A Newly Identified Flower-Specific Splice Variant of AUXIN RESPONSE FACTOR8 Regulates Stamen Elongation and Endothecium Lignification in Arabidopsis

Alternative splicing of DSP1 enhances snRNA accumulation by promoting transcription termination and recycle of the processing complex

Small nuclear RNAs (snRNAs) are the basal components of the spliceosome and play crucial roles in splicing. Their biogenesis is spatiotemporally regulated. However, related mechanisms are still poorly understood. Defective in snRNA processing (DSP1) is an essential component of the DSP1 complex that catalyzes plant snRNA 3'-end maturation by cotranscriptional endonucleolytic cleavage of the primary snRNA transcripts (presnRNAs). Here, we show that DSP1 is subjected to alternative splicing in pollens and embryos, resulting in two splicing variants, DSP1α and DSP1β. Unlike DSP1α, DSP1β is not required for presnRNA 3'-end cleavage. Rather, it competes with DSP1α for the interaction with CPSF73-I, the catalytic subunit of the DSP1 complex, which promotes efficient release of CPSF73-I and the DNA-dependent RNA polymerease II (Pol II) from the 3' end of snRNA loci thereby facilitates snRNA transcription termination, resulting in increased snRNA levels in pollens. Taken together, this study uncovers a mechanism that spatially regulates snRNA accumulation.

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.

Conserved exitrons of FLAGELLIN-SENSING 2 (FLS2) across dicot plants and their functions

The alternative splicing of pattern recognition receptor genes regulates immune signalling in mammals, but in plants its role is still unknown. Here, we detected alternatively spliced introns (exitrons) in the first annotated exons of FLAGELLIN-SENSING 2 (FLS2) genes in all the examined dicot plants across nine families. The 5' splice site (SS) regions were conserved and with rare synonymous substitutions. Point mutations and gene swaps indicated that the position and efficiency of exitron splicing primarily depended on the nucleotide sequences of FLS2 genes. Single-nucleotide mutations in the invariable codon carrying 5' SS dramatically altered the accumulation of poplar and tomato FLS2 transcripts, indicating the 5'-proximal exitrons of FLS2 function as stimulatory introns on gene expression. The 3' SSs of exitrons are diverse and can be changed by 1-2 nucleotide mutations in Salicaceae FLS2. The alternative transcripts (ATs) of poplar and tobacco FLS2, which encode small secreted proteins, were specifically induced by flg22, and one such AT from tobacco FLS2 suppressed flg22-induced response. Our results indicated that the exitrons of FLS2 genes regulate the accumulation of transcripts by an intron mediated enhancement (IME) mechanism and some ATs have the potential to encode suppressors for FLS2 pathway.

Light controls stamen elongation via cryptochromes, phytochromes and COP1 through HY5 and HYH

In Arabidopsis, stamen elongation, which ensures male fertility, is controlled by the auxin response factor ARF8, which regulates the expression of the auxin repressor IAA19. Here, we uncover a role for light in controlling stamen elongation. By an extensive genetic and molecular analysis we show that the repressor of light signaling COP1, through its targets HY5 and HYH, controls stamen elongation, and that HY5 - oppositely to ARF8 - directly represses the expression of IAA19 in stamens. In addition, we show that in closed flower buds, when light is shielded by sepals and petals, the blue light receptors CRY1/CRY2 repress stamen elongation. Coherently, at flower disclosure and in subsequent stages, stamen elongation is repressed by the red and far-red light receptors PHYA/PHYB. In conclusion, different light qualities - sequentially perceived by specific photoreceptors - and the downstream COP1-HY5/HYH module finely tune auxin-induced stamen elongation and thus male fertility.

