Temporal responses of conserved miRNAs to drought and their associations with drought tolerance and productivity in rice

Comparative miRNome and transcriptome analyses reveal the expression of novel miRNAs in the panicle of rice implicated in sustained agronomic performance under terminal drought stress

MAIN CONCLUSION

Differential expression of 128 known and 111 novel miRNAs in the panicle of Nagina 22 under terminal drought stress targeting transcription factors, stress-associated genes, etc., enhances drought tolerance and helps sustain agronomic performance under terminal drought stress. Drought tolerance is a complex multigenic trait, wherein the genes are fine-tuned by coding and non-coding components in mitigating deleterious effects. MicroRNA (miRNA) controls gene expression at post-transcriptional level either by cleaving mRNA (transcript) or by suppressing its translation. miRNAs are known to control developmental processes and abiotic stress tolerance in plants. To identify terminal drought-responsive novel miRNA in contrasting rice cultivars, we constructed small RNA (sRNA) libraries from immature panicles of drought-tolerant rice [Nagina 22 (N 22)] and drought-sensitive (IR 64) cultivars grown under control and terminal drought stress. Our analysis of sRNA-seq data resulted in the identification of 169 known and 148 novel miRNAs in the rice cultivars. Among the novel miRNAs, 68 were up-regulated while 43 were down-regulated in the panicle of N 22 under stress. Interestingly, 31 novel miRNAs up-regulated in N 22 were down-regulated in IR 64, whereas 4 miRNAs down-regulated in N 22 were up-regulated in IR 64 under stress. To detect the effects of miRNA on mRNA expression level, transcriptome analysis was performed, while differential expression of miRNAs and their target genes was validated by RT-qPCR. Targets of the differentially expressed miRNAs include transcription factors and stress-associated genes involved in cellular/metabolic/developmental processes, response to abiotic stress, programmed cell death, photosynthesis, panicle/seed development, and grain yield. Differential expression of the miRNAs could be validated in an independent set of the samples. The findings might be useful in genetic improvement of drought-tolerant rice.

Integration of miRNA dynamics and drought tolerant QTLs in rice reveals the role of miR2919 in drought stress response

To combat drought stress in rice, a major threat to global food security, three major quantitative trait loci for 'yield under drought stress' (qDTYs) were successfully exploited in the last decade. However, their molecular basis still remains unknown. To understand the role of secondary regulation by miRNA in drought stress response and their relation, if any, with the three qDTYs, the miRNA dynamics under drought stress was studied at booting stage in two drought tolerant (Sahbaghi Dhan and Vandana) and one drought sensitive (IR 20) cultivars. In total, 53 known and 40 novel differentially expressed (DE) miRNAs were identified. The primary drought responsive miRNAs were Osa-MIR2919, Osa-MIR3979, Osa-MIR159f, Osa-MIR156k, Osa-MIR528, Osa-MIR530, Osa-MIR2091, Osa-MIR531a, Osa-MIR531b as well as three novel ones. Sixty-one target genes that corresponded to 11 known and 4 novel DE miRNAs were found to be co-localized with the three qDTYs, out of the 1746 target genes identified. We could validate miRNA-mRNA expression under drought for nine known and three novel miRNAs in eight different rice genotypes showing varying degree of tolerance. From our study, Osa-MIR2919, Osa-MIR3979, Osa-MIR528, Osa-MIR2091-5p and Chr01_11911S14Astr and their target genes LOC_Os01g72000, LOC_Os01g66890, LOC_Os01g57990, LOC_Os01g56780, LOC_Os01g72834, LOC_Os01g61880 and LOC_Os01g72780 were identified as the most promising candidates for drought tolerance at booting stage. Of these, Osa-MIR2919 with 19 target genes in the qDTYs is being reported for the first time. It acts as a negative regulator of drought stress tolerance by modulating the cytokinin and brassinosteroid signalling pathway.

Comparative Genome-Wide Analysis of MicroRNAs and Their Target Genes in Roots of Contrasting Indica Rice Cultivars under Reproductive-Stage Drought

