Small RNA sequencing reveals various microRNAs involved in piperine biosynthesis in black pepper (Piper nigrum L.)

Noncoding RNAs in regulation of plant secondary metabolism

Plant secondary metabolites (PSMs) are a large class of structurally diverse molecules, mainly consisting of terpenoids, phenolic compounds, and nitrogen-containing compounds, which play active roles in plant development and stress responses. The biosynthetic processes of PSMs are governed by a sophisticated regulatory network at multiple levels. Noncoding RNAs (ncRNAs) such as microRNAs (miRNAs), long ncRNAs (lncRNAs), and circular RNAs (circRNAs) may serve as post-transcriptional regulators for plant secondary metabolism through acting on genes encoding either transcription factors or participating enzymes in relevant metabolic pathways. High-throughput sequencing technologies have facilitated the large-scale identifications of ncRNAs potentially involved in plant secondary metabolism in model plant species as well as certain species with enriched production of specific types of PSMs. Moreover, a series of miRNA-target modules have been functionally characterized to be responsible for regulating PSM biosynthesis and accumulation in plants under abiotic or biotic stresses. In this review, we will provide an overview of current findings on the ncRNA-mediated regulation of plant secondary metabolism with special attention to its participation in plant stress responses, and discuss possible issues to be addressed in future fundamental research and breeding practice.

Genome-Wide Identification and Characterization of miRNAs and Natural Antisense Transcripts Show the Complexity of Gene Regulatory Networks for Secondary Metabolism in Aristolochia contorta

Aristolochia contorta Bunge is an academically and medicinally important plant species. It belongs to the magnoliids, with an uncertain phylogenetic position, and is one of the few plant species lacking a whole-genome duplication (WGD) event after the angiosperm-wide WGD. A. contorta has been an important traditional Chinese medicine material. Since it contains aristolochic acids (AAs), chemical compounds with nephrotoxity and carcinogenicity, the utilization of this plant has attracted widespread attention. Great efforts are being made to increase its bioactive compounds and reduce or completely remove toxic compounds. MicroRNAs (miRNAs) and natural antisense transcripts (NATs) are two classes of regulators potentially involved in metabolism regulation. Here, we report the identification and characterization of 223 miRNAs and 363 miRNA targets. The identified miRNAs include 51 known miRNAs belonging to 20 families and 172 novel miRNAs belonging to 107 families. A negative correlation between the expression of miRNAs and their targets was observed. In addition, we identified 441 A. contorta NATs and 560 NAT-sense transcript (ST) pairs, of which 12 NATs were targets of 13 miRNAs, forming 18 miRNA-NAT-ST modules. Various miRNAs and NATs potentially regulated secondary metabolism through the modes of miRNA-target gene-enzyme genes, NAT-STs, and NAT-miRNA-target gene-enzyme genes, suggesting the complexity of gene regulatory networks in A. contorta. The results lay a solid foundation for further manipulating the production of its bioactive and toxic compounds.

Mechanism of action of microRNA166 on nitric oxide in alfalfa (Medicago sativa L.) under drought stress

BACKGROUND

Alfalfa is a perennial forage crop of high importance, but its cultivation is often affected by drought stress. Currently, the investigation of drought-related small RNAs is a popular research topic to uncover plant drought resistance mechanisms. Among these small RNAs, microRNA166 (miR166) is associated with drought in numerous plant species. Initial small RNA sequencing studies have shown that miR166 is highly responsive to exogenous nitric oxide (NO) and drought. Therefore, analyzing the expression of Msa-miR166 under nitric oxide and drought treatment is significant.

RESULT

Bioinformatics analysis revealed that the miR166 family is widely distributed among plants, ranging from mosses to eudicots, with significant distribution differences between species. The evolutionary degree of Msa-miR166s is highly similar to that of Barrel medic (Medicago truncatula) and Soybean (Glycine max), but significantly different from the model plant Arabidopsis (Arabidopsis thaliana). It is suggested that there are no significant differences in miR166s within the species, and members of Msa-miR166s can form a typical stem-loop. The lowest level of exogenous nitric oxide was observed in Msa-miR166s under drought stress, followed by individual drought, and the highest level was observed after removing endogenous nitric oxide.

CONCLUSION

In response to short-term drought, Msa-miR166s down-regulate expression in alfalfa (Medicago sativa L.). Exogenous nitric oxide can reduce the expression of Msa-miR166s in response to short-term drought. These findings suggest that Msa-miR166e-5p is responsive to environmental changes. The expression levels of target genes showed an opposite trend to Msa-miR166s, verifying the accuracy of Degradome sequencing in the early stage. This suggests that alfalfa experiences drought stress when regulated by exogenous nitric oxide, targeting HD ZIP-III, FRI, and CoA ligase genes. Additionally, the expression of Msa-miR166s in response to drought stress varies between leaves and roots, indicating spatiotemporal specificity.