Class-I TCP Transcription Factors Activate the SAUR63 Gene Subfamily in Gibberellin-Dependent Stamen Filament Elongation

In autogamous plants like Arabidopsis (Arabidopsis thaliana), stamen filament elongation must be finely regulated to ensure that anthers reach the pistil at the correct developmental stage. In this work, we studied the roles of Arabidopsis TEOSINTE BRANCHED1, CYCLOIDEA, PCF15 (TCP15), and related class-I TCP transcription factors in stamen filament elongation. Plants with decreased expression of class-I TCPs and plants that express a fusion of TCP15 to a repressor domain (pTCP15::TCP15-EAR) had shorter stamens, indicating that class-I TCPs stimulate filament growth. These plants also showed reduced expression of several SMALL AUXIN UP RNA (SAUR)63 subfamily genes, which contain TCP target motifs in their promoters. Mutational analysis indicated that the TCP target motif in the SAUR63 promoter is required for expression of SAUR63 in stamen filaments. Moreover, TCP15 directly binds to the SAUR63 promoter region that contains the TCP target motif in vivo, highlighting the role of the TCPs in this process. Class-I TCPs are also required for the induction of SAUR63 subfamily genes by gibberellins (GAs). In addition, overexpression of SAUR63 restores filament growth in pTCP15::TCP15-EAR plants, whereas overexpression of TCP15 rescues the short stamen phenotype of GA-deficient plants. The results indicate that TCP15 and related class-I TCPs modulate GA-dependent stamen filament elongation by direct activation of SAUR63 subfamily genes through conserved target sites in their promoters. This work provides insight into GA-dependent stamen filament elongation.

Investigation of the interaction of DAD1-LIKE LIPASE 3 (DALL3) with Selenium Binding Protein 1 (SBP1) in Arabidopsis thaliana

Phospholipase PLA₁-Iγ2 or otherwise DAD1-LIKE LIPASE 3 (DALL3) is a member of class I phospholipases and has a role in JA biosynthesis. AtDALL3 was previously identified in a yeast two-hybrid screening as an interacting protein of the Arabidopsis Selenium Binding Protein 1 (SBP1). In this work, we have studied AtDALL3 as an interacting partner of the Arabidopsis Selenium Binding Protein 1 (SBP1). Phylogenetic analysis showed that DALL3 appears in the PLA1-Igamma1, 2 group, paired with PLA1-Igammma1. The highest level of expression of AtDALL3 was observed in 10-day-old roots and in flowers, while constitutive levels were maintained in seedlings, cotyledons, shoots and leaves. In response to abiotic stress, DALL3 was shown to participate in the network of genes regulated by cadmium, selenite and selenate compounds. DALL3 promoter driven GUS assays revealed that the expression patterns defined were overlapping with the patterns reported for AtSBP1 gene, indicating that DALL3 and SBP1 transcripts co-localize. Furthermore, quantitative GUS assays showed that these compounds elicited changes in activity in specific cells files, indicating the differential response of DALL3 promoter. GFP::DALL3 studies by confocal microscopy demonstrated the localization of DALL3 in the plastids of the root apex, the plastids of the central root and the apex of emerging lateral root primordia. Additionally, we confirmed by yeast two hybrid assays the physical interaction of DALL3 with SBP1 and defined a minimal SBP1 fragment that DALL3 binds to. Finally, by employing bimolecular fluorescent complementation we demonstrated the in planta interaction of the two proteins.

Improved reproductive growth of euhalophyte Suaeda salsa under salinity is correlated with altered phytohormone biosynthesis and signal transduction

