Plant nonsense-mediated mRNA decay is controlled by different autoregulatory circuits and can be induced by an EJC-like complex

Regulation of natural mRNAs by the nonsense-mediated mRNA decay pathway

The nonsense-mediated mRNA decay (NMD) pathway is a specialized mRNA degradation pathway that degrades select mRNAs. This pathway is conserved in all eukaryotes examined so far, and it triggers the degradation of mRNAs that prematurely terminate translation. Originally identified as a pathway that degrades mRNAs with premature termination codons as a result of errors during transcription, splicing, or damage to the mRNA, NMD is now also recognized as a pathway that degrades some natural mRNAs. The degradation of natural mRNAs by NMD has been identified in multiple eukaryotes, including Saccharomyces cerevisiae, Drosophila melanogaster, Arabidopsis thaliana, and humans. S. cerevisiae is used extensively as a model to study natural mRNA regulation by NMD. Inactivation of the NMD pathway in S. cerevisiae affects approximately 10% of the transcriptome. Similar percentages of natural mRNAs in the D. melanogaster and human transcriptomes are also sensitive to the pathway, indicating that NMD is important for the regulation of gene expression in multiple organisms. NMD can either directly or indirectly regulate the decay rate of natural mRNAs. Direct NMD targets possess NMD-inducing features. This minireview focuses on the regulation of natural mRNAs by the NMD pathway, as well as the features demonstrated to target these mRNAs for decay by the pathway in S. cerevisiae. We also compare NMD-targeting features identified in S. cerevisiae with known NMD-targeting features in other eukaryotic organisms.

Uncovering Divergence of Rice Exon Junction Complex Core Heterodimer Gene Duplication Reveals Their Essential Role in Growth, Development, and Reproduction

The exon junction complex (EJC) plays important developmental roles in animals; however, its role in plants is not well known. Here, we show various aspects of the divergence of each duplicated MAGO NASHI (MAGO) and Y14 gene pair in rice (Oryza sativa) encoding the putative EJC core subunits that form the obligate MAGO-Y14 heterodimers. OsMAGO1, OsMAGO2, and OsY14a were constitutively expressed in all tissues, while OsY14b was predominantly expressed in embryonic tissues. OsMAGO2 and OsY14b were more sensitive to different stresses than OsMAGO1 and OsY14a, and their encoded protein pair shared 93.8% and 46.9% sequence identity, respectively. Single MAGO down-regulation in rice did not lead to any phenotypic variation; however, double gene knockdowns generated short rice plants with abnormal flowers, and the stamens of these flowers showed inhibited degradation and absorption of both endothecium and tapetum, suggesting that OsMAGO1 and OsMAGO2 were functionally redundant. OsY14a knockdowns phenocopied OsMAGO1OsMAGO2 mutants, while down-regulation of OsY14b failed to induce plantlets, suggesting the functional specialization of OsY14b in embryogenesis. OsMAGO1OsMAGO2OsY14a triple down-regulation enhanced the phenotypes of OsMAGO1OsMAGO2 and OsY14a down-regulated mutants, indicating that they exert developmental roles in the MAGO-Y14 heterodimerization mode. Modified gene expression was noted in the altered developmental pathways in these knockdowns, and the transcript splicing of UNDEVELOPED TAPETUM1 (OsUDT1), a key regulator in stamen development, was uniquely abnormal. Concomitantly, MAGO and Y14 selectively bound to the OsUDT1 premessenger RNA, suggesting that rice EJC subunits regulate splicing. Our work provides novel insights into the function of the EJC locus in growth, development, and reproduction in angiosperms and suggests a role for these genes in the adaptive evolution of cereals.

SMG1 is an ancient nonsense-mediated mRNA decay effector

Nonsense-mediated mRNA decay (NMD) is a eukaryotic process that targets selected mRNAs for destruction, for both quality control and gene regulatory purposes. SMG1, the core kinase of the NMD machinery in animals, phosphorylates the highly conserved UPF1 effector protein to activate NMD. However, SMG1 is missing from the genomes of fungi and the model flowering plant Arabidopsis thaliana, leading to the conclusion that SMG1 is animal-specific and questioning the mechanistic conservation of the pathway. Here we show that SMG1 is not animal-specific, by identifying SMG1 in a range of eukaryotes, including all examined green plants with the exception of A. thaliana. Knockout of SMG1 by homologous recombination in the basal land plant Physcomitrella patens reveals that SMG1 has a conserved role in the NMD pathway across kingdoms. SMG1 has been lost at various points during the evolution of eukaryotes from multiple lineages, including an early loss in the fungal lineage and a very recent observable gene loss in A. thaliana. These findings suggest that the SMG1 kinase functioned in the NMD pathway of the last common eukaryotic ancestor.