Molecular mechanism of miRNA mediated biosynthesis of secondary metabolites in medicinal plants

Plant secondary metabolites are important raw materials for the pharmaceutical industry, and their biosynthetic processes are subject to diverse and precise regulation by miRNA. The identification of miRNA molecules in medicinal plants and exploration of their mechanisms not only contribute to a deeper understanding of the molecular genetic mechanisms of plant growth, development and resistance to stress, but also provide a theoretical basis for elucidating the pharmacological effects of authentic medicinal materials and constructing bioreactors for the synthesis of medicinal secondary metabolite components. This paper summarizes the research reports on the discovery of miRNA in medicinal plants and their regulatory mechanisms on the synthesis of secondary metabolites by searching the relevant literature in public databases. It summarizes the currently discovered miRNA and their functions in medicinal plants, and summarizes the molecular mechanisms regulating the synthesis and degradation of secondary metabolites. Furthermore, it provides a prospect for the research and development of medicinal plant miRNA. The compiled information contributes to a comprehensive understanding of the research progress on miRNA in medicinal plants and provides a reference for the industrial development of related secondary metabolite biosynthesis.

A comprehensive review on genomic resources in medicinally and industrially important major spices for future breeding programs: Status, utility and challenges

In the global market, spices possess a high-value but low-volume commodities of commerce. The food industry depends largely on spices for taste, flavor, and therapeutic properties in replacement of cheap synthetic ones. The estimated growth rate for spices demand in the world is ∼3.19%. Since spices grow in limited geographical regions, India is one of the leading producer of spices, contributing 25-30 percent of total world trade. Hitherto, there has been no comprehensive review of the genomic resources of industrially important major medicinal spices to overcome major impediments in varietal improvement and management. This review focuses on currently available genomic resources of 24 commercially significant spices, namely, Ajwain, Allspice, Asafoetida, Black pepper, Cardamom large, Cardamom small, Celery, Chillies, Cinnamon, Clove, Coriander, Cumin, Curry leaf, Dill seed, Fennel, Fenugreek, Garlic, Ginger, Mint, Nutmeg, Saffron, Tamarind, Turmeric and Vanilla. The advent of low-cost sequencing machines has contributed immensely to the voluminous data generation of these spices, cracking the complex genomic architecture, marker discovery, and understanding comparative and functional genomics. This review of spice genomics resources concludes the perspective and way forward to provide footprints by uncovering genome assemblies, sequencing and re-sequencing projects, transcriptome-based studies, non-coding RNA-mediated regulation, organelles-based resources, developed molecular markers, web resources, databases and AI-directed resources in candidate spices for enhanced breeding potential in them. Further, their integration with molecular breeding could be of immense use in formulating a strategy to protect and expand the production of the spices due to increased global demand.

Genome-wide identification and characterization of tissue-specific non-coding RNAs in black pepper (Piper nigrum L.)

Long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs) are the two classes of non-coding RNAs (ncRNAs) present predominantly in plant cells and have various gene regulatory functions at pre- and post-transcriptional levels. Previously deemed as "junk", these ncRNAs have now been reported to be an important player in gene expression regulation, especially in stress conditions in many plant species. Black pepper, scientifically known as Piper nigrum L., despite being one of the most economically important spice crops, lacks studies related to these ncRNAs. From a panel of 53 RNA-Seq datasets of black pepper from six tissues, namely, flower, fruit, leaf, panicle, root, and stem of six black pepper cultivars, covering eight BioProjects across four countries, we identified and characterized a total of 6406 lncRNAs. Further downstream analysis inferred that these lncRNAs regulated 781 black pepper genes/gene products via miRNA-lncRNA-mRNA network interactions, thus working as competitive endogenous RNAs (ceRNAs). The interactions may be various mechanisms like miRNA-mediated gene silencing or lncRNAs acting as endogenous target mimics (eTMs) of the miRNAs. A total of 35 lncRNAs were also identified to be potential precursors of 94 miRNAs after being acted upon by endonucleases like Drosha and Dicer. Tissue-wise transcriptome analysis revealed 4621 circRNAs. Further, miRNA-circRNA-mRNA network analysis showed 432 circRNAs combining with 619 miRNAs and competing for the binding sites on 744 mRNAs in different black pepper tissues. These findings can help researchers to get a better insight to the yield regulation and responses to stress in black pepper in endeavor of higher production and improved breeding programs in black pepper varieties.

Regulation of the regulators: Transcription factors controlling biosynthesis of plant secondary metabolites during biotic stresses and their regulation by miRNAs

Biotic stresses threaten to destabilize global food security and cause major losses to crop yield worldwide. In response to pest and pathogen attacks, plants trigger many adaptive cellular, morphological, physiological, and metabolic changes. One of the crucial stress-induced adaptive responses is the synthesis and accumulation of plant secondary metabolites (PSMs). PSMs mitigate the adverse effects of stress by maintaining the normal physiological and metabolic functioning of the plants, thereby providing stress tolerance. This differential production of PSMs is tightly orchestrated by master regulatory elements, Transcription factors (TFs) express differentially or undergo transcriptional and translational modifications during stress conditions and influence the production of PSMs. Amongst others, microRNAs, a class of small, non-coding RNA molecules that regulate gene expression post-transcriptionally, also play a vital role in controlling the expression of many such TFs. The present review summarizes the role of stress-inducible TFs in synthesizing and accumulating secondary metabolites and also highlights how miRNAs fine-tune the differential expression of various stress-responsive transcription factors during biotic stress.