Recurrent occurrence of drought stress in varying intensity has become a common phenomenon in the present era of global climate change, which not only causes severe yield losses but also challenges the cultivation of rice. This raises serious concerns for sustainable food production and global food security. The root of a plant is primarily responsible to perceive drought stress and acquire sufficient water for the survival/optimal growth of the plant under extreme climatic conditions. Earlier studies reported the involvement/important roles of microRNAs (miRNAs) in plants' responses to environmental/abiotic stresses. A number (738) of miRNAs is known to be expressed in different tissues under varying environmental conditions in rice, but our understanding of the role, mode of action, and target genes of the miRNAs are still elusive. Using contrasting rice [IR-64 (reproductive-stage drought sensitive) and N-22 (drought-tolerant)] cultivars, imposed with terminal (reproductive-stage) drought stress, we demonstrate differential expression of 270 known and 91 novel miRNAs in roots of the contrasting rice cultivars in response to the stress. Among the known miRNAs, osamiR812, osamiR166, osamiR156, osamiR167, and osamiR396 were the most differentially expressed miRNAs between the rice cultivars. In the root of N-22, 18 known and 12 novel miRNAs were observed to be exclusively expressed, while only two known (zero novels) miRNAs were exclusively expressed in the roots of IR-64. The majority of the target gene(s) of the miRNAs were drought-responsive transcription factors playing important roles in flower, grain development, auxin signaling, root development, and phytohormone-crosstalk. The novel miRNAs identified in this study may serve as good candidates for the genetic improvement of rice for terminal drought stress towards developing climate-smart rice for sustainable food production.

An Integration of MicroRNA and Transcriptome Sequencing Analysis Reveal Regulatory Roles of miRNAs in Response to Chilling Stress in Wild Rice

A chromosome single segment substitution line (CSSL) DC90, which was generated by introgressing CTS-12, a locus derived from common wild rice (Oryza rufipogon Griff.), into the 9311 (Oryza sativa L. ssp. indica) background, exhibits a chilling tolerance phenotype under chilling stress. Here, an integration of microRNA (miRNA) deep sequencing and transcriptomic sequencing analysis was performed to explore the expression profiles of miRNAs and their target genes mediated by CTS-12 under chilling stress, and to reveal the possible regulatory mechanisms of miRNAs that are involved in chilling tolerance. Integration analysis revealed that a number of differentially expressed miRNAs (DEMs) and putative target genes with different expression patterns and levels were identified in 9311 and DC90 under chilling stress. KEGG enrichment analysis revealed that the target genes that are regulated by chilling-induced miRNAs are involved in the regulation of various biological processes/pathways, including protein biosynthesis, redox process, photosynthetic process, and chloroplast development in two genotypes. CRISPR/Cas9 editing of the target genes of the key DEMs in a chilling tolerant rice variety Zhonghua 11 (ZH11) found that LOC_Os11g48020 (OsGL1-11), one of the putative target genes of osa-miR1846a/b-5p and encoding a wax synthesis protein, is correlated with a chilling stress tolerance phenotype, implying osa-miR1846a/b-5p/OsGL1-11 plays an important role in CTS-12-mediated chilling stress tolerance regulatory pathway(s). Therefore, we speculate that the CTS-12 may regulate the key miRNA target genes in response to chilling stress by differential regulation of miRNAs in wild rice, thereby resulting in the variation of chilling tolerance phenotype between 9311 and DC90.

The miR169n-NF-YA8 regulation module involved in drought resistance in Brassica napus L

As an ancient and conserved plant microRNA (miRNA) family, miR169 targets nuclear factor Y subunit alpha (NF-YA) family members. The miR169-NF-YA module is associated with plant development and various stress responses. However, the function of miR169 in response to drought stress in rapeseed (Brassica napus L.) is unclear. In the present study, we showed that miR169n acted as a negative regulator of drought resistance in rapeseed by targeting a nuclear factor Y-A gene, NF-YA8. miR169n was strongly down-regulated by drought stress. Expression of a miR169n target mimicry construct (MIM169n) which functioned as a sponge to trap miR169n resulted in enhanced resistance of transgenic plants to both osmotic stress at the post-germination stage and drought stress at the seedling stage. MIM169n plants had a higher relative water content (RWC) and proline content, lower relative electrolyte leakage (REL), and showed higher antioxidative capability compared with those of control (CK) plants under drought stress. Moreover, NF-YA8 was verified as a target of miR169n, and overexpression of NF-YA8 led to improved tolerance of rapeseed to osmotic stress at the post-germination stage. Overall, our findings implied that the miR169n-NF-YA8 regulatory module could serve as a potential target for genetic improvement of drought resistance in B. napus.

