Elevated carbon dioxide and drought modulate physiology and storage-root development in sweet potato by regulating microRNAs

Unravelling structural, functional, evolutionary and genetic basis of SWEET transporters regulating abiotic stress tolerance in maize

Sugars Will Eventually be Exported Transporters (SWEETs) are the novel sugar transporters widely distributed among living systems. SWEETs play a crucial role in various bio-physiological processes, viz., plant developmental, nectar secretion, pollen development, and regulation of biotic and abiotic stresses, in addition to their prime sugar-transporting activity. Thus, in-depth structural, evolutionary, and functional characterization of maize SWEET transporters was performed for their utility in maize improvement. The mining of SWEET genes in the latest maize genome release (v.5) showed an uneven distribution of 20 ZmSWEETs. The comprehensive structural analyses and docking of ZmSWEETs with four sugars, viz., fructose, galactose, glucose, and sucrose, revealed frequent amino acid residues forming hydrogen (asparagine, valine, serine) and hydrophobic (tryptophan, glycine, and phenylalanine) interactions. Evolutionary analyses of SWEETs showed a mixed lineage with 50-100 % commonality of ortho-groups and -sequences evolved under strong purifying selection (Ka/Ks < 0.5). The duplication analysis showed non-functionalization (ZmSWEET18 in B73) and neo- and sub-functionalization (ZmSWEET3, ZmSWEET6, ZmSWEET9, ZmSWEET19, and ZmSWEET20) events in maize. Functional analyses of ZmSWEET genes through co-expression, in silico expression and qRT-PCR assays showed the relevance of ZmSWEETs expression in regulating drought, heat, and waterlogging stress tolerances in maize. The first ever ZmSWEET-regulatory network revealed 286 direct (ZmSWEET-TF: 140 ZmSWEET-miRNA: 146) and 1226 indirect (TF-TF: 597; TF-miRNA: 629) edges. The present investigation has given new insights into the complex transcriptional and post-transcriptional regulation and the regulatory and functional relevance of ZmSWEETs in assigning stress tolerance in maize.

Plant responses to drought stress: microRNAs in action

Drought is common in most regions of the world, and it has a significant impact on plant growth and development. Plants, on the other hand, have evolved their own defense systems to deal with the extreme weather. The reprogramming of gene expression by microRNAs (miRNAs) is one of these defense mechanisms. miRNAs are short noncoding RNAs that have emerged as key post-transcriptional gene regulators in a variety of species. Drought stress modulates the expression of certain miRNAs that are functionally conserved across plant species. These characteristics imply that miRNA-based genetic changes might improve drought resistance in plants. This study highlights current knowledge of plant miRNA biogenesis, regulatory mechanisms and their role in drought stress responses. miRNAs functions and their adaptations by plants during drought stress has also been explained that can be exploited to promote drought-resistance among economically important crops.

What Do We Know about Barley miRNAs?

Plant miRNAs are powerful regulators of gene expression at the post-transcriptional level, which was repeatedly proved in several model plant species. miRNAs are considered to be key regulators of many developmental, homeostatic, and immune processes in plants. However, our understanding of plant miRNAs is still limited, despite the fact that an increasing number of studies have appeared. This systematic review aims to summarize our current knowledge about miRNAs in spring barley (Hordeum vulgare), which is an important agronomical crop worldwide and serves as a common monocot model for studying abiotic stress responses as well. This can help us to understand the connection between plant miRNAs and (not only) abiotic stresses in general. In the end, some future perspectives and open questions are summarized.

Effects of Combined Abiotic Stresses Related to Climate Change on Root Growth in Crops

Climate change is a major threat to crop productivity that negatively affects food security worldwide. Increase in global temperatures are usually accompanied by drought, flooding and changes in soil nutrients composition that dramatically reduced crop yields. Against the backdrop of climate change, human population increase and subsequent rise in food demand, finding new solutions for crop adaptation to environmental stresses is essential. The effects of single abiotic stress on crops have been widely studied, but in the field abiotic stresses tend to occur in combination rather than individually. Physiological, metabolic and molecular responses of crops to combined abiotic stresses seem to be significantly different to individual stresses. Although in recent years an increasing number of studies have addressed the effects of abiotic stress combinations, the information related to the root system response is still scarce. Roots are the underground organs that directly contact with the soil and sense many of these abiotic stresses. Understanding the effects of abiotic stress combinations in the root system would help to find new breeding tools to develop more resilient crops. This review will summarize the current knowledge regarding the effects of combined abiotic stress in the root system in crops. First, we will provide a general overview of root responses to particular abiotic stresses. Then, we will describe how these root responses are integrated when crops are challenged to the combination of different abiotic stress. We will focus on the main changes on root system architecture (RSA) and physiology influencing crop productivity and yield and convey the latest information on the key molecular, hormonal and genetic regulatory pathways underlying root responses to these combinatorial stresses. Finally, we will discuss possible directions for future research and the main challenges needed to be tackled to translate this knowledge into useful tools to enhance crop tolerance.

