Targeted degradation of the cyclin-dependent kinase inhibitor ICK4/KRP6 by RING-type E3 ligases is essential for mitotic cell cycle progression during Arabidopsis gametogenesis

Regulation of cell cycle in plant gametes: when is the right time to divide?

Cell division is a fundamental process shared across diverse life forms, from yeast to humans and plants. Multicellular organisms reproduce through the formation of specialized types of cells, the gametes, which at maturity enter a quiescent state that can last decades. At the point of fertilization, signalling lifts the quiescent state and triggers cell cycle reactivation. Studying how the cell cycle is regulated during plant gamete development and fertilization is challenging, and decades of research have provided valuable, yet sometimes contradictory, insights. This Review summarizes the current understanding of plant cell cycle regulation, gamete development, quiescence, and fertilization-triggered reactivation.

The Arabidopsis KASH protein SINE3 is involved in male and female gametogenesis

KEY MESSAGE

The Arabidopsis KASH protein SINE3 is involved in male and female gametophyte development, likely affecting the first post-meiotic mitosis in both cases, and is required for full seed set. Linker of nucleoskeleton and cytoskeleton (LINC) complexes are protein complexes spanning the inner and outer membranes of the nuclear envelope (NE) and are key players in nuclear movement and positioning. Through their roles in nuclear movement and cytoskeletal reorganization, plant LINC complexes affect processes as diverse as pollen tube rupture and stomatal development and function. KASH proteins are the outer nuclear membrane component of the LINC complex, with conserved C-termini but divergent N-terminal cytoplasmic domains. Of the known Arabidopsis KASH proteins, SUN-INTERACTING NUCLEAR ENVELOPE PROTEIN 3 (SINE3) has not been functionally characterized. Here, we show that SINE3 is expressed at all stages of male and female gametophyte development. It is located at the NE in male and female gametophytes. Loss of SINE3 results in a female-derived seed set defect, with sine3 mutant ovules arresting at stage FG1. Pollen viability is also significantly reduced, with microspores arresting prior to pollen mitosis I. In addition, sine3 mutants have a minor male meiosis defect, with some tetrads containing more than four spores. Together, these results demonstrate that the KASH protein SINE3 plays a crucial role in male and female gametophyte development, likely affecting the first post-meiotic nuclear division in both cases.

Genome-wide association studies of bundle and single fiber length traits reveal the genetic basis of within-sample variation in upland cotton fiber length

Within-sample variation in cotton fiber length is a major factor influencing the production and quality of yarns. The textile industry has been searching for approaches of improving the long fiber fraction and minimizing the short fiber fraction within a cotton sample to produce superior fiber and yarn quality. USTER® High Volume Instrument (HVI) has been widely used for a rapid assessment of cotton fiber length traits from a fiber bundle. However, its effectiveness for genetic studies has been questioned due to the indirect estimations of the cotton fiber traits that cannot be measured from a fiber bundle. To overcome the limits of the HVI fiber length traits, we utilized the Advanced Fiber Information System (AFIS) measuring fiber length traits directly from individual fibers based on weight or number. Comparative fiber length analyses showed AFIS provided higher sensitivity in detecting the fiber length variations within and among cotton samples than HVI. The weight-based AFIS length traits were strongly correlated with the corresponding HVI lengths, whereas the number-based AFIS mean length showed a relatively weaker correlation with the HVI lengths. Integrations of the weight based-length traits with genome-wide association studies (GWAS) enabled classifying the QTLs specifically associated with long, mean, or short fiber length traits and identified a false positive associated with the indirectly estimated HVI short fiber trait. Unlike the weight based-AFIS length traits, the number-based AFIS length trait did not show a negative correlation with a weight related-HVI property, and identified a single QTL that was not detected by the corresponding HVI trait. These results suggested that integrating the AFIS method with GWAS helped discoveries of the genome loci involved in the within-sample variation in cotton fiber length and characterizations of the fiber length QTLs.

