**1. Introduction**

RNA is the ribonucleic acid, one of the earliest formed molecules to shape life on earth. Being single-stranded, these are known to have a shorter life span and integrity compared to DNA. But, from other angles, these properties make RNA a magical molecule with a unique ability to work inside and outside the nucleus, leading to diverse sorts of roles in structural to regulatory aspects. In the case of the prokaryotes, most of the transcribed RNA is translated because of the smaller size of the genome and the variety and load of work endowed on it for the organism's survivability. But, in the case of the eukaryotes, as the genome size gets bigger, the actual coding part of the chromosomes becomes sparse and rather scattered, mostly. Secondly, the coding genes occupy only 1–2% of the genome. A very high percentage of eukaryotic genomes around up to 90% undergo transcription to produce RNA, nevertheless only a little portion of transcripts get actually translated into proteins [1].

Out of the total RNAs, non-coding RNA (ncRNA) pertains to active and functional RNA molecules that are not translated into proteins, although being transcribed from DNA. They encompass a wide range of RNA molecules having the potential to play in the regulation of gene expression [1]. They can act as long ncRNAs or be converted into s very smaller size RNA molecule proteins in eukaryotic cells. Mostly ncRNA is categorized worldwide based on its size, function, or genetic origin [1].

Based on the size of the molecule, their origin, functionality, and ncRNA are sorted into either the long non-coding RNAs (lncRNAs) having nucleotide lengths of more than 200 nucleotides or the small RNAs (sRNAs), which are comparatively quite smaller in size. Again the sRNAs are further classified into different types of RNA, such as miRNA, siRNA, piRNA, which will be disused further in detail with the various aspects of their origin, features, functions, and other aspects in the later section of the chapter.

ncRNAs are responsible for a wide range of biological functions. They control gene expression at the transcriptional, RNA processing, and translational levels by the particular structural aspects of RNA itself. Recent discoveries also show their role in various epigenetic phenomena affecting multiple physiological pathways and expression of particular phenotypes in a different situation. The fraction of the coding region of the protein genome varies considerably and is oppositely proportionate to the genome's size and complexity [2]. Many regulatory ncRNAs do have relatively high specificity of the target, whereas others serve as a major modulator of extensive regulatory signaling networks by acting across the genome [3]. They keep alien nucleic acids out of genomes and safeguard the genome against them. Non-coding RNAs target a single gene and pathways involving multiple genes at the genome level through distinct molecular mechanisms. Hence, these regulatory ncRNAs could be potential breeding targets for advanced breeding programs in plants [4]. They can regulate the synthesis of DNA and also take part in the reorganization of the genome. The biological activity of ribozymes and riboswitches is served by several ncRNAs that use the power of base pairing to interact with other nucleic acids preferentially [5].

Non-coding RNAs (ncRNAs), which act as a natural defense mechanism against—attacking viruses, have also been found as effectors in RNA-mediated gene silencing and hence now utilized in crop genetic modification [6]. The role of ncRNA has been observed in RNA interference and other regulatory mechanisms in plants; these provide a huge scope for the use of advanced molecular biology tools on these for enhancing the production potential of plants and modulation of growth and development of a plant to a certain extent. These have also been reported to influence the genes and biochemical pathways involving important traits like floral growth, maturation of seed, various biotic and abiotic stresses, along with pest and disease resistance processes. The ncRNA and their detailed structural to functional aspects are narrated meticulously in the upcoming sections of the chapter.
