PRP4KA, a Putative Spliceosomal Protein Kinase, Is Important for Alternative Splicing and Development in Arabidopsis thaliana

Regulation of plant resistance to salt stress by the SnRK1-dependent splicing factor SRRM1L

Most splicing factors are extensively phosphorylated but their physiological functions in plant salt resistance are still elusive. We found that phosphorylation by SnRK1 kinase is essential for SRRM1L nuclear speckle formation and its splicing factor activity in plant cells. In Arabidopsis, loss-of-function of SRRM1L leads to the occurrence of alternative pre-mRNA splicing events and compromises plant resistance to salt stress. In Arabidopsis srrm1l mutant line, we identified an intron-retention Nuclear factor Y subunit A 10 (NFYA10) mRNA variant by RNA-Seq and found phosphorylation-dependent RNA-binding of SRRM1L is indispensable for its alternative splicing activity. In the wild-type Arabidopsis, salt stress can activate SnRK1 to phosphorylate SRRM1L, triggering enrichment of functional NFYA10.1 variant to enhance plant salt resistance. By contrast, the Arabidopsis srrm1l mutant accumulates nonfunctional NFYA10.3 variant, sensitizing plants to salt stress. In summary, this work deciphered the molecular mechanisms and physiological functions of SnRK1-SRRM1L-NFYA10 module, shedding light on a regulatory pathway to fine-tune plant adaptation to abiotic stress at the post-transcriptional and post-translational levels.

An RRM domain protein SOE suppresses transgene silencing in rice

Gene expression is regulated at multiple levels, including RNA processing and DNA methylation/demethylation. How these regulations are controlled remains unclear. Here, through analysis of a suppressor for the OsEIN2 over-expressor, we identified an RNA recognition motif protein SUPPRESSOR OF EIN2 (SOE). SOE is localized in nuclear speckles and interacts with several components of the spliceosome. We find SOE associates with hundreds of targets and directly binds to a DNA glycosylase gene DNG701 pre-mRNA for efficient splicing and stabilization, allowing for subsequent DNG701-mediated DNA demethylation of the transgene promoter for proper gene expression. The V81M substitution in the suppressor mutant protein mSOE impaired its protein stability and binding activity to DNG701 pre-mRNA, leading to transgene silencing. SOE mutation enhances grain size and yield. Haplotype analysis in c. 3000 rice accessions reveals that the haplotype 1 (Hap 1) promoter is associated with high 1000-grain weight, and most of the japonica accessions, but not indica ones, have the Hap 1 elite allele. Our study discovers a novel mechanism for the regulation of gene expression and provides an elite allele for the promotion of yield potentials in rice.

Phosphorylation mediated regulation of RNA splicing in plants

For the past two decades, the study of alternative splicing (AS) and its involvement in plant development and stress response has grown in popularity. Only recently however, has the focus shifted to the study of how AS regulation (or lack-thereof) affects downstream mRNA and protein landscapes and how these AS regulatory events impact plant development and stress tolerance. In humans, protein phosphorylation represents one of the predominant mechanisms by which AS is regulated and thus the protein kinases governing these phosphorylation events are of interest for further study. Large-scale phosphoproteomic studies in plants have consistently found that RNA splicing-related proteins are extensively phosphorylated, however, the signaling pathways involved in AS regulation have not been resolved. In this mini-review, we summarize our current knowledge of the three major splicing-related protein kinase families in plants that are suggested to mediate AS phospho-regulation and draw comparisons to their metazoan orthologs. We also summarize and contextualize the phosphorylation events identified as occurring on splicing-related protein families to illustrate the high degree to which splicing-related proteins are modified, placing a new focus on elucidating the impacts of AS at the protein and PTM-level.