GhATL68b regulates cotton fiber cell development by ubiquitinating the enzyme required for β-oxidation of polyunsaturated fatty acids

E3 ligases are key enzymes required for protein degradation. Here, we identified a C3H2C3 RING domain-containing E3 ubiquitin ligase gene named GhATL68b. It is preferentially and highly expressed in developing cotton fiber cells and shows greater conservation in plants than in animals or archaea. The four orthologous copies of this gene in various diploid cottons and eight in the allotetraploid G. hirsutum were found to have originated from a single common ancestor that can be traced back to Chlamydomonas reinhardtii at about 992 million years ago. Structural variations in the GhATL68b promoter regions of G. hirsutum, G. herbaceum, G. arboreum, and G. raimondii are correlated with significantly different methylation patterns. Homozygous CRISPR-Cas9 knockout cotton lines exhibit significant reductions in fiber quality traits, including upper-half mean length, elongation at break, uniformity, and mature fiber weight. In vitro ubiquitination and cell-free protein degradation assays revealed that GhATL68b modulates the homeostasis of 2,4-dienoyl-CoA reductase, a rate-limiting enzyme for the β-oxidation of polyunsaturated fatty acids (PUFAs), via the ubiquitin proteasome pathway. Fiber cells harvested from these knockout mutants contain significantly lower levels of PUFAs important for production of glycerophospholipids and regulation of plasma membrane fluidity. The fiber growth defects of the mutant can be fully rescued by the addition of linolenic acid (C18:3), the most abundant type of PUFA, to the ovule culture medium. This experimentally characterized C3H2C3 type E3 ubiquitin ligase involved in regulating fiber cell elongation may provide us with a new genetic target for improved cotton lint production.

The miR159a-DUO1 module regulates pollen development by modulating auxin biosynthesis and starch metabolism in citrus

Achieving seedlessness in citrus varieties is one of the important objectives of citrus breeding. Male sterility associated with abnormal pollen development is an important factor in seedlessness. However, our understanding of the regulatory mechanism underlying the seedlessness phenotype in citrus is still limited. Here, we determined that the miR159a-DUO1 module played an important role in regulating pollen development in citrus, which further indirectly modulated seed development and fruit size. Both the overexpression of csi-miR159a and the knocking out of DUO1 in Hong Kong kumquat (Fortunella hindsii) resulted in small and seedless fruit phenotypes. Moreover, pollen was severely aborted in both transgenic lines, with arrested pollen mitotic I and abnormal pollen starch metabolism. Through additional cross-pollination experiments, DUO1 was proven to be the key target gene for miR159a to regulate male sterility in citrus. Based on DNA affinity purification sequencing (DAP-seq), RNA-seq, and verified interaction assays, YUC2/YUC6, SS4 and STP8 were identified as downstream target genes of DUO1, those were all positively regulated by DUO1. In transgenic F. hindsii lines, the miR159a-DUO1 module down-regulated the expression of YUC2/YUC6, which decreased indoleacetic acid (IAA) levels and modulated auxin signaling to repress pollen mitotic I. The miR159a-DUO1 module reduced the expression of the starch synthesis gene SS4 and sugar transport gene STP8 to disrupt starch metabolism in pollen. Overall, this work reveals a new mechanism by which the miR159a-DUO1 module regulates pollen development and elucidates the molecular regulatory network underlying male sterility in citrus.

ABSCISIC ACID-INSENSITIVE 5-KIP-RELATED PROTEIN 1-SHOOT MERISTEMLESS modulates reproductive development of Arabidopsis

Soil (or plant) water deficit accelerates plant reproduction. However, the underpinning molecular mechanisms remain unknown. By modulating cell division/number, ABSCISIC ACID-INSENSITIVE 5 (ABI5), a key bZIP (basic (region) leucine zippers) transcription factor, regulates both seed development and abiotic stress responses. The KIP-RELATED PROTEIN (KRP) cyclin-dependent kinases (CDKs) play an essential role in controlling cell division, and SHOOT MERISTEMLESS (STM) plays a key role in the specification of flower meristem identity. Here, our findings show that abscisic acid (ABA) signaling and/or metabolism in adjust reproductive outputs (such as rosette leaf number and open flower number) under water-deficient conditions in Arabidopsis (Arabidopsis thaliana) plants. Reproductive outputs increased under water-sufficient conditions but decreased under water-deficient conditions in the ABA signaling/metabolism mutants abscisic acid2-1 (aba2-1), aba2-11, abscisic acid insensitive3-1 (abi3-1), abi4-1, abi5-7, and abi5-8. Further, under water-deficient conditions, ABA induced-ABI5 directly bound to the promoter of KRP1, which encodes a CDK that plays an essential role in controlling cell division, and this binding subsequently activated KRP1 expression. In turn, KRP1 physically interacted with STM, which functions in the specification of flower meristem identity, promoting STM degradation. We further demonstrate that reproductive outputs are adjusted by the ABI5-KRP1-STM molecular module under water-deficient conditions. Together, our findings reveal the molecular mechanism by which ABA signaling and/or metabolism regulate reproductive development under water-deficient conditions. These findings provide insights that may help guide crop yield improvement under water deficiency.