Phytohormones are essential for plant reproductive growth. Salinity limits crop reproductive growth and yield, but improves reproductive growth of euhalophytes. However, little is known about the mechanisms underlying salinity's effects on plant reproductive growth. To elucidate the role of plant hormones in flower development of the euhalophyte Suaeda salsa under saline conditions, we analysed endogenous gibberellic acid (GA3,4), indoleacetic acid (IAA), zeatin riboside (ZR), abscisic acid (ABA), and brassinosteroids (BRs) during flowering in control (0 mM) and NaCl-treated (200 mM) plants. At the end of vegetative growth, endogenous GA3, GA4, ABA and BR contents in stems of NaCl-treated plants were significantly higher than those in controls. During flowering, GA3, GA4, IAA and ZR contents showed the most significant enhancement in flower organs of plants treated with NaCl when compared with controls. Additionally, genes related to ZR, IAA, GA, BR and ABA biosynthesis and plant hormone signal transduction, such as those encoding CYP735A, CYP85A, GID1, NCED, PIF4, AHP, TCH4, SnRK2 and ABF, were upregulated in S. salsa flowers from NaCl-treated plants. These results suggest that coordinated upregulation of genes involved in phytohormone biosynthesis and signal transduction contributes to the enhanced reproductive growth of S. salsa under salinity.

Regulation of flowering transition by alternative splicing: the role of the U2 auxiliary factor

Flowering transition is regulated by complex genetic networks in response to endogenous and environmental signals. Pre-mRNA splicing is an essential step for the post-transcriptional regulation of gene expression. Alternative splicing of key flowering genes has been investigated in detail over the past decade. However, few splicing factors have been identified as being involved in flowering transition. Human heterodimeric splicing factor U2 snRNP auxiliary factor (U2AF) consists of two subunits, U2AF35 and U2AF65, and functions in 3' splice site recognition in mRNA splicing. Recent studies reveal that Arabidopsis U2AF65a/b and U2AF35a/b play important roles in the splicing of key flowering genes. We summarize recent advances in research on splicing-regulated flowering transition by focusing on the role of Arabidopsis U2AF in the splicing of key flowering-related genes at ambient temperature and in the abscisic acid signaling pathways.

Comparative transcriptomics of stem bark reveals genes associated with bast fiber development in Boehmeria nivea L. gaud (ramie)

BACKGROUND

Boehmeria nivea L. Gaud (Ramie) produces one of the longest natural fibers in nature. The bark of ramie mainly comprises of the phloem tissue of stem and is the raw material for fiber. Therefore, identifying the molecular regulation of phloem development is important for understanding of bast fiber biosynthesis and improvement of fiber quality in ramie.

RESULTS

In this study, we collected top bud (TB), bark from internode elongating region (ER) and bark from internode fully elongated region (FER) from the ramie variety Zhongzhu No. 1. Histological study indicated that these samples contain phloem tissues at different developmental and maturation stages, with a higher degree of maturation of phloem tissue in FER. RNA sequencing (RNA-seq) was performed and de novo transcriptome was assembled. Unigenes and differentially expressed genes (DEGs) in these three samples were identified. The analysis of DEGs by using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) revealed clear differences in gene expression between ER and FER. Some unigenes involved in secondary cell wall biosynthesis were up-regulated in both ER and FER, while unigenes for some cell wall components or cell wall modifications showed differential expression between ER and FER. In addition, the ethylene respond factors (ERFs) in the ethylene signaling pathway were up-regulated in FER, and ent-kaurenoic acid oxidase (KAO) and GA 20-oxidase (GA20ox) for gibberellins biosynthesis were up-regulated while GA 2-oxidase (GA2ox) for gibberellin inactivation was down-regulated in FER.

CONCLUSIONS

Both morphological study and gene expression analysis supported a burst of phloem and vascular developmental processes during the fiber maturation in the ramie stem, and ethylene and gibberellin are likely to be involved in this process. Our findings provide novel insights into the phloem development and fiber maturation in ramie, which could be useful for fiber improvement in ramie and other fiber crops.

