**Author details**

in *Arabidopsis thaliana* [51]. RNA editing is a rare process where RNA polymerase is involved in insertion, deletion, and base substitution of nucleotide within the transcript [52–55]. Many studies reported the evidence of RNA editing in tRNA, rRNA, and mRNA. However, RNA editing has also been reported in noncoding RNA, like microRNAs of eukaryotes. The RNA editing occurs in all DNA-containing organelles like nucleus, mitochondria, and plastids. In nucleus, chloroplast and mitochondria RNA editing occurs during the process of transcription and posttranscriptional modifications [56, 57]. Caseinolytic protease complex component (CLPC1) plays a crucial role in RNA homeostasis [58]. Anyhow, discrete changes in RNA before its translation into protein occur by RNA editing. Besides this, RNA editing is also a

In chloroplast gene expression system, RNA editing is an important posttranscriptional modification. The use of pentatricopeptide repeat (PPR) protein family for RNA editing in chloroplast has been reported [51]. Mostly genes in chloroplast are cotranscribed and arranged in clusters. To control gene expression, posttranscriptional RNA editing is an essential step, and this step is also required for gene function [52]. It has been studied that C-to-U editing is the major type of RNA editing in chloroplasts. In chloroplast, etioplast, and amyloplast of maize, expression of almost 15 different genes has been affected by 27 C-to-U RNA editing sites. In chloroplast, RNA editing plays an important role to correct harmful mutations instead of producing protein diversity. Genomic DNA sequence is not changed by C-to-U editing because this editing changes the nucleotide sequence only within RNA molecule. RNA polymerase is used to produce RNA editing [60]. Insertion, deletion, and base substitution are events of RNA editing. That is why RNA editing can reverse harmful genomic mutations in consistent RNA transcript. In chloroplast, different sites are edited by C-to-U RNA editing enzymes as well [61]. Around 126 C-to-U editing events and 11 U-to-C editing events were identified in the chloroplast DNA of moth orchid (*P. aphrodite* subsp. Formosana). In leaf tissues, 110 editing events and in floral tissue, 106 editing events were identified. In non-protein-coding RNA such as introns, tRNA, and regulatory sequences, RNA editing occurred [62]. Besides C-to-U editing, which is mostly reported in chloroplast of plants, adenosine-to-inosine editing in plastid tRNA of *Arabidopsis thaliana* has also been characterized. Adenosine-to-inosine editing was recognized in the anticodon of the plastid tRNA-Arg (ACG). AtTadA gene expression is

Chloroplasts are the most important solar-energy-capturing natural systems on earth. They not only capture it but also convert it into a form useful for all living organism on earth. Molecular oxygen is liberated as a by-product, which is a vital source for respiration of all aerobic organisms. Chloroplasts are believed to be evolved from prokaryotic ancestors through a process known as endosymbiosis. Chloroplast contains circular genome having compactly arranged genes, which are involved in not only photosynthesis but also many other vital biological processes. Keeping in view its utmost physiological importance, plant as well as algal plastome has been engineered for a number of agronomic as well as pharmaceutical traits [63, 64]. Advancements in molecular biology and transgenic technology have further groomed importance of the organelle, and they

vibrant mechanism to produce functional and molecular diversity [59].

68 Transgenic Crops - Emerging Trends and Future Perspectives

involved in adenosine-to-inosine editing in the chloroplast [51].

**8. Conclusions and future directions**

Muhammad Sarwar Khan, Ghulam Mustafa\* and Faiz Ahmad Joyia

\*Address all correspondence to: drmustafa8@gmail.com

Centre of Agricultural Biochemistry and Biotechnology (CABB), University of Agriculture, Faisalabad, Pakistan