QTG-Miner aids rapid dissection of the genetic base of tassel branch number in maize

Genetic dissection of agronomic traits is important for crop improvement and global food security. Phenotypic variation of tassel branch number (TBN), a major breeding target, is controlled by many quantitative trait loci (QTLs). The lack of large-scale QTL cloning methodology constrains the systematic dissection of TBN, which hinders modern maize breeding. Here, we devise QTG-Miner, a multi-omics data-based technique for large-scale and rapid cloning of quantitative trait genes (QTGs) in maize. Using QTG-Miner, we clone and verify seven genes underlying seven TBN QTLs. Compared to conventional methods, QTG-Miner performs well for both major- and minor-effect TBN QTLs. Selection analysis indicates that a substantial number of genes and network modules have been subjected to selection during maize improvement. Selection signatures are significantly enriched in multiple biological pathways between female heterotic groups and male heterotic groups. In summary, QTG-Miner provides a large-scale approach for rapid cloning of QTGs in crops and dissects the genetic base of TBN for further maize breeding.

Genome-scale analysis of Arabidopsis splicing-related protein kinase families reveals roles in abiotic stress adaptation

Nearly 60 - 80 % of intron-containing plant genes undergo alternative splicing in response to either stress or plant developmental cues. RNA splicing is performed by a large ribonucleoprotein complex called the spliceosome in conjunction with associated subunits such as serine arginine (SR) proteins, all of which undergo extensive phosphorylation. In plants, there are three main protein kinase families suggested to phosphorylate core spliceosome subunits and related splicing factors based on orthology to human splicing-related kinases: the SERINE/ARGININE PROTEIN KINASES (SRPK), ARABIDOPSIS FUS3 COMPLEMENT (AFC), and Pre-mRNA PROCESSING FACTOR 4 (PRP4K) protein kinases. To better define the conservation and role(s) of these kinases in plants, we performed a genome-scale analysis of the three families across photosynthetic eukaryotes, followed by extensive transcriptomic and bioinformatic analysis of all Arabidopsis thaliana SRPK, AFC, and PRP4K protein kinases to elucidate their biological functions. Unexpectedly, this revealed the existence of SRPK and AFC phylogenetic groups with distinct promoter elements and patterns of transcriptional response to abiotic stress, while PRP4Ks possess no phylogenetic sub-divisions, suggestive of functional redundancy. We also reveal splicing-related kinase families are both diel and photoperiod regulated, implicating different orthologs as discrete time-of-day RNA splicing regulators. This foundational work establishes a number of new hypotheses regarding how reversible spliceosome phosphorylation contributes to both diel plant cell regulation and abiotic stress adaptation in plants.

MDF is a conserved splicing factor and modulates cell division and stress response in Arabidopsis

The coordination of cell division with stress response is essential for maintaining genome stability in plant meristems. Proteins involved in pre-mRNA splicing are important for these processes in animal and human cells. Based on its homology to the splicing factor SART1, which is implicated in the control of cell division and genome stability in human cells, we analyzed if MDF has similar functions in plants. We found that MDF associates with U4/U6.U5 tri-snRNP proteins and is essential for correct splicing of 2,037 transcripts. Loss of MDF function leads to cell division defects and cell death in meristems and was associated with up-regulation of stress-induced genes and down-regulation of mitotic regulators. In addition, the mdf-1 mutant is hypersensitive to DNA damage treatment supporting its role in coordinating stress response with cell division. Our analysis of a dephosphomutant of MDF suggested how its protein activity might be controlled. Our work uncovers the conserved function of a plant splicing factor and provides novel insight into the interplay of pre-mRNA processing and genome stability in plants.

Dynamics of Protein Phosphorylation during Arabidopsis Seed Germination

Seed germination is critical for early plantlet development and is tightly controlled by environmental factors. Nevertheless, the signaling networks underlying germination control remain elusive. In this study, the remodeling of Arabidopsis seed phosphoproteome during imbibition was investigated using stable isotope dimethyl labeling and nanoLC-MS/MS analysis. Freshly harvested seeds were imbibed under dark or constant light to restrict or promote germination, respectively. For each light regime, phosphoproteins were extracted and identified from dry and imbibed (6 h, 16 h, and 24 h) seeds. A large repertoire of 10,244 phosphopeptides from 2546 phosphoproteins, including 110 protein kinases and key regulators of seed germination such as Delay Of Germination 1 (DOG1), was established. Most phosphoproteins were only identified in dry seeds. Early imbibition led to a similar massive downregulation in dormant and non-dormant seeds. After 24 h, 411 phosphoproteins were specifically identified in non-dormant seeds. Gene ontology analyses revealed their involvement in RNA and protein metabolism, transport, and signaling. In addition, 489 phosphopeptides were quantified, and 234 exhibited up or downregulation during imbibition. Interaction networks and motif analyses revealed their association with potential signaling modules involved in germination control. Our study provides evidence of a major role of phosphosignaling in the regulation of Arabidopsis seed germination.