Comprehensive Atlas of Wheat (Triticum aestivum L.) AUXIN RESPONSE FACTOR Expression During Male Reproductive Development and Abiotic Stress

AUXIN RESPONSE FACTOR (ARF) proteins regulate a wide range of signaling pathways, from general plant growth to abiotic stress responses. Here, we performed a genome-wide survey in wheat (Triticum aestivum) and identified 69 TaARF members that formed 24 homoeologous groups. Phylogenetic analysis clustered TaARF genes into three clades, similar to ARF genes in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa). Structural characterization suggested that ARF gene structure and domain composition are well conserved between plant species. Expression profiling revealed diverse patterns of TaARF transcript levels across a range of developmental stages, tissues, and abiotic stresses. A number of TaARF genes shared similar expression patterns and were preferentially expressed in anthers. Moreover, our systematic analysis identified three anther-specific TaARF genes (TaARF8, TaARF9, and TaARF21) whose expression was significantly altered by low temperature in thermosensitive genic male-sterile (TGMS) wheat; these TaARF genes are candidates to participate in the cold-induced male sterility pathway, and offer potential applications in TGMS wheat breeding and hybrid seed production. Moreover, we identified putative functions for a set of TaARFs involved in responses to abscisic acid and abiotic stress. Overall, this study characterized the wheat ARF gene family and generated several hypotheses for future investigation of ARF function during anther development and abiotic stress.

Alternative Splicing Enhances the Transcriptome Complexity of Liriodendron chinense

Alternative splicing (AS) plays pivotal roles in regulating plant growth and development, flowering, biological rhythms, signal transduction, and stress responses. However, no studies on AS have been performed in Liriodendron chinense, a deciduous tree species that has high economic and ecological value. In this study, we used multiple tools and algorithms to analyze transcriptome data derived from seven tissues via hybrid sequencing. Although only 17.56% (8,503/48,408) of genes in L. chinense were alternatively spliced, these AS genes occurred in 37,844 AS events. Among these events, intron retention was the most frequent AS event, producing 1,656 PTC-containing and 3,310 non-PTC-containing transcripts. Moreover, 183 long noncoding RNAs (lncRNAs) also underwent AS events. Furthermore, weighted gene coexpression network analysis (WGCNA) revealed that there were great differences in the activities of transcription and post-transcriptional regulation between pistils and leaves, and AS had an impact on many physiological and biochemical processes in L. chinense, such as photosynthesis, sphingolipid metabolism, fatty acid biosynthesis and metabolism. Moreover, our analysis showed that the features of genes may affect AS, as AS genes and non-AS genes had differences in the exon/intron length, transcript length, and number of exons/introns. In addition, the structure of AS genes may impact the frequencies and types of AS because AS genes with more exons or introns tended to exhibit more AS events, and shorter introns tended to be retained, whereas shorter exons tended to be skipped. Furthermore, eight AS genes were verified, and the results were consistent with our analysis. Overall, this study reveals that AS and gene interaction are mutual-on one hand, AS can affect gene expression and translation, while on the other hand, the structural characteristics of the gene can also affect AS. This work is the first to comprehensively report on AS in L. chinense, and it can provide a reference for further research on AS in L. chinense.

Jasmonate Signaling during Arabidopsis Stamen Maturation

The last stages of stamen development, collectively called stamen maturation, encompass pollen viability, filament elongation and anther dehiscence or opening. These processes are essential for male fertility in Arabidopsis and require the function of jasmonate signaling. There is a good understanding of jasmonate synthesis, perception and transcriptional outputs in Arabidopsis stamens. In addition, the spatiotemporal localization of jasmonate signaling components at the tissue and cellular levels has started to emerge in recent years. However, the ultimate cellular functions activated by jasmonate to promote stamen maturation remain unknown. The hormones auxin and gibberellin have been proposed to control the activation of jasmonate synthesis to promote stamen maturation, although we hypothesize that this action is rather indirect. In this review, we examine these different areas, attempt to clarify some confusing aspects found in the literature and raise testable hypothesis that may help to further understand how jasmonate controls male fertility in Arabidopsis.