The SIAMESE family of cell-cycle inhibitors in the response of plants to environmental stresses

Environmental abiotic constraints are known to reduce plant growth. This effect is largely due to the inhibition of cell division in the leaf and root meristems caused by perturbations of the cell cycle machinery. Progression of the cell cycle is regulated by CDK kinases whose phosphorylation activities are dependent on cyclin proteins. Recent results have emphasized the role of inhibitors of the cyclin-CDK complexes in the impairment of the cell cycle and the resulting growth inhibition under environmental constraints. Those cyclin-CDK inhibitors (CKIs) include the KRP and SIAMESE families of proteins. This review presents the current knowledge on how CKIs respond to environmental changes and on the role played by one subclass of CKIs, the SIAMESE RELATED proteins (SMRs), in the tolerance of plants to abiotic stresses. The SMRs could play a central role in adjusting the balance between growth and stress defenses in plants exposed to environmental stresses.

Identification of short protein-destabilizing sequences in Arabidopsis cyclin-dependent kinase inhibitors, ICKs

Plants have a family of cyclin-dependent kinase (CDK) inhibitors called interactors/inhibitors of CDK (ICKs) or Kip-related proteins (KRPs). ICK proteins have important functions in cell proliferation, endoreduplication, plant growth, and reproductive development, and their functions depend on the protein levels. However, understanding of how ICK protein levels are regulated is very limited. We fused Arabidopsis ICK sequences to green fluorescent protein (GFP) and determined their effects on the fusion proteins in plants, yeast, and Escherichia coli. The N-terminal regions of ICKs drastically reduced GFP fusion protein levels in Arabidopsis plants. A number of short sequences of 10-20 residues were found to decrease GFP fusion protein levels when fused at the N-terminus or C-terminus. Three of the four short sequences from ICK3 showed a similar function in yeast. Intriguingly, three short sequences from ICK1 and ICK3 caused the degradation of the fusion proteins in E. coli. In addition, computational analyses showed that ICK proteins were mostly disordered and unstructured except for the conserved C-terminal region, suggesting that ICKs are intrinsically disordered proteins. This study has identified a number of short protein-destabilizing sequences, and evidence suggests that some of them may cause protein degradation through structural disorder and instability.

QTL analysis of traits related to seed size and shape in sesame (Sesamum indicum L.)

Seed size and shape are important traits that determine seed yield in sesame. Understanding the genetic basis of seed size and shape is essential for improving the yield of sesame. In this study, F2 and BC1 populations were developed by crossing the Yuzhi 4 and Bengal small-seed (BS) lines for detecting the quantitative trait loci (QTLs) of traits related to seed size and shape. A total of 52 QTLs, including 13 in F2 and 39 in BC1 populations, for seed length (SL), seed width (SW), and length to width ratio (L/W) were identified, explaining phenotypic variations from 3.68 to 21.64%. Of these QTLs, nine stable major QTLs were identified in the two populations. Notably, three major QTLs qSL-LG3-2, qSW-LG3-2, and qSW-LG3-F2 that accounted for 4.94-16.34% of the phenotypic variations were co-localized in a 2.08 Mb interval on chromosome 1 (chr1) with 279 candidate genes. Three stable major QTLs qSL-LG6-2, qLW-LG6, and qLW-LG6-F2 that explained 8.14-33.74% of the phenotypic variations were co-localized in a 3.27 Mb region on chr9 with 398 candidate genes. In addition, the stable major QTL qSL-LG5 was co-localized with minor QTLs qLW-LG5-3 and qSW-LG5 to a 1.82 Mb region on chr3 with 195 candidate genes. Gene annotation, orthologous gene analysis, and sequence analysis indicated that three genes are likely involved in sesame seed development. These results obtained herein provide valuable in-formation for functional gene cloning and improving the seed yield of sesame.

