There are many types of RNA: some like messenger (m) RNA are translated into proteins, transfer (t) RNA and ribosomal (r) RNA have a housekeping role during mRNA translation and small (s) RNAs are not translated into proteins but regulate various biological processes like biotic and abiotic stress responses. Small RNAs come in different forms, the best understood classes being small interfering RNAs (siRNAs), microRNAs (miRNAs) and Piwi-associated RNAs (piRNAs). Small RNAs play a critical role in regulating the interaction of pathogens with plants thereby providing innate immunity to plants. Plant miRNA have important functions in organ development such as leaf morphogenesis, floral organ identity and root development. They are also involved in feedback regulation in small RNA pathway and in directing some siRNAs biogenesis. Moreover, they are involved in various stress responses, such as oxidative, mineral nutrient, dehydration stress and even mechanical stress.
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