The Complex Genetic Architecture of Early Root and Shoot Traits in Flax Revealed by Genome-Wide Association Analyses

Roots are fundamental organs for water and nutrient uptake as well as for signal transduction in response to biotic and abiotic stresses. Flax has a shallow tap root system that relies mostly on top soil nutrient and moisture resources. The crop can easily be outcompeted by weeds or other crops in intercropping systems, especially in moisture deficit conditions. However, there is a wide range of variation among genotypes in terms of performance under scarce resources such as moisture limitation. Here we phenotyped 15 root, two shoot traits and shoot to root dry weight ratio on 115 flax accessions grown in a hydroponic pouch system and performed a genome-wide association study (GWAS) based on seven different models to identify quantitative trait loci underlying these traits. Significant variation among genotypes was observed for the two shoot and 12 of the 14 root traits. Shoot dry weight was correlated with root network volume, length, surface area, and root dry weight (r > 0.5, P < 0.001) but not significantly correlated with root depth (r = 0.033, P > 0.05). The seven GWAS models detected a total of 228 quantitative trait nucleotides (QTNs) for 16 traits. Most loci, defined by an interval of 100 kb up and downstream of the QTNs, harbored genes known to play role(s) in root and shoot development, suggesting them as candidates. Examples of candidate genes linked to root network QTNs included genes encoding GRAS transcription factors, mitogen-activated protein kinases, and auxin related lateral organ boundary proteins while QTN loci for shoot dry weight harbored genes involved in photomorphogenesis and plant immunity. These results provide insights into the genetic bases of early shoot and root development traits in flax that could be capitalized upon to improve its root architecture, particularly in view of better withstanding water limiting conditions during the cropping season.

A GARP transcription factor anther dehiscence defected 1 (OsADD1) regulates rice anther dehiscence

Anther dehiscence, one of the essential steps in pollination and double fertilization, is regulated by a complex signaling pathway encompassing hormones and environmental factors. However, key components underlying the signaling pathway that regulate anther dehiscence remain largely elusive. Here, we isolated a rice mutant anther dehiscence defected 1 (Osadd1) that exhibited defects in anther dehiscence and glume open. Map-based cloning revealed that OsADD1 encoded a GARP (Golden2, ARR-B and Psr1) transcription factor. Sequence analysis showed that a single base deletion in Osadd1 mutant resulted in pre-termination of the GARP domain. OsADD1 was constitutively expressed in various tissues, with more abundance in the panicles. The major genes associated with anther dehiscence were affected in the Osadd1 mutant, and the expression level of the cellulose synthase-like D sub-family 4 (OsCSLD4) was significantly decreased. We demonstrate that OsADD1 regulated the expression of OsCSLD4 by binding to its promoter, and affects rice anther dehiscence.

AUXIN RESPONSE FACTOR17 Directly Regulates MYB108 for Anther Dehiscence

The timely release of mature pollen following anther dehiscence is essential for reproduction in flowering plants. AUXIN RESPONSE FACTOR17 (ARF17) plays a crucial role in pollen wall pattern formation, tapetum development, and auxin signal transduction in anthers. Here, we showed that ARF17 is also involved in anther dehiscence. The Arabidopsis (Arabidopsis thaliana) arf17 mutant exhibits defective endothecium lignification, which leads to defects in anther dehiscence. The expression of MYB108, which encodes a transcription factor important for anther dehiscence, was dramatically down-regulated in the flower buds of arf17 Chromatin immunoprecipitation assays and electrophoretic mobility shift assays showed ARF17 directly binds to the MYB108 promoter. In an ARF17-GFP transgenic line, in which ARF17-GFP fully complements the arf17 phenotype, ARF17-GFP was observed in the endothecia at anther stage 11. The GUS signal driven by the MYB108 promoter was also detected in endothecia at late anther stages in transgenic plants expressing promoterMYB108::GUS Thus, the expression pattern of both ARF17 and MYB108 is consistent with the function of these genes in anther dehiscence. Furthermore, the expression of MYB108 driven by the ARF17 promoter successfully restored the defects in anther dehiscence of arf17 These results demonstrated that ARF17 regulates the expression of MYB108 for anther dehiscence. Together with its function in microcytes and tapeta, ARF17 likely coordinates the development of different sporophytic cell layers in anthers. The ARF17-MYB108 pathway involved in regulating anther dehiscence is also discussed.