HECT-type ubiquitin ligase KAKTUS mediates the proteasome-dependent degradation of cyclin-dependent kinase inhibitor KRP2 during trichome morphogenesis in Arabidopsis

Trichome development is a fascinating model to elaborate the plant cell differentiation and growth processes. A wealth of information has pointed to the contributions of the components associated with cell cycle control and ubiquitin/26S proteasome system (UPS) to trichome morphogenesis, but how these two pathways are connected remains obscure. Here, we report that HECT‐type ubiquitin ligase KAKTUS (KAK) targets the cyclin‐dependent kinase (CDK) inhibitor KRP2 (for kip‐related protein 2) for proteasome‐dependent degradation during trichome branching in Arabidopsis. We show that over‐expression of KRP2 promotes trichome branching and endoreduplication which is similar to kak loss of function mutants. KAK directly interacts with KRP2 and mediates KRP2 degradation. Mutation of KAK results in the accumulation of steady‐state KRP2. Consistently, in kak pKRP2:KRP2‐GFP plants, the trichome branching is further induced compared with the single mutant. Taken together, our studies bridge the cell cycle control and UPS pathways during trichome development and underscore the importance of post‐translational control in epidermal differentiation.

SCFOsFBK1 E3 ligase mediates jasmonic acid-induced turnover of OsATL53 and OsCCR14 to regulate lignification of rice anthers and roots

The rice F-box protein OsFBK1, which mediates the turnover of a cinnamoyl CoA-reductase, OsCCR14, has previously been shown to regulate anther and root lignification. Here, we identify OsATL53, a member of the ATL family of RING-H2 proteins that interacts with OsCCR14 in the cytoplasm. OsATL53 was identified in the same yeast two-hybrid library screening as reported previously for OsCCR14, and we show it to have cytoplasmic localization and E3 ligase ubiquitination properties. SCFOsFBK1 mediates turnover of OsATL53 in the cytoplasm and the nucleus, and that of OsCCR14 only in the nucleus, as shown by cell-free degradation assays. Confocal fluorescence lifetime imaging microscopy analyses demonstrate that in presence of jasmonic acid (JA), which plays a role in anther dehiscence, OsATL53-OsCCR14 undergoes conformational changes that trigger the complex to accumulate around the nuclear periphery and signals OsFBK1 to initiate degradation of the proteins in the respective cellular compartments. OsATL53 decreases the enzymatic activity of OsCCR14 and sequesters it in the cytoplasm, thereby regulating the lignification process. Transgenic rice with knockdown of OsATL53 display increased lignin deposition in the anthers and roots compared to the wild type, whilst knockdown of OsCCR14 results in decreased lignin content. Our results show that OsATL53 affects the activity of OsCCR14, and that their JA-induced degradation by SCFOsFBK1 regulates lignification of rice anthers and roots.

Spatiotemporal formation of the large vacuole regulated by the BIN2-VLG module is required for female gametophyte development in Arabidopsis

In Arabidopsis thaliana, female gametophyte (FG) development is accompanied by the formation and expansion of the large vacuole in the FG; this is essential for FG expansion, nuclear polar localization, and cell fate determination. Arabidopsis VACUOLELESS GAMETOPHYTES (VLG) facilitates vesicular fusion to form large vacuole in the FG, but the regulation of VLG remains largely unknown. Here, we found that gain-of-function mutation of BRASSINOSTEROID INSENSITIVE2 (BIN2) (bin2-1) increases VLG abundance to induce the vacuole formation at stage FG1, and leads to abortion of FG. Loss-of-function mutation of BIN2 and its homologs (bin2-3 bil1 bil2) reduced VLG abundance and mimicked vlg/VLG phenotypes. Knocking down VLG in bin2-1 decreased the ratio of aberrant vacuole formation at stage FG1, whereas FG1-specific overexpression of VLG mimicked the bin2-1 phenotype. VLG partially rescued the bin2-3 bil1 bil2 phenotype, demonstrating that VLG acts downstream of BIN2. Mutation of VLG residues that are phosphorylated by BIN2 altered VLG stability and a phosphorylation mimic of VLG causes similar defects as did bin2-1. Therefore, BIN2 may function by interacting with and phosphorylating VLG in the FG to enhance its stability and abundance, thus facilitating vacuole formation. Our findings provide mechanistic insight into how the BIN2-VLG module regulates the spatiotemporal formation of the large vacuole in FG development.

