et al. 2006; Yamasaki et al. 2007; Beauclair et al. 2010; Brousse et al. 2014).
miR408 targets plantacyanin gene family members in rice (Li et al. 2010; Zhou et al.
2010; Mutum et al. 2016). miR397 is involved in the downregulation of the laccase
gene family members (Abdel-Ghany and Pilon 2008; Lu et al. 2013; Wang et al.
2014). The above described copper Cu-miRNA:targets are organized into
intertwined feedback loops that regulate the cellular Cu levels (Pilon 2017). The
SPL7 TF governs the abundance of Cu-miRNA:target modules, by sensing any
change in the available pool of Cu. Lower levels of Cu lead to activation of SPL7
which enhances the transcription of Cu-miRNAs, which, in turn, lead to the reduction in target gene expression causing a re-adjustment of the system by diminishing
the Cu utilization by target genes (Pilon 2017). The above regulatory module helps
plants to regulate the local and systemic distribution of Cu making the plant to
modulate the Cu protein expression and development (Pilon 2017). The Cu-miRNA
shows a wide range of stress response and developmental roles suggesting the
critical involvement of their regulatory functions across diverse plant species
(Pilon 2017).
8.9
TFs-miR395: Regulating Sulfate Homeostasis
Sulfur is another essential macronutrient available in soil in the form of sulfate. On
absorption, by plants, sulfate is initially assimilated into the amino acid cysteine and
subsequently into glutathione, phytoalexins, and glucosinolates required by plant for
optimum growth under normal and stress conditions (Rausch and Wachter 2005).
During sulfate deprivation, plants tightly regulate sulfur uptake and metabolism at
different levels and miRNAs are also involved in this regulatory network
(Lewandowska and Sirko 2008). In sulfate deprived conditions, the SLIM1 (SULFUR LIMITATION 1) TF gets accumulated which induces the expression of
miR395 (Liang et al. 2010). Enhanced miR395 levels further target the APS (ATP
SULFURYLASE) genes and thus regulate the accumulation of sulfate in shoot.
miR395 also cleaves the transporter SULTR2;1 (SULFATE TRANSPORTER 2;1)
gene which finally affects the translocation of sulfate between the leaves. Besides
miR395, levels of miR160, miR164, miR167, miR168, miR156, and miR394 are
also altered during sulfate deprivation in Brassica rapa (Huang et al. 2010),
suggesting possible roles of these miRNAs in modulating necessary growth and
developmental adjustments during sulfate deprived conditions (Fig. 8.2 and
Table 8.1).
8.10 Conclusion and Future Perspective
Plant stress response is a complex trait involving some mechanisms that are
conserved across plant species while other processes that are specific to only a few
plants. The role of TFs in various stress responses is well worked out in many plant
266
S. Rao et al.