Identification of Auxin Response Factor-Encoding Genes Expressed in Distinct Phases of Leaf Vein Development and with Overlapping Functions in Leaf Formation

Based on mutant phenotypes the MONOPTEROS (MP)/Auxin Response Factor 5 (ARF5) gene acts in several developmental processes including leaf vein development. Since overlapping functions among ARF genes are common, we assessed the related ARF 3-8 and 19 genes for potential overlap in expression during vein development using in-situ hybridization. Like MP/ARF5, ARF3 was expressed in preprocambial and procambial cells. ARF7 was also expressed in procambial cells, close to and during vein differentiation. ARF19 was expressed in differentiating vessel elements. To assess if genes with vein expression have overlapping functions, double mutants were generated. While arf3, 5 and 7 mutants formed leaves normally, double mutant combinations of mp/arf5 with arf3 or arf7 resulted in a breakdown of leaf formation. Instead, novel structures not present in any of the single mutants formed. The results implicate ARF3 and ARF7 in rosette leaf formation and suggest that their functions overlap and act in parallel with MP/ARF5 in this process. The observed vascular expression patterns suggest unique functions (ARF7 and 19) and potentially overlapping functions (ARF3 and 5) in vein development. Since arf3 arf5 double mutants do not form leaves, assessment of their potential combined action in vein development will require the use of conditional mutants.

The BAP Module: A Multisignal Integrator Orchestrating Growth

Coordination of cell proliferation, cell expansion, and differentiation underpins plant growth. To maximise reproductive success, growth needs to be fine-tuned in response to endogenous and environmental cues. This developmental plasticity relies on a cellular machinery that integrates diverse signals and coordinates the downstream responses. In arabidopsis, the BAP regulatory module, which includes the BRASSINAZOLE RESISTANT 1 (BZR1), AUXIN RESPONSE FACTOR 6 (ARF6), and PHYTOCHROME INTERACTING FACTOR 4 (PIF4) transcription factors (TFs), has been shown to coordinate growth in response to multiple growth-regulating signals. In this Opinion article, we provide an integrative view on the BAP module control of cell expansion and discuss whether its function is conserved or diversified, thus providing new insights into the molecular control of growth.

Imbalance of tyrosine by modulating TyrA arogenate dehydrogenases impacts growth and development of Arabidopsis thaliana

l-Tyrosine is an essential aromatic amino acid required for the synthesis of proteins and a diverse array of plant natural products; however, little is known on how the levels of tyrosine are controlled in planta and linked to overall growth and development. Most plants synthesize tyrosine by TyrA arogenate dehydrogenases, which are strongly feedback-inhibited by tyrosine and encoded by TyrA1 and TyrA2 genes in Arabidopsis thaliana. While TyrA enzymes have been extensively characterized at biochemical levels, their in planta functions remain uncertain. Here we found that TyrA1 suppression reduces seed yield due to impaired anther dehiscence, whereas TyrA2 knockout leads to slow growth with reticulate leaves. The tyra2 mutant phenotypes were exacerbated by TyrA1 suppression and rescued by the expression of TyrA2, TyrA1 or tyrosine feeding. Low-light conditions synchronized the tyra2 and wild-type growth, and ameliorated the tyra2 leaf reticulation. After shifting to normal light, tyra2 transiently decreased tyrosine and subsequently increased aspartate before the appearance of the leaf phenotypes. Overexpression of the deregulated TyrA enzymes led to hyper-accumulation of tyrosine, which was also accompanied by elevated aspartate and reticulate leaves. These results revealed that TyrA1 and TyrA2 have distinct and overlapping functions in flower and leaf development, respectively, and that imbalance of tyrosine, caused by altered TyrA activity and regulation, impacts growth and development of Arabidopsis. The findings provide critical bases for improving the production of tyrosine and its derived natural products, and further elucidating the coordinated metabolic and physiological processes to maintain tyrosine levels in plants.

