miR319. Overexpression of miR319 downregulates TCP transcription factor genes
and enhances cold tolerance in both species (Thiebaut et al. 2012; Yang et al. 2013),
suggesting that miR319 acts as a positive regulator of cold tolerance. Plants
overexpressing miR397 are tolerant to chilling stress of 4
C for 2 months (Dong
and Pei 2014). Higher transcript levels of cold induced CBF (C-repeat/dehydration
binding factor) TF genes and downstream cold responsive genes in miR397a
overexpression plants indicate possible role of miR397a in the CBF-regulon
(Dong and Pei 2014). Overexpression of miR394a and its target F-box gene LCR
(LEAF CURLING RESPONSIVENESS) in Arabidopsis has demonstrated the positive role of this miRNA-target pair in response to low temperature stress (Song et al.
2016). MIR394a overexpressing plants have higher levels of CBF1, CBF2, and
CBF3 TF transcripts relative to the wild-type plants, suggesting the involvement
of miR394 in CBF-dependent cold tolerance pathway. In rice, miR535 negatively
regulates cold tolerance by aggravating cell death and ROS accumulation.
Overexpression of osa-miR535 downregulates the expression of SPL TF genes
during cold conditions in rice, thereby negatively regulating cold tolerance (Sun
et al. 2020) (Table 8.1).
8.5
miRNA-TFs: Regulating Heavy Metal Stress
Plants acquire essential heavy-metal elements like iron (Fe), zinc (Zn), copper (Cu),
and manganese (Mn) from the soil. These are required for many physiological and
biochemical processes of the plant in small concentrations. However, when present
in higher concentrations, they become harmful for the plant, often leading to oxidative stress and stunted growth (Noman et al. 2019). Certain non-essential
heavy metals like lead (Pb), mercury (Hg), and cadmium (Cd) are also present in
the heavy metal contaminated soils and can be taken up by the plants, thus leading to
heavy metal toxicity (Gupta et al. 2014). Plants have devised various mechanisms to
prevent the accumulation of these heavy metal ions at harmful concentrations.
miRNAs play an important role in protecting the plant from heavy metal toxicity
by targeting specific TFs involved in pathways which ultimately lead to translocation
and compartmentalization of heavy metals, heavy metal chelation or control of
oxidative damage (Noman and Aqeel 2017). Gao et al. (2019) have shown that
miR156, miR166, miR167, and miR171, all of which target different TFs, are
downregulated in response to cadmium stress in maize. The downregulation of
miR171 (and thus accumulation of its target GRAS transcription factor transcripts)
shows the highest fold-difference by qPCR and is also confirmed by in situ
hybridization. Furthermore, overexpression of miR166 in rice improves Cd tolerance by reducing the Cd-induced oxidative burst in transgenic rice plants (Ding et al.
2018). Overexpression of miR166 reduces both Cd translocation from roots to
shoots and Cd accumulation in the grains by targeting class-III HD-Zip transcription
factor HOMEODOMAIN CONTAINING PROTEIN4 (OsHB4) transcripts in rice
(Ding et al. 2018). In Medicago truncatula, high Cd, Hg, and Al induce miR319
8 Orchestration of MicroRNAs and Transcription Factors in the Regulation of Plant. . .
261
and enhances cold tolerance in both species (Thiebaut et al. 2012; Yang et al. 2013),
suggesting that miR319 acts as a positive regulator of cold tolerance. Plants
overexpressing miR397 are tolerant to chilling stress of 4
C for 2 months (Dong
and Pei 2014). Higher transcript levels of cold induced CBF (C-repeat/dehydration
binding factor) TF genes and downstream cold responsive genes in miR397a
overexpression plants indicate possible role of miR397a in the CBF-regulon
(Dong and Pei 2014). Overexpression of miR394a and its target F-box gene LCR
(LEAF CURLING RESPONSIVENESS) in Arabidopsis has demonstrated the positive role of this miRNA-target pair in response to low temperature stress (Song et al.
2016). MIR394a overexpressing plants have higher levels of CBF1, CBF2, and
CBF3 TF transcripts relative to the wild-type plants, suggesting the involvement
of miR394 in CBF-dependent cold tolerance pathway. In rice, miR535 negatively
regulates cold tolerance by aggravating cell death and ROS accumulation.
Overexpression of osa-miR535 downregulates the expression of SPL TF genes
during cold conditions in rice, thereby negatively regulating cold tolerance (Sun
et al. 2020) (Table 8.1).
8.5
miRNA-TFs: Regulating Heavy Metal Stress
Plants acquire essential heavy-metal elements like iron (Fe), zinc (Zn), copper (Cu),
and manganese (Mn) from the soil. These are required for many physiological and
biochemical processes of the plant in small concentrations. However, when present
in higher concentrations, they become harmful for the plant, often leading to oxidative stress and stunted growth (Noman et al. 2019). Certain non-essential
heavy metals like lead (Pb), mercury (Hg), and cadmium (Cd) are also present in
the heavy metal contaminated soils and can be taken up by the plants, thus leading to
heavy metal toxicity (Gupta et al. 2014). Plants have devised various mechanisms to
prevent the accumulation of these heavy metal ions at harmful concentrations.
miRNAs play an important role in protecting the plant from heavy metal toxicity
by targeting specific TFs involved in pathways which ultimately lead to translocation
and compartmentalization of heavy metals, heavy metal chelation or control of
oxidative damage (Noman and Aqeel 2017). Gao et al. (2019) have shown that
miR156, miR166, miR167, and miR171, all of which target different TFs, are
downregulated in response to cadmium stress in maize. The downregulation of
miR171 (and thus accumulation of its target GRAS transcription factor transcripts)
shows the highest fold-difference by qPCR and is also confirmed by in situ
hybridization. Furthermore, overexpression of miR166 in rice improves Cd tolerance by reducing the Cd-induced oxidative burst in transgenic rice plants (Ding et al.
2018). Overexpression of miR166 reduces both Cd translocation from roots to
shoots and Cd accumulation in the grains by targeting class-III HD-Zip transcription
factor HOMEODOMAIN CONTAINING PROTEIN4 (OsHB4) transcripts in rice
(Ding et al. 2018). In Medicago truncatula, high Cd, Hg, and Al induce miR319
8 Orchestration of MicroRNAs and Transcription Factors in the Regulation of Plant. . .
261