Abiotic Stress Responses in Plants: Unraveling the Complexity of Genes and Networks to Survive - Abiotic Stress: Plant Responses and Applications in Agriculture - page 58

Sumoylation was also reported to participate in responses to phosphate starvation, and to the
tolerance to low and high temperatures [117]. An increase in the levels of SUMO-protein
conjugates was also detected in water-deprived plants [118].
The concerted actions of the transcriptional, post-transcriptional and post-translational
mechanisms ensures temporally and spatially appropriate patterns of downstream gene
expression and ultimately the shaping of transcriptome and proteome of stress-exposed plants
to switch on adaptive response. The complete understanding of the interplay of these three
regulatory systems is crucial for the understanding of the molecular mechanisms governing
plant adaptation to environment as well as for plant improvement for stress tolerance.
3.2. miRNAs in plant responses to abiotic stress — An additional post-transcriptional
regulation layer may apply
Plant responses to abiotic stress such as water deficit involve an intricate regulation of gene
expression at the transcriptional and post-transcriptional levels. MicroRNAs (miRNAs) are a
class of small non-coding RNAs molecules (21-24 nt) involved in post-transcriptional regula‐
tion of gene expression. miRNAs were shown to be involved in plant development [119-124],
biotic [125, 126] and abiotic stress responses [108, 110, 127-130].
In plants, microRNAs repress gene expression by directing mRNA degradation or trans‐
lational arrest: miRNAs guide Argonaute (AGO) proteins to bind to matching target
mRNAs in a RNA-induced silencing complex (RISC), promoting cleavage of mRNAs
with near perfect base complementarity and/or inhibiting translation of those with lower
complementarity [131-133].
The first reports assigning miRNAs to have a role in shaping plant responses to abiotic stresses
were based on small RNA cloning and sequencing [134], complemented with analyses of
miRNA expression profiles and miRNA target prediction [108]. Since then, the application of
high-throughput sequencing technology and genomic approaches like microarray analyses to
evaluate the profile of miRNA expression in various tissues and conditions, associated to
improved bioinformatic tools to identify miRNAs and their targets, have allowed an extensive
recognition of stress-responsive small RNAs and their targets in various plant species (re‐
viewed in [107]).
Sequencing of miRNAs in Legumes was first reported in Medicago truncatula [135] and Glycine
max [136] but there are references to small RNAs in other Legumes back to 2004, with a size
population of small RNA molecules being identified in the phloem sap of Lupinus albus [137].
These findings were the basis of a systemic signalling mechanism in which small RNAs
movement is facilitated by chaperone proteins to exert their action at a distance.
One of the most extensively studied miRNAs in the context of abiotic stresses have been
the miRNAs involved in nutrient deprivation miR395, miR398 and mir399, all identified
in the phloem sap of nutrient deprived plants. In fact, studies in Arabidopsis have estab‐
lished that miR395 (sulphate), miR399 (phosphate) and miR398 (copper) regulate these
nutrients homeostasis by moving along the phloem to inform the roots of the nutrient
status of the shoot [138-139].