Integrative Proteome and Phosphoproteome Profiling of Early Cold Response in Maize Seedlings

Cold limits the growth and yield of maize in temperate regions, but the molecular mechanism of cold adaptation remains largely unexplored in maize. To identify early molecular events during cold shock, maize seedlings were treated under 4 °C for 30 min and 2 h, and analyzed at both the proteome and phosphoproteome levels. Over 8500 proteins and 19,300 phosphopeptides were quantified. About 660 and 620 proteins were cold responsive at protein abundance or site-specific phosphorylation levels, but only 65 proteins were shared between them. Functional enrichment analysis of cold-responsive proteins and phosphoproteins revealed that early cold response in maize is associated with photosynthesis light reaction, spliceosome, endocytosis, and defense response, consistent with similar studies in Arabidopsis. Thirty-two photosynthesis proteins were down-regulated at protein levels, and 48 spliceosome proteins were altered at site-specific phosphorylation levels. Thirty-one kinases and 33 transcriptional factors were cold responsive at protein, phosphopeptide, or site-specific phosphorylation levels. Our results showed that maize seedlings respond to cold shock rapidly, at both the proteome and phosphoproteome levels. This study provides a comprehensive landscape at the cold-responsive proteome and phosphoproteome in maize seedlings that can be a significant resource to understand how C₄ plants respond to a sudden temperature drop.

PRP4KA phosphorylates SERRATE for degradation via 20S proteasome to fine-tune miRNA production in Arabidopsis

Phosphorylation can quickly switch on/off protein functions. Here, we reported pre-mRNA processing 4 kinase A (PRP4KA), and its paralogs interact with Serrate (SE), a key factor in RNA processing. PRP4KA phosphorylates at least five residues of SE in vitro and in vivo. Hypophosphorylated, but not hyperphosphorylated, SE variants could readily rescue se phenotypes in vivo. Moreover, hypophosphorylated SE variants had stronger binding affinity to microprocessor component HYL1 and were more resistant to degradation by 20S proteasome than hyperphosphorylated counterparts. Knockdown of the kinases enhanced the accumulation of hypophosphorylated SE. However, the excessive SE interfered with the assembly and function of SE-scaffolded macromolecule complexes, causing the se-like defects in the mutant and wild-type backgrounds. Thus, phosphorylation of SE via PRP4KA can quickly clear accumulated SE to secure its proper amount. This study provides new insight into how protein phosphorylation regulates miRNA metabolism through controlling homeostasis of SE accumulation in plants.

How alternative splicing changes the properties of plant proteins

Most plant primary transcripts undergo alternative splicing (AS), and its impact on protein diversity is a subject of intensive investigation. Several studies have uncovered various mechanisms of how particular protein splice isoforms operate. However, the common principles behind the AS effects on protein function in plants have rarely been surveyed. Here, on the selected examples, we highlight diverse tissue expression patterns, subcellular localization, enzymatic activities, abilities to bind other molecules and other relevant features. We describe how the protein isoforms mutually interact to underline their intriguing roles in altering the functionality of protein complexes. Moreover, we also discuss the known cases when these interactions have been placed inside the autoregulatory loops. This review is particularly intended for plant cell and developmental biologists who would like to gain inspiration on how the splice variants encoded by their genes of interest may coordinately work.

Low molecular weight protein phosphatase APH mediates tyrosine dephosphorylation and ABA response in Arabidopsis