Mitigating tradeoffs in plant breeding

Tradeoffs among plant traits help maintain relative fitness under unpredictable conditions and maximize reproductive success. However, modifying tradeoffs is a breeding challenge since many genes of minor effect are involved. The intensive crosstalk and fine-tuning between growth and defense responsive phytohormones via transcription factors optimizes growth, reproduction, and stress tolerance. There are regulating genes in grain crops that deploy diverse functions to overcome tradeoffs, e.g., miR-156-IPA1 regulates crosstalk between growth and defense to achieve high disease resistance and yield, while OsALDH2B1 loss of function causes imbalance among defense, growth, and reproduction in rice. GNI-A1 regulates seed number and weight in wheat by suppressing distal florets and altering assimilate distribution of proximal seeds in spikelets. Knocking out ABA-induced transcription repressors (AITRs) enhances abiotic stress adaptation without fitness cost in Arabidopsis. Deploying AITRs homologs in grain crops may facilitate breeding. This knowledge suggests overcoming tradeoffs through breeding may expose new ones.

Current status of microRNA-mediated regulation of drought stress responses in cereals

Drought is one of the most important abiotic stress factors impeding crop productivity. With the uncovering of their role as potential regulators of gene expression, microRNAs (miRNAs) have been recognized as new targets for developing stress resistance. MicroRNAs are small noncoding RNAs whose abundance is significantly altered under stress conditions. Interestingly, plant miRNAs predominantly targets transcription factors (TFs), and some of which are also the most critical drought-responsive genes that in turn could regulate the expression of numerous loci with drought-adaptive potential. The phytohormone ABA plays important roles in regulating stomatal conductance and in initiating an adaptive response to drought stress. miRNAs are implicated in regulating ABA-(abscisic acid) and non-ABA-mediated drought resistance pathways. For instance, miR159-MYB module and miR169-NFYA module participates in an ABA-dependent pathway, whereas several other ABA-independent miRNA-target modules (miR156-SPL; miR393-TIR1; miR160-ARF10, ARF16, ARF17; miR167-ARF6 and ARF8; miR390/TAS3siRNA-ARF2, ARF3, ARF4) collectively regulate drought responses in plants. Overall, miRNA-mediated drought response manifests diverse molecular, biochemical and physiological processes. Because of their immense role in controlling gene expression, miRNA manipulation has significant potential to augment plant tolerance to drought stress. This review compiles the current understanding of drought-responsive miRNAs in major cereals. Also, potential miRNA manipulation strategies currently in use along with the challenges and future perspectives are discussed.

Emerging Molecular Strategies for Improving Rice Drought Tolerance

Rice occupies a pre-eminent position as a food crop in the world. Its production, how- ever, entails up to 3000 liters of water per kilogram of grain produced. Such high demand makes rice prone to drought easily. Sustainable rice cultivation with limited water resources requires the deployment of a suitable strategy for better water use efficiency and improved drought tolerance. Several drought-related genes have been evaluated in rice for their mode of action in conferring drought tolerance. Manipulation of components of abscisic acid signal transduction, stomatal density, deposition of cuticular wax, and protein modification pathways are emerging as priority targets. Gene reprogramming by microRNAs is also being explored to achieve drought tolerance. Genetically dissected Quantitative Trait Loci (QTLs) and their constituent genes are being deployed to develop drought-tolerant rice varieties. Progressive research and challenges include a better understanding of crucial components of drought response and search for new targets and the deployment of improved varieties in the field.

Temporal transcriptomic differences between tolerant and susceptible genotypes contribute to rice drought tolerance

BACKGROUND

Drought-tolerance ensures a crop to maintain life activities and protect cell from damages under dehydration. It refers to diverse mechanisms temporally activated when the crop adapts to drought. However, knowledge about the temporal dynamics of rice transcriptome under drought is limited.

RESULTS

Here, we investigated temporal transcriptomic dynamics in 12 rice genotypes, which varied in drought tolerance (DT), under a naturally occurred drought in fields. The tolerant genotypes possess less differentially expressed genes (DEGs) while they have higher proportions of upregulated DEGs. Tolerant and susceptible genotypes have great differences in temporally activated biological processes (BPs) during the drought period and at the recovery stage based on their DEGs. The DT-featured BPs, which are activated specially (e.g. raffinose, fucose, and trehalose metabolic processes, etc.) or earlier in the tolerant genotypes (e.g. protein and histone deacetylation, protein peptidyl-prolyl isomerization, transcriptional attenuation, ferric iron transport, etc.) shall contribute to DT. Meanwhile, the tolerant genotypes and the susceptible genotypes also present great differences in photosynthesis and cross-talks among phytohormones under drought. A certain transcriptomic tradeoff between DT and productivity is observed. Tolerant genotypes have a better balance between DT and productivity under drought by activating drought-responsive genes appropriately. Twenty hub genes in the gene coexpression network, which are correlated with DT but without potential penalties in productivity, are recommended as good candidates for DT.

CONCLUSIONS

Findings of this study provide us informative cues about rice temporal transcriptomic dynamics under drought and strengthen our system-level understandings in rice DT.