Phosphorylation of the N- and C-terminal UPF1 domains plays a critical role in plant nonsense-mediated mRNA decay

Nonsense-mediated mRNA decay (NMD) is an essential quality control system that degrades aberrant transcripts containing premature termination codons and regulates the expression of several normal transcripts. Targets for NMD are selected during translational termination. If termination is slow, the UPF1 NMD factor binds the eRF3 protein of the termination complex and then recruits UPF2 and UPF3. Consequently, the UPF1-2-3 NMD complex induces SMG7-mediated degradation of the target mRNA. It is unknown how formation of the NMD complex and transcript degradation are linked in plants. Previously we have shown that the N- and C-terminal domains of UPF1 act redundantly and that the N-terminal domain is phosphorylated. To clarify the role of UPF1 phosphorylation in plant NMD, we generated UPF1 mutants and analyzed their phosphorylation status and the NMD competency of the mutants. We show that although several residues in the N-terminal domain of UPF1 are phosphorylated, only three phosphorylated amino acids, S3, S13 and T29, play a role in NMD. Moreover, we found that the C-terminal domain consists of redundant S/TQ-rich segments and that S1076 is involved in NMD. All NMD-relevant phosphorylation sites were in the S/TQ context. Co-localization and fluorescence resonance energy transfer-fluorescence lifetime imaging assays suggest that N-terminal and probably also C-terminal phosphorylated S/TQ residues are the binding platform for SMG7. Our data support the hypothesis that phosphorylation of UPF1 connects NMD complex formation and the SMG7-mediated target transcript degradation steps of NMD. SMG7 binds the phosphorylated S/TQ sites of the UPF1 component of the NMD complex, and then it induces the degradation of the NMD target.

Nonsense-mediated decay of alternative precursor mRNA splicing variants is a major determinant of the Arabidopsis steady state transcriptome

The nonsense-mediated decay (NMD) surveillance pathway can recognize erroneous transcripts and physiological mRNAs, such as precursor mRNA alternative splicing (AS) variants. Currently, information on the global extent of coupled AS and NMD remains scarce and even absent for any plant species. To address this, we conducted transcriptome-wide splicing studies using Arabidopsis thaliana mutants in the NMD factor homologs UP FRAMESHIFT1 (UPF1) and UPF3 as well as wild-type samples treated with the translation inhibitor cycloheximide. Our analyses revealed that at least 17.4% of all multi-exon, protein-coding genes produce splicing variants that are targeted by NMD. Moreover, we provide evidence that UPF1 and UPF3 act in a translation-independent mRNA decay pathway. Importantly, 92.3% of the NMD-responsive mRNAs exhibit classical NMD-eliciting features, supporting their authenticity as direct targets. Genes generating NMD-sensitive AS variants function in diverse biological processes, including signaling and protein modification, for which NaCl stress-modulated AS-NMD was found. Besides mRNAs, numerous noncoding RNAs and transcripts derived from intergenic regions were shown to be NMD responsive. In summary, we provide evidence for a major function of AS-coupled NMD in shaping the Arabidopsis transcriptome, having fundamental implications in gene regulation and quality control of transcript processing.

A global profiling of uncapped mRNAs under cold stress reveals specific decay patterns and endonucleolytic cleavages in Brachypodium distachyon

Background

mRNA degradation is a critical factor in determining mRNA abundance and enables rapid adjustment of gene expression in response to environmental stress. The involvement of processing bodies in stress response suggests a role for decapping-mediated mRNA degradation. However, little is known about the role of mRNA degradation under stressful environmental conditions.

Results

Here, we perform a global study of uncapped mRNAs, via parallel analysis of RNA ends (PARE), under cold stress in Brachypodium distachyon. Enrichment analysis indicates that degradation products detected by PARE are mainly generated by the decapping pathway. Endonucleolytic cleavages are detected, uncovering another way of modulating gene expression. PARE and RNA-Seq analyses identify four types of mRNA decay patterns. Type II genes, for which light-harvesting processes are over-represented in gene ontology analyses, show unchanged transcript abundance and altered uncapped transcript abundance. Uncapping-mediated transcript stability of light harvesting-related genes changes significantly in response to cold stress, which may allow rapid adjustments in photosynthetic activity in response to cold stress. Transcript abundance and uncapped transcript abundance for type III genes changes in opposite directions in response to cold stress, indicating that uncapping-mediated mRNA degradation plays a role in regulating gene expression.

Conclusion

To our knowledge, this is the first global analysis of mRNA degradation under environmental stress conditions in Brachypodium distachyon. We uncover specific degradation and endonucleolytic cleavage patterns under cold stress, which will deepen our understanding of mRNA degradation under stressful environmental conditions, as well as the cold stress response mechanism in monocots.