The Role of miRNA in Plant Adaptation to Abiotic Stress

Extended Abstract Global food security has become an urgent concern due to rapid climate change. Plants, the foundation of the food chain, are constantly under environmental pressures such as drought, salinity, and extreme and low temperatures. These stressors threaten plant growth and productivity, jeopardizing global food supplies. Plants can sense environmental stimuli and activate defense mechanisms through various regulatory networks, including small RNAs, to combat abiotic stressors. These changes trigger a cascade of defense responses, including the utilization of small RNAs, to protect themselves from damage. MicroRNAs (miRNAs) were first identified in plants less than two decades ago and have since been recognized as crucial controllers of various developmental processes. These processes include leaf morphogenesis (the formation of leaves), vegetative phase change (the transition from vegetative growth to flowering), flowering time, and the ability to respond to environmental signals. miRNAs, recognized as one of the most crucial RNA molecules, play a pivotal role by modulating gene function through post-transcriptional and translational mechanisms. RNA interference is a group of 18-25 nucleotide sequence-specific RNAs that are found abundantly in plant genomes. These RNAs play an important role in various processes, including plant growth and development, cell behavior, biochemical and physiological activities, defense against threats to the genome, and tolerance to abiotic stresses. Despite their small size, they wield immense power in regulating gene expression networks. miRNAs negatively regulate the expression of a wide range of genes at the transcription levels (DNA methylation), post-transcription, and translation. They act as post-transcriptional regulators, binding to specific sequences on messenger RNA (mRNA) molecules. This binding cleaves the target mRNA, effectively silencing the gene it encodes, or inhibits its translation into protein. Short interfering RNAs (siRNAs) are derived from the processing of long double-stranded RNAs (dsRNAs). Then, a specific guide strand is chosen and integrated into the RNA-induced silencing complex (RISC). Once this complex is inside RISC, a member of the Argonaute (AGO) protein family binds with the guide strand, directing RISC to target RNAs with complete sequence complementarity. This interaction leads to the precise cleavage of the target RNAs by the Argonaute protein. This process, known as RNA interference (RNAi), plays a fundamental role in gene regulation and defense responses in plants. Plants employ a sophisticated regulatory system called gene silencing, which controls gene expression by inactivating specific genes. Two key mechanisms in this system are post-transcriptional gene silencing (PTGS), which inactivates genes by targeting RNA molecules, and transcriptional gene silencing (TGS), which prevents RNA production from the DNA template. miRNAs can influence PTGS by promoting the degradation of specific mRNA transcripts and TGS by recruiting DNA methylation machinery to target genes. PTGS acts in the cytoplasm, targeting messenger RNA (mRNA) molecules. PTGS can be triggered by dsRNAs or miRNAs. These dsRNAs can originate from viral infection, transgene insertion, and inverted repeats within plant genes. Dicer, an RNase III enzyme complex, recognizes and cleaves relevant dsRNAs into small interfering RNAs (siRNAs) for RNA interference (RNAi). The siRNAs then guide a protein complex called RISC (RNA-induced silencing complex) to complementary mRNA sequences. RISC cleaves the targeted mRNAs, silencing them and preventing their translation into proteins. Similar to siRNAs, miRNAs can also regulate gene expression in PTGS by targeting mRNA molecules, although they often function through imperfect base pairing. TGS operates by modifying DNA in the nucleus, making it less accessible for transcription and thereby preventing mRNA production. TGS relies on mechanisms such as DNA methylation and histone modifications to silence gene expression in the nucleus. These modifications create a repressive chromatin environment that hinders RNA polymerase from accessing and transcribing the DNA. While the primary role of miRNAs lies in PTGS, some studies suggest that they might also influence TGS. Some miRNAs could interact with proteins involved in DNA methylation or chromatin remodeling, indirectly leading to transcriptional silencing. The interplay between PTGS and TGS is intricate. While they are distinct pathways, they can be interconnected. In some instances, PTGS might influence TGS through mechanisms. Degraded mRNAs from PTGS might serve as signals that guide DNA methylation machinery to homologous DNA sequences, potentially leading to long-term transcriptional silencing. Conversely, TGS-mediated gene silencing could prevent the formation of dsRNAs or aberrant transcripts that trigger PTGS. miRNAs act as molecular switches by strategically targeting specific mRNAs and fine-tuning the production of proteins essential for environmental stress tolerance. This precise control allows plants to adapt to a dynamic environment, tailoring their gene expression to meet the specific challenges they face. Plant miRNAs act as mediators for silencing or direct cleavage of target mRNAs. While some miRNAs perfectly match their mRNA targets, others can function with some mismatches. miRNA families are grouped into conserved and non-conserved miRNAs based on conserved spots and variation during processing. Each group of these miRNAs has its targets. Today, RNA interference (RNAi), triggered by dsRNA, is a widely used and valuable tool for researchers to specifically silence genes and understand their function in various biological processes. Despite ongoing research in genetically engineering plants to manipulate miRNAs for improved tolerance to biotic and abiotic stresses, knowledge remains limited regarding their functional and regulatory networks in this context. Understanding the regulatory function of this group of RNAs opens up new avenues for applied research in genomic fields, enhancing resistance to plant diseases, bolstering tolerance to various stresses such as drought, salinity, heat, and cold, as well as improving product quality and increasing food production. The review's objective is to assess the existing knowledge concerning plant small RNAs and elucidate their significance in enhancing resilience to abiotic stressors.