Molecular Characterization and Target Prediction of Candidate miRNAs Related to Abiotic Stress Responses and/or Storage Root Development in Sweet Potato

Sweet potato is a tuberous root crop with strong environmental stress resistance. It is beneficial to study its storage root formation and stress responses to identify sweet potato stress- and storage-root-thickening-related regulators. Here, six conserved miRNAs (miR156g, miR157d, miR158a-3p, miR161.1, miR167d and miR397a) and six novel miRNAs (novel 104, novel 120, novel 140, novel 214, novel 359 and novel 522) were isolated and characterized in sweet potato. Tissue-specific expression patterns suggested that miR156g, miR157d, miR158a-3p, miR167d, novel 359 and novel 522 exhibited high expression in fibrous roots or storage roots and were all upregulated in response to storage-root-related hormones (indole acetic acid, IAA; zeaxanthin, ZT; abscisic acid, ABA; and gibberellin, GAs). The expression of miR156g, miR158a-3p, miR167d, novel 120 and novel 214 was induced or reduced dramatically by salt, dehydration and cold or heat stresses. Moreover, these miRNAs were all upregulated by ABA, a crucial hormone modulator in regulating abiotic stresses. Additionally, the potential targets of the twelve miRNAs were predicted and analyzed. Above all, these results indicated that these miRNAs might play roles in storage root development and/or stress responses in sweet potato as well as provided valuable information for the further investigation of the roles of miRNA in storage root development and stress responses.

MicroRNA-mediated bioengineering for climate-resilience in crops

Global projections on the climate change and the dynamic environmental perturbations indicate severe impacts on food security in general, and crop yield, vigor and the quality of produce in particular. Sessile plants respond to environmental challenges such as salt, drought, temperature, heavy metals at transcriptional and/or post-transcriptional levels through the stress-regulated network of pathways including transcription factors, proteins and the small non-coding endogenous RNAs. Amongs these, the miRNAs have gained unprecedented attention in recent years as key regulators for modulating gene expression in plants under stress. Hence, tailoring of miRNAs and their target pathways presents a promising strategy for developing multiple stress-tolerant crops. Plant stress tolerance has been successfully achieved through the over expression of microRNAs such as Os-miR408, Hv-miR82 for drought tolerance; OsmiR535A and artificial DST miRNA for salinity tolerance; and OsmiR535 and miR156 for combined drought and salt stress. Examples of miR408 overexpression also showed improved efficiency of irradiation utilization and carbon dioxide fixation in crop plants. Through this review, we present the current understanding about plant miRNAs, their roles in plant growth and stress-responses, the modern toolbox for identification, characterization and validation of miRNAs and their target genes including in silico tools, machine learning and artificial intelligence. Various approaches for up-regulation or knock-out of miRNAs have been discussed. The main emphasis has been given to the exploration of miRNAs for development of bioengineered climate-smart crops that can withstand changing climates and stressful environments, including combination of stresses, with very less or no yield penalties.

Identification of miRNAs and Their Targets Involved in Flower and Fruit Development across Domesticated and Wild Capsicum Species

