The Molecular Basis of ABA-Mediated Plant Response to Drought - Abiotic Stress: Plant Responses and Applications in Agriculture - page 113

are processed by DCL1 (DICER-LIKE 1) HYL1 (HYPONASTIC LEAVES 1), SE (SERRATED)
proteins into pre-miRNA hairpin precursors which are finally converted into short duplexes
- mature miRNAs. The duplexes are then methylated at the 3' terminus and exported to the
cytoplasm. In the cytoplasm, single-stranded miRNAs are incorporated in the AGO (ARGO‐
NAUTE) protein, the catalytic compound of the RISC (RNA-INDUCED SILENCING COM‐
PLEX) complex, and guide the RISC to the target mRNAs by sequence complementarity to
negatively regulate their expression [112].
Plant microRNAs are involved in various developmental processes including flowering, and
leaf, stem and root development [113-115]. Jones-Rhoades and Bartel [116] drew the atten‐
tion of plant biologists to the miRNA engagement in stress response for the first time. To
gain an insight into the role of miRNAs in the regulation of transcripts in response to
drought, several projects on the identification of the miRNAs related to stress response in
crops were undertaken. Using deep sequencing techniques, Zhou et al [117] identified nine‐
teen new miRNAs that are induced by drought in rice, among them eleven down-regulated
and eight up-regulated miRNAs. In addition, they identified nine miRNAs that showed an
opposite expression to that observed in drought-stressed Arabidopsis (Table 3). A similar
approach was used by Kulcheski et al. [118] in soybean, which revealed 11 miRNAs that are
related to drought stress (Table 3). Based on bioinformatic prediction and then verification
of the obtained results using RT-qPCR, Xu et al. [119] identified 21 miRNAs differently ex‐
pressed during water stress in maize (Table 3). A similar approach using bioinformatic pre‐
diction of miRNAs on dehydration stress was undertaken by Kantar et al. [7], who found
four miRNAs that are related to drought stress in barley (Table 3). Deep sequencing of a
small RNA library in the case of barley was performed by Lv et al. [8]. They showed that six
miRNAs specific for stress response. hvu-MIRn026a, hvu-MIRn029, hvu-MIR035, hvu-
MIR156d exhibited higher expression in response to salt and drought stress, whereas hvu-
MIR396d and hvu-MIR399b showed a higher expression only in drought-stressed plants.
Additionally, the authors observed that hvu-mir029 was highly expressed after drought
treatment and at a very low level under non-stressed conditions, which suggests the impor‐
tant role of this molecule in water deficit response (Table 3).
To understand the function of newly identified miRNAs, the putative target transcripts have
to be predicted. In order to identify microRNAs target transcripts, Kantar et al [7] performed
computational studies and a modified 5' RLM-RACE (RNA ligase-mediated 5' rapid ampli‐
fication of cDNA ends) in barley. Seven cleaved miRNA transcripts were retrieved from
drought-stressed leaf samples as targets for hvu-MIR165, hvu-MIR166, hvu-MIR156, hvu-
MIR2055, hvu-MIR171, hvu-MIR172, hvu-MIR397 and hvu-MIR159. The identified targets
are mainly transcription factors that play a role in plant development, morphology and de‐
termination of the flowering time. SCRL6 ( SCARECROW LIKE 6 ) encodes a transcription fac‐
tor that is involved in diverse plant developmental processes such as leaf or root growth and
is the target of hvu-MIR171, ARF10 ( AUXIN RESPONSIVE FACTOR 10 ) encodes a transcrip‐
tion factor that negatively regulates auxin signaling and is the target of hvu-MIR160, SBP
( SQUAMOSA PROMOTER BINDING PROTEIN ) is a transcription factor that is mainly im‐
portant for leaf development and is the target of hvu-MIR156a, and MYB33 ( MYB DOMAIN