Low molecular weight protein tyrosine phosphatase (LWM-PTP), also known as acid phosphatase, is a highly conserved tyrosine phosphatase in living organisms. However, the function of LWM-PTP homolog has not been reported yet in plants. Here, we revealed a homolog of acid phosphatase, APH, in Arabidopsis plants, is a functional protein tyrosine phosphatase. The aph mutants are hyposensitive to ABA in post-germination growth. We performed an anti-phosphotyrosine antibody-based quantitative phosphoproteomics in wild-type and aph mutant and identified hundreds of putative targets of APH, including multiple splicing factors and other transcriptional regulators. Consistently, RNA-seq analysis revealed that the expression of ABA-highly-responsive genes is suppressed in aph mutants. Thus, APH regulates the ABA-responsive gene expressions by regulating the tyrosine phosphorylation of multiple splicing factors and other post-transcriptional regulators. We also revealed that Tyr383 in RAF9, a member of B2 and B3 RAF kinases that phosphorylate and activate SnRK2s in the ABA signaling pathway, is a direct target site of APH. Phosphorylation of Tyr383 is essential for RAF9 activity. Our results uncovered a crucial function of APH in ABA-induced tyrosine phosphorylation in Arabidopsis.

Supplementary Information

The online version contains supplementary material available at 10.1007/s44154-022-00041-6.

A novel strategy to uncover specific GO terms/phosphorylation pathways in phosphoproteomic data in Arabidopsis thaliana

BACKGROUND

Proteins are the workforce of the cell and their phosphorylation status tailors specific responses efficiently. One of the main challenges of phosphoproteomic approaches is to deconvolute biological processes that specifically respond to an experimental query from a list of phosphoproteins. Comparison of the frequency distribution of GO (Gene Ontology) terms in a given phosphoproteome set with that observed in the genome reference set (GenRS) is the most widely used tool to infer biological significance. Yet, this comparison assumes that GO term distribution between the phosphoproteome and the genome are identical. However, this hypothesis has not been tested due to the lack of a comprehensive phosphoproteome database.

RESULTS

In this study, we test this hypothesis by constructing three phosphoproteome databases in Arabidopsis thaliana: one based in experimental data (ExpRS), another based in in silico phosphorylation protein prediction (PredRS) and a third that is the union of both (UnRS). Our results show that the three phosphoproteome reference sets show default enrichment of several GO terms compared to GenRS, indicating that GO term distribution in the phosphoproteomes does not match that of the genome. Moreover, these differences overshadow the identification of GO terms that are specifically enriched in a particular condition. To overcome this limitation, we present an additional comparison of the sample of interest with UnRS to uncover GO terms specifically enriched in a particular phosphoproteome experiment. Using this strategy, we found that mRNA splicing and cytoplasmic microtubule compounds are important processes specifically enriched in the phosphoproteome of dark-grown Arabidopsis seedlings.

CONCLUSIONS

This study provides a novel strategy to uncover GO specific terms in phosphoproteome data of Arabidopsis that could be applied to any other organism. We also highlight the importance of specific phosphorylation pathways that take place during dark-grown Arabidopsis development.

Polyphenol oxidase genes as integral part of the evolutionary history of domesticated tetraploid wheat

Studying and understanding the genetic basis of polyphenol oxidases (PPO)-related traits plays a crucial role in genetic improvement of crops. A tetraploid wheat collection (T. turgidum ssp., TWC) was analyzed using the 90K wheat SNP iSelect assay and phenotyped for PPO activity. A total of 21,347 polymorphic SNPs were used to perform genome-wide association analysis (GWA) in TWC and durum wheat sub-groups, detecting 23 and 85 marker-trait associations (MTA). In addition, candidate genes responsible for PPO activity were predicted. Based on the 23 MTAs detected in TWC, two haplotypes associated with low and high PPO activity were identified. Four SNPs were developed and validated providing one reliable marker (IWB75732) for marker assisted selection. The 23 MTAs were used to evaluate the genetic divergence (F_ST > 0.25) between the T. turgidum subspecies, providing new information important for understanding the domestication process of Triticum turgidum ssp. and in particular of ssp. carthlicum.