The Casparian strip-one ring to bring cell biology to lignification?

Lignin research has long been motivated by the outstanding importance of wood for human societies. The annual, non-woody Arabidopsis thaliana, has nevertheless contributed greatly to our understanding of lignification, due to its unrivalled genetic resources. Arabidopsis is also great for cell and developmental biology, allowing precise imaging and tracking of cell types. Root endodermis differentiation involves the precise lignification of the Casparian Strip, as an apoplastic barrier; while barrier damage triggers a less localized, compensatory lignification. Transcriptional reprogramming and peptide-induced signalling emerge as promising tools for the study of endodermal lignification. We argue that endodermis lignification is an attractive model complementary to equally powerful, cellular xylem differentiation systems, as it might better represent the restricted - often localized - lignification seen in non-vascular cells.

INDUCER OF CBF EXPRESSION 1 is a male fertility regulator impacting anther dehydration in Arabidopsis

INDUCER OF CBF EXPRESSION 1 (ICE1) encodes a MYC-like basic helix-loop-helix (bHLH) transcription factor playing a critical role in plant responses to chilling and freezing stresses and leaf stomata development. However, no information connecting ICE1 and reproductive development has been reported. In this study, we show that ICE1 controls plant male fertility via impacting anther dehydration. The loss-of-function mutation in ICE1 gene in Arabidopsis caused anther indehiscence and decreased pollen viability as well as germination rate. Further analysis revealed that the anthers in the mutant of ICE1 (ice1-2) had the structure of stomium, though the epidermis did not shrink to dehisce. The anther indehiscence and influenced pollen viability as well as germination in ice1-2 were due to abnormal anther dehydration, for most of anthers dehisced with drought treatment and pollen grains from those dehydrated anthers had similar viability and germination rates compared with wild type. Accordingly, the sterility of ice1-2 could be rescued by ambient dehydration treatments. Likewise, the stomatal differentiation of ice1-2 anther epidermis was disrupted in a different manner compared with that in leaves. ICE1 specifically bound to MYC-recognition elements in the promoter of FAMA, a key regulator of guard cell differentiation, to activate FAMA expression. Transcriptome profiling in the anther tissues further exhibited ICE1-modulated genes associated with water transport and ion exchange in the anther. Together, this work reveals the key role of ICE1 in male fertility control and establishes a regulatory network mediated by ICE1 for stomata development and water movement in the anther.

Position-specific intron retention is mediated by the histone methyltransferase SDG725

BACKGROUND

Intron retention (IR), the most prevalent alternative splicing form in plants, plays a critical role in gene expression during plant development and stress response. However, the molecular mechanisms underlying IR regulation remain largely unknown.

RESULTS

Knockdown of SDG725, a histone H3 lysine 36 (H3K36)-specific methyltransferase in rice, leads to alterations of IR in more than 4700 genes. Surprisingly, IR events are globally increased at the 5' region but decreased at the 3' region of the gene body in the SDG725-knockdown mutant. Chromatin immunoprecipitation sequencing analyses reveal that SDG725 depletion results in a genome-wide increase of the H3K36 mono-methylation (H3K36me1) but, unexpectedly, promoter-proximal shifts of H3K36 di- and tri-methylation (H3K36me2 and H3K36me3). Consistent with the results in animals, the levels of H3K36me1/me2/me3 in rice positively correlate with gene expression levels, whereas shifts of H3K36me2/me3 coincide with position-specific alterations of IR. We find that either H3K36me2 or H3K36me3 alone contributes to the positional change of IR caused by SDG725 knockdown, although IR shift is more significant when both H3K36me2 and H3K36me3 modifications are simultaneously shifted.

CONCLUSIONS

Our results revealed that SDG725 modulates IR in a position-specific manner, indicating that H3K36 methylation plays a role in RNA splicing, probably by marking the retained introns in plants.