Molecular regulation of tomato male reproductive development

The reproductive success of flowering plants, which directly affects crop yield, is sensitive to environmental changes. A thorough understanding of how crop reproductive development adapts to climate changes is vital for ensuring global food security. In addition to being a high-value vegetable crop, tomato is also a model plant used for research on plant reproductive development. Tomato crops are cultivated under highly diverse climatic conditions worldwide. Targeted crosses of hybrid varieties have resulted in increased yields and abiotic stress resistance; however, tomato reproduction, especially male reproductive development, is sensitive to temperature fluctuations, which can lead to aborted male gametophytes, with detrimental effects on fruit set. We herein review the cytological features as well as genetic and molecular pathways influencing tomato male reproductive organ development and responses to abiotic stress. We also compare the shared features among the associated regulatory mechanisms of tomato and other plants. Collectively, this review highlights the opportunities and challenges related to characterizing and exploiting genic male sterility in tomato hybrid breeding programs.

Cytokinin regulates female gametophyte development by cell cycle modulation in Arabidopsis thaliana

Male and female gametophyte development, double fertilization, and embryogenesis are key to alternating generations in angiosperms. The female gametophyte of Arabidopsis is an eight-nucleate haploid structure developed from functional megaspores (FMs) through three flawless mitoses regulated by a series of cell cycle genes. Cytokinin, an important phytohormone, plays a critical role in the regulation of plant growth and development. However, the mechanisms by which cytokinins regulate female gametophyte development remain largely unknown. In this study, we constructed transgenic plants (pES1::CKX1) with low cytokinin levels in the embryo sac. Phenotypic analysis showed that pES1::CKX1 inhibits female gametophyte development. Microscopic observation revealed that female gametophyte development of pES1::CKX1 was delayed. The promoters of all cell cycle genes were cloned and transformed into wild-type (WT). We crossed these transgenic plants of cell cycle genes expressed in ovules with pES1::CKX1 and compared the expression level of β-glucuronidase (GUS) in pES1::CKX1 and WT. Many cell cycle-regulated genes were up or downregulated in pES1::CKX1 compared with WT, and the embryo sac development cell cycle in cycd2;1/+ cycd3;3 was defective. Our results demonstrated that cytokinin affects cell division in the female gametophyte by affecting the expression of cell cycle genes.

Arabidopsis RAN GTPases are critical for mitosis during male and female gametogenesis

The development of male and female gametophytes is a prerequisite for successful propagation of angiosperms. The small GTPases RAN play fundamental roles in numerous cellular processes. Although RAN GTPases have been characterized in plants, their roles in cellular processes are far from understood. We report here that RAN GTPases in Arabidopsis are critical for gametophytic development. RAN1 loss-of-function showed no defects in gametophytic development likely due to redundancy. However, the expression of a dominant negative or constitutively active RAN1 resulted in gametophytic lethality. Genetic interference of RAN GTPases caused the arrest of pollen mitosis I and of mitosis of functional megaspores, implying a key role of properly regulated RAN activity in mitosis during gametophytic development.

Phytochrome interacting factor 3 regulates pollen mitotic division through auxin signalling and sugar metabolism pathways in tomato

The development of viable pollen determines male fertility, and is crucial for reproduction in flowering plants. Phytochrome interacting factor 3 (PIF3) acts as a central regulator of plant growth and development, but its relationship with pollen development has not been determined. Through genetic, histological and transcriptomic analyses, we identified an essential role for SlPIF3 in regulating tomato (Solanum lycopersicum) pollen development. Knocking out SlPIF3 using clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 resulted in pollen mitosis I arrest, and a failure to form viable pollen. We further demonstrated that both glutamate synthase 1 (SlGLT1) and cell wall invertase 9 (SlCWIN9), involved in auxin and sugar homeostasis, respectively, colocalised with SlPIF3 in the anthers and were directly regulated by SlPIF3. Knockout of either SlGLT1 or SlCWIN9 phenocopied the pollen phenotype of SlPIF3 knockout (Slpif3) lines. Slpif3 fertility was partially restored by exogenous auxin indole-3-acetic acid in a dose-dependent manner. This study reveals a mechanism by which SlPIF3 regulates pollen development and highlights a new strategy for creating hormone-regulated genic male sterile lines for tomato hybrid seed production.