Daily temperature cycles promote alternative splicing of RNAs encoding SR45a, a splicing regulator in maize

Elevated temperatures enhance alternative RNA splicing in maize (Zea mays) with the potential to expand the repertoire of plant responses to heat stress. Alternative RNA splicing generates multiple RNA isoforms for many maize genes, and here we observed changes in the pattern of RNA isoforms with temperature changes. Increases in maximum daily temperature elevated the frequency of the major modes of alternative splices (AS), in particular retained introns and skipped exons. The genes most frequently targeted by increased AS with temperature encode factors involved in RNA processing and plant development. Genes encoding regulators of alternative RNA splicing were themselves among the principal AS targets in maize. Under controlled environmental conditions, daily changes in temperature comparable to field conditions altered the abundance of different RNA isoforms, including the RNAs encoding the splicing regulator SR45a, a member of the SR45 gene family. We established an "in protoplast" RNA splicing assay to show that during the afternoon on simulated hot summer days, SR45a RNA isoforms were produced with the potential to encode proteins efficient in splicing model substrates. With the RNA splicing assay, we also defined the exonic splicing enhancers that the splicing-efficient SR45a forms utilize to aid in the splicing of model substrates. Hence, with rising temperatures on hot summer days, SR45a RNA isoforms in maize are produced with the capability to encode proteins with greater RNA splicing potential.

In vivo Reporters for Visualizing Alternative Splicing of Hormonal Genes

Rapid progress in plant molecular biology in recent years has uncovered the main players in hormonal pathways and characterized transcriptomic networks associated with hormonal response. However, the role of RNA processing, in particular alternative splicing (AS), remains largely unexplored. Here, using example genes involved in cytokinin signaling, brassinosteroid synthesis and auxin transport, we present a set of reporters devised to visualize their AS events in vivo. These reporters show a differential tissue-specific expression of certain transcripts and reveal that expression of some of the them can be changed by the application of the exogenous hormone. Finally, based on the characterized AS event of the PIN7 auxin efflux carrier, we designed a system that allows a rapid genetic screening for the factors upstream of this AS event. Our innovative toolset can be therefore highly useful for exploring novel regulatory nodes of hormonal pathways and potentially helpful for plant researchers focusing on developmental aspects of AS.

A Collection of Pre-mRNA Splicing Mutants in Arabidopsis thaliana

To investigate factors influencing pre-mRNA splicing in plants, we conducted a forward genetic screen using an alternatively-spliced GFP reporter gene in Arabidopsis thaliana. This effort generated a collection of sixteen mutants impaired in various splicing-related proteins, many of which had not been recovered in any prior genetic screen or implicated in splicing in plants. The factors are predicted to act at different steps of the spliceosomal cycle, snRNP biogenesis pathway, transcription, and mRNA transport. We have described eleven of the mutants in recent publications. Here we present the final five mutants, which are defective, respectively, in RNA-BINDING PROTEIN 45D (rbp45d), DIGEORGE SYNDROME CRITICAL REGION 14 (dgcr14), CYCLIN-DEPENDENT KINASE G2 (cdkg2), INTERACTS WITH SPT6 (iws1) and CAP BINDING PROTEIN 80 (cbp80). We provide RNA-sequencing data and analyses of differential gene expression and alternative splicing patterns for the cbp80 mutant and for several previously published mutants, including smfa and new alleles of cwc16a, for which such information was not yet available. Sequencing of small RNAs from the cbp80 mutant highlighted the necessity of wild-type CBP80 for processing of microRNA (miRNA) precursors into mature miRNAs. Redundancy tests of paralogs encoding several of the splicing factors revealed their functional non-equivalence in the GFP reporter gene system. We discuss the cumulative findings and their implications for the regulation of pre-mRNA splicing efficiency and alternative splicing in plants. The mutant collection provides a unique resource for further studies on a coherent set of splicing factors and their roles in gene expression, alternative splicing and plant development.

Alternative Splicing and DNA Damage Response in Plants

Plants are exposed to a variety of abiotic and biotic stresses that may result in DNA damage. Endogenous processes - such as DNA replication, DNA recombination, respiration, or photosynthesis - are also a threat to DNA integrity. It is therefore essential to understand the strategies plants have developed for DNA damage detection, signaling, and repair. Alternative splicing (AS) is a key post-transcriptional process with a role in regulation of gene expression. Recent studies demonstrate that the majority of intron-containing genes in plants are alternatively spliced, highlighting the importance of AS in plant development and stress response. Not only does AS ensure a versatile proteome and influence the abundance and availability of proteins greatly, it has also emerged as an important player in the DNA damage response (DDR) in animals. Despite extensive studies of DDR carried out in plants, its regulation at the level of AS has not been comprehensively addressed. Here, we provide some insights into the interplay between AS and DDR in plants.