Regulation of pri-MIRNA processing: mechanistic insights into the miRNA homeostasis in plant.

miRNAs in plant plays crucial role in controlling proper growth, development and fitness by modulating the expression of their target genes. Therefore to modulate the expression of any stress/development related gene specifically, it is better to modulate expression of the miRNA that can target that gene. To modulate the expression level of miRNA, it is prerequisite to uncover the underlying molecular mechanism of its biogenesis. The biogenesis pathway consists of two major steps, transcription of MIR gene to pri-MIRNA and processing of pri-MIRNA into mature miRNA via sequential cleavage steps. Both of these pathways are tightly controlled by several different factors involving structural and functional molecules. This review is mainly focused on different aspects of pri-MIRNA processing mechanism to emphasize on the fact that to modulate the level of a miRNA in the cell only over-expression or knock-down of that MIR gene is not always sufficient rather it is also crucial to take processing regulation into consideration. The data collected from the recent and relevant literatures depicts that processing regulation is controlled by several aspects like structure and size of the pri-MIRNA, presence of introns in MIR gene and their location, interaction of processing factors with the core components of processing machinery etc. These detailed information can be utilized to figure out the particular point which can be utilized to modulate the expression of the miRNA which would ultimately be beneficial for the scientist and researcher working in this field to generate protocol for engineering plant with improved yield and stress tolerance.

miRNA-mediated regulation of auxin signaling pathway during plant development and stress responses.

Auxin is one of the most important plant growth hormones, playing a crucial role in development as well as in stress responses. Auxin biosynthesis and signaling pathway comprises a series of events including auxin perception by the receptor, activation, and function of auxin response factors and control by auxin repressors. All these factors are regulated by several different microRNAs during leaf, flower and fruit development, anther development, nodulation, lateral and adventitious root development, potato tuber development as well as during heat stress, submergence, boron toxicity, aluminium stress responses, etc., as depicted in the available literature. In this review a thorough study on miRNA-mediated regulation of auxin biosynthesis and signaling has been done in various plant species. The data gathered can be utilized to point out the particular miRNAmediated regulation module which can be utilized to modulate the expression of the miRNA and thereby modulation of the auxin pathway. Information in this review would be beneficial to utilize the miRNA expression to generate the protocol for engineering plants with altered auxin signaling pathway to obtain better yield and improved stress tolerance.

Distinct transcriptional and processing regulations control miR167a level in tomato during stress.

Besides their definite role in plant developmental processes miR167 also serve as mediator of stress response. Although differential expression of miR167 occurs during stresses, the regulatory-mechanism of biogenesis remained elusive. Therefore, using tomato as the model plant we have explored the mechanism of regulation of miR167a expression during stresses. Fungus or virus infections and exposure to cold stress raised the level of miR167a expression. Whereas, salt, drought and heat treatments resulted in the downregulation, indicating different stresses activated alternative mechanisms for miR167a regulation. Interestingly, the relative expression level of precursors in control versus temperature stressed plants differed from the pattern observed in the mature miR167a expression, suggesting that both transcriptional and processing regulation were important for biogenesis. The promoter-regulatory sequence of the major isoform MIR167a harbours several development and stress-related regulatory sites. Accordingly, promoter assays using transient transformation and transgenic tobacco plants proved stress-dependent regulation of the promoter. Further analyses corroborated the role of tomato DREB2A protein in the transcriptional regulation during temperature stress. Finally, in vitro assays established the importance of processing factors in cold-stress dependent efficient processing of MIR167a precursors. These data confirm distinct role of transcriptional and processing machinery in stress-influenced regulation of tomato miR167a biogenesis.