Cycling in a crowd: Coordination of plant cell division, growth, and cell fate

The reiterative organogenesis that drives plant growth relies on the constant production of new cells, which remain encased by interconnected cell walls. For these reasons, plant morphogenesis strictly depends on the rate and orientation of both cell division and cell growth. Important progress has been made in recent years in understanding how cell cycle progression and the orientation of cell divisions are coordinated with cell and organ growth and with the acquisition of specialized cell fates. We review basic concepts and players in plant cell cycle and division, and then focus on their links to growth-related cues, such as metabolic state, cell size, cell geometry, and cell mechanics, and on how cell cycle progression and cell division are linked to specific cell fates. The retinoblastoma pathway has emerged as a major player in the coordination of the cell cycle with both growth and cell identity, while microtubule dynamics are central in the coordination of oriented cell divisions. Future challenges include clarifying feedbacks between growth and cell cycle progression, revealing the molecular basis of cell division orientation in response to mechanical and chemical signals, and probing the links between cell fate changes and chromatin dynamics during the cell cycle.

Plant casein kinases phosphorylate and destabilize a cyclin-dependent kinase inhibitor to promote cell division

Cell cycle is one of the most fundamentally conserved biological processes of plants and mammals. Casein kinase1s (CK1s) are critical for cell proliferation in mammalian cells; however, how CK1s coordinate cell division in plants remains unknown. Through genetic and biochemical studies, here we demonstrated that plant CK1, Arabidopsis (Arabidopsis thaliana) EL1-like (AELs), regulate cell cycle/division by modulating the stability and inhibitory effects of Kip-related protein6 (KRP6) through phosphorylation. Cytological analysis showed that AELs deficiency results in suppressed cell-cycle progression mainly due to the decreased DNA replication rate at S phase and increased period of G2 phase. AELs interact with and phosphorylate KRP6 at serines 75 and 109 to stimulate KRP6's interaction with E3 ligases, thus facilitating the KRP6 degradation through the proteasome. These results demonstrate the crucial roles of CK1s/AELs in regulating cell division through modulating cell-cycle rates and elucidate how CK1s/AELs regulate cell division by destabilizing the stability of cyclin-dependent kinase inhibitor KRP6 through phosphorylation, providing insights into the plant cell-cycle regulation through CK1s-mediated posttranslational modification.

Parallel adaptation in autopolyploid Arabidopsis arenosa is dominated by repeated recruitment of shared alleles

Relative contributions of pre-existing vs de novo genomic variation to adaptation are poorly understood, especially in polyploid organisms. We assess this in high resolution using autotetraploid Arabidopsis arenosa, which repeatedly adapted to toxic serpentine soils that exhibit skewed elemental profiles. Leveraging a fivefold replicated serpentine invasion, we assess selection on SNPs and structural variants (TEs) in 78 resequenced individuals and discover significant parallelism in candidate genes involved in ion homeostasis. We further model parallel selection and infer repeated sweeps on a shared pool of variants in nearly all these loci, supporting theoretical expectations. A single striking exception is represented by TWO PORE CHANNEL 1, which exhibits convergent evolution from independent de novo mutations at an identical, otherwise conserved site at the calcium channel selectivity gate. Taken together, this suggests that polyploid populations can rapidly adapt to environmental extremes, calling on both pre-existing variation and novel polymorphisms.

Relative contributions of pre-existing versus de novo genomic variation to adaptation remain unclear. Here, the authors address this problem by examining the adaptation of autotetraploid Arabidopsis arenosa to serpentine soils and find that both types of variations contribute to rapid adaptation.

EMBRYO SAC DEVELOPMENT 1 affects seed setting rate in rice by controlling embryo sac development

Seed setting rate is one of the critical factors that determine rice yield. Grain formation is a complex biological process, whose molecular mechanism is yet to be improved. Here we investigated the function of an OVATE family protein, Embryo Sac Development 1 (ESD1