acquired thermo-tolerance. In Medicago sativa, miR156 overexpression and its
target SPL13 RNAi plants show enhanced tolerance to HS (Matthews 2018).
Contrary to the roles of miR398 and miR156, miR159 acts as a negative regulator
of HS response by targeting gibberellic acid (GA) regulated MYB (GAMYB)-like
family TFs in plants. Overexpression of wheat miR159 leads to enhanced sensitivity
to HS in rice (Wang et al. 2012b). Similarly, heat-mediated downregulation of
csa-miR159b is also reported in cucumber (Cucumis sativa) plants (Li et al. 2016).
Overexpression of csa-miR159b leads to decreased heat tolerance in Arabidopsis,
further confirming the negative role of miR159 in heat tolerance. Giacomelli et al.
(2012) have identified a WRKY transcription factor, HaWRKY6 as a novel miR396
target in sunflower (Helianthus annuus) that is unique to the Asteraceae family.
Transgenic plants expressing the miR396-resistant version of HaWRKY6 are sensitive to HS indicating the positive role of this recently evolved miR396:HaWRKY6
miRNA:target module has in tolerance to high temperatures. These results also show
how a miRNA (miR396) that is normally known for regulating plant development
via its canonical targets the GROWTH-REGULATING FACTOR (GRF) TF genes
(Rodriguez et al. 2010), can also be recruited for HS tolerance. Similarly, miR172
known for regulating the juvenile to adult phase transition has been reported to be
downregulated in heat stress in Arabidopsis, wheat, and sunflower, while its target
TF genes TOE1 and TOE2 (Target of early activation tagged 1 and 2) are
upregulated (May et al. 2013; Li et al. 2014). In addition, other developmentally
regulated miRNA:TF target modules such as miR164:NAC1 (NAM, ATAF1/2, and
CUC2), miR166:PHV/REV/HOX9 (PHAVOLUTA/REVOLUTA/HOMEOBOX A9),
miR169:NF-YA (Nuclear factor-YA), and miR171:SCL6-III (Scarecrow-like protein
6-III) have also been shown to be heat stress responsive in genome-wide large data
studies (Barku et al. 2013; May et al. 2013; Li et al. 2014; Kumar et al. 2014:
Kruszka et al. 2014; Rao et al. 2020). Lin et al. (2018) have reported that
overexpression of miR160 improves seed germination and seedling survival under
heat stress. miR160 overexpression and its target mimic MIM160 plants show heat
tolerance and heat sensitivity, respectively, via regulating the expression of target
Auxin Response Factor (ARF) TFs, namely ARF10, ARF16, and ARF17. The authors
have shown that miR160 alters the expression of heat shock proteins as well as plant
development to allow plants to survive heat stress. Furthermore, a study has
delineated a Brassicaceae-specific heat-mediated regulation of miR824 in integrating
the HS signals to modulate the MADS-box TF, AGAMOUS LIKE 16 (AGL16),
leading to the derepression of FLOWERING LOCUS T (FT) gene (Szaker et al.
2019). During HS the HSFA1 and HSFA2 mediated transcriptional induction of
miR824 regulates AGL16 post-transcriptionally, which alters the FT levels to finetune the post-stress development of the plants (Szaker et al. 2019).
258
S. Rao et al.
target SPL13 RNAi plants show enhanced tolerance to HS (Matthews 2018).
Contrary to the roles of miR398 and miR156, miR159 acts as a negative regulator
of HS response by targeting gibberellic acid (GA) regulated MYB (GAMYB)-like
family TFs in plants. Overexpression of wheat miR159 leads to enhanced sensitivity
to HS in rice (Wang et al. 2012b). Similarly, heat-mediated downregulation of
csa-miR159b is also reported in cucumber (Cucumis sativa) plants (Li et al. 2016).
Overexpression of csa-miR159b leads to decreased heat tolerance in Arabidopsis,
further confirming the negative role of miR159 in heat tolerance. Giacomelli et al.
(2012) have identified a WRKY transcription factor, HaWRKY6 as a novel miR396
target in sunflower (Helianthus annuus) that is unique to the Asteraceae family.
Transgenic plants expressing the miR396-resistant version of HaWRKY6 are sensitive to HS indicating the positive role of this recently evolved miR396:HaWRKY6
miRNA:target module has in tolerance to high temperatures. These results also show
how a miRNA (miR396) that is normally known for regulating plant development
via its canonical targets the GROWTH-REGULATING FACTOR (GRF) TF genes
(Rodriguez et al. 2010), can also be recruited for HS tolerance. Similarly, miR172
known for regulating the juvenile to adult phase transition has been reported to be
downregulated in heat stress in Arabidopsis, wheat, and sunflower, while its target
TF genes TOE1 and TOE2 (Target of early activation tagged 1 and 2) are
upregulated (May et al. 2013; Li et al. 2014). In addition, other developmentally
regulated miRNA:TF target modules such as miR164:NAC1 (NAM, ATAF1/2, and
CUC2), miR166:PHV/REV/HOX9 (PHAVOLUTA/REVOLUTA/HOMEOBOX A9),
miR169:NF-YA (Nuclear factor-YA), and miR171:SCL6-III (Scarecrow-like protein
6-III) have also been shown to be heat stress responsive in genome-wide large data
studies (Barku et al. 2013; May et al. 2013; Li et al. 2014; Kumar et al. 2014:
Kruszka et al. 2014; Rao et al. 2020). Lin et al. (2018) have reported that
overexpression of miR160 improves seed germination and seedling survival under
heat stress. miR160 overexpression and its target mimic MIM160 plants show heat
tolerance and heat sensitivity, respectively, via regulating the expression of target
Auxin Response Factor (ARF) TFs, namely ARF10, ARF16, and ARF17. The authors
have shown that miR160 alters the expression of heat shock proteins as well as plant
development to allow plants to survive heat stress. Furthermore, a study has
delineated a Brassicaceae-specific heat-mediated regulation of miR824 in integrating
the HS signals to modulate the MADS-box TF, AGAMOUS LIKE 16 (AGL16),
leading to the derepression of FLOWERING LOCUS T (FT) gene (Szaker et al.
2019). During HS the HSFA1 and HSFA2 mediated transcriptional induction of
miR824 regulates AGL16 post-transcriptionally, which alters the FT levels to finetune the post-stress development of the plants (Szaker et al. 2019).
258
S. Rao et al.