MicroRNAs (miRNAs) are regulators of the post-transcription stage of gene activity documented to play central roles in flower and fruit development in model plant species. However, little is known about their roles and differences in domesticated and wild Capsicum species. In this study, we used high-throughput sequencing to analyze the miRNA content at three developmental stages (flower, small fruit, and middle fruit) from two cultivated (C. baccatum and C. annuum) and two wild (C. chacoense and C. eximium) pepper species. This analysis revealed 22 known and 27 novel miRNAs differentially expressed across species and tissues. A number of stage- and species-specific miRNAs were identified, and Gene Ontology terms were assigned to 138 genes targeted by the miRNAs. Most Gene Ontology terms were for the categories "genetic information processing", "signaling and cellular processes", "amino acid metabolism", and "carbohydrate metabolism". Enriched KEGG analysis revealed the pathways amino acids, sugar and nucleotide metabolism, starch and sucrose metabolism, and fructose-mannose metabolism among the principal ones regulated by miRNAs during pepper fruit ripening. We predicted miRNA-target gene interactions regulating flowering time and fruit development, including miR156/157 with SPL genes, miR159 with GaMYB proteins, miR160 with ARF genes, miR172 with AP2-like transcription factors, and miR408 with CLAVATA1 gene across the different Capsicum species. In addition, novel miRNAs play an important role in regulating interactions potentially controlling plant pathogen defense and fruit quality via fructokinase, alpha-L-arabinofuranosidase, and aromatic and neutral amino acid transporter. Overall, the small RNA-sequencing results from this study represent valuable information that provides a solid foundation for uncovering the miRNA-mediated mechanisms of flower and fruit development between domesticated and wild Capsicum species.

MicroRNAs: Potential Targets for Developing Stress-Tolerant Crops

Crop yield is challenged every year worldwide by changing climatic conditions. The forecasted climatic scenario urgently demands stress-tolerant crop varieties to feed the ever-increasing global population. Molecular breeding and genetic engineering approaches have been frequently exploited for developing crops with desired agronomic traits. Recently, microRNAs (miRNAs) have emerged as powerful molecules, which potentially serve as expression markers during stress conditions. The miRNAs are small non-coding endogenous RNAs, usually 20-24 nucleotides long, which mediate post-transcriptional gene silencing and fine-tune the regulation of many abiotic- and biotic-stress responsive genes in plants. The miRNAs usually function by specifically pairing with the target mRNAs, inducing their cleavage or repressing their translation. This review focuses on the exploration of the functional role of miRNAs in regulating plant responses to abiotic and biotic stresses. Moreover, a methodology is also discussed to mine stress-responsive miRNAs from the enormous amount of transcriptome data available in the public domain generated using next-generation sequencing (NGS). Considering the functional role of miRNAs in mediating stress responses, these molecules may be explored as novel targets for engineering stress-tolerant crop varieties.

Phylogenetic Analysis of the SQUAMOSA Promoter-Binding Protein-Like Genes in Four Ipomoea Species and Expression Profiling of the IbSPLs During Storage Root Development in Sweet Potato (Ipomoea batatas)

As a major plant-specific transcription factor family, SPL genes play a crucial role in plant growth, development, and stress tolerance. The SPL transcription factor family has been widely studied in various plant species; however, systematic studies on SPL genes in the genus Ipomoea are lacking. Here, we identified a total of 29, 27, 26, and 23 SPLs in Ipomoea batatas, Ipomoea trifida, Ipomoea triloba, and Ipomoea nil, respectively. Based on the phylogenetic analysis of SPL proteins from model plants, the Ipomoea SPLs were classified into eight clades, which included conserved gene structures, domain organizations and motif compositions. Moreover, segmental duplication, which is derived from the Ipomoea lineage-specific whole-genome triplication event, was speculated to have a predominant role in Ipomoea SPL expansion. Particularly, tandem duplication was primarily responsible for the expansion of SPL subclades IV-b and IV-c. Furthermore, 25 interspecific orthologous groups were identified in Ipomoea, rice, Arabidopsis, and tomato. These findings support the expansion of SPLs in Ipomoea genus, with most of the SPLs being evolutionarily conserved. Of the 105 Ipomoea SPLs, 69 were predicted to be the targets of miR156, with seven IbSPLs being further verified as targets using degradome-seq data. Using transcriptomic data from aboveground and underground sweet potato tissues, IbSPLs showed diverse expression patterns, including seven highly expressed IbSPLs in the underground tissues. Furthermore, the expression of 11 IbSPLs was validated using qRT-PCR, and two (IbSPL17/IbSPL28) showed significantly increased expression during root development. Additionally, the qRT-PCR analysis revealed that six IbSPLs were strongly induced in the roots under phytohormone treatments, particularly zeatin and abscisic acid. Finally, the transcriptomic data of storage roots from 88 sweet potato accessions were used for weighted gene co-expression network analysis, which revealed four IbSPLs (IbSPL16/IbSPL17/IbSPL21/IbSPL28) clusters with genes involved in "regulation of root morphogenesis," "cell division," "cytoskeleton organization," and "plant-type cell wall organization or biogenesis," indicating their potential role in storage root development. This study not only provides novel insights into the evolutionary and functional divergence of the SPLs in the genus Ipomoea but also lays a foundation for further elucidation of the potential functional roles of IbSPLs on storage root development.

Optimization of reference genes for qRT-PCR analysis of microRNA expression under abiotic stress conditions in sweetpotato

Sweetpotato (Ipomoea batatas. L) is an important food crop, harvested for its nutrient-rich tuberous roots. Drought and salt stresses are two major factors limiting the sweetpotato production. Since microRNAs (miRNAs) are well known to play crucial roles in regulation of plant stress responses, quantitative profiling of miRNA expression under stress conditions will facilitate identification and genetic manipulation of novel miRNAs to improve stress tolerance. Real-time quantitative reverse transcription PCR (qRT-PCR) is a commonly used tool for this purpose, but not without challenges. Although stem-loop and poly(A)-tail modified qRT-PCR methods were developed for characterizing miRNA expression, accurate profiling of miRNAs is still difficult in many plant species because of a lack of reliable reference genes for normalizing miRNA transcripts. To identify reference genes that are suitable for normalizing miRNA expression in sweetpotato, the expression stability of eight candidate miRNAs and two commonly used reference genes were tested in 96 samples involving four tissues and two cultivars under drought and salt stress treatments. Data analysis using the geNorm, NormFinder and Bestkeeper algorithms demonstrated that miRn60, miR482, and their combination were reliable references. We further validated the reference genes by expression analysis of the well-characterized miR319 and miR156 that regulate drought and salt stress responses, respectively. The reference genes identified in this study will facilitate future miRNA analysis under abiotic stress conditions in sweetpotato.

Molecular and Physiological Alterations in Chickpea under Elevated CO2 Concentrations

The present study reports profiling of the elevated carbon dioxide (CO2) concentration responsive global transcriptome in chickpea, along with a combinatorial approach for exploring interlinks between physiological and transcriptional changes, important for the climate change scenario. Various physiological parameters were recorded in two chickpea cultivars (JG 11 and KAK 2) grown in open top chambers under ambient [380 parts per million (ppm)] and two stressed/elevated CO2 concentrations (550 and 700 ppm), at different stages of plant growth. The elevated CO2 concentrations altered shoot and root length, nodulation (number of nodules), total chlorophyll content and nitrogen balance index, significantly. RNA-Seq from 12 tissues representing vegetative and reproductive growth stages of both cultivars under ambient and elevated CO2 concentrations identified 18,644 differentially expressed genes including 9,687 transcription factors (TF). The differential regulations in genes, gene networks and quantitative real-time polymerase chain reaction (qRT-PCR) -derived expression dynamics of stress-responsive TFs were observed in both cultivars studied. A total of 138 pathways, mainly involved in sugar/starch metabolism, chlorophyll and secondary metabolites biosynthesis, deciphered the crosstalk operating behind the responses of chickpea to elevated CO2 concentration.

Identification of candidate miRNAs related in storage root development of sweet potato by high throughput sequencing

Sweet potato (Ipomoea batatas L.) is a food consumed worldwide, an industrial raw material and new energy crop. The storage root is the most economical part of the crop. However, the mechanism of storage root initiation and development is still unclear. In this study, conserved and novel miRNAs during storage root development were identified by high-throughput sequencing technology by constructing small RNA libraries from sweet potato fibrous roots (F) and storage roots at four different developmental stages (storage roots with different diameters: 1 cm, D1; 3 cm, D3; 5 cm, D5 and 10 cm, D10). A total of 61 known miRNAs and 471 novel miRNAs were identified. In addition, 145 differentially expressed miRNAs were identified in the F library compared with the four storage root libraries, with 30 known miRNAs and 115 novel miRNAs. Moreover, the targets of the differentially expressed miRNAs were predicted and their network was further investigated by GO analysis using our previous transcriptome data. The GO analysis revealed that antioxidant activity and binding process were the most enriched terms of the target genes. The secondary structure and expression of six candidate miRNAs including three conserved miRNAs and three novel miRNAs were investigated and their predicted targets were validated by qRT-PCR. The results showed that the expression levels of the miRNAs were all consistent with the sequencing data. Most of the miRNAs and their corresponding targets had obvious negative correlations. This study contributed to elucidating the potential miRNA mediated regulatory mechanism of storage root development in sweet potato. The specific differentially expressed miRNAs in sweet potato storage roots can be used to breed high-yield sweet potatoes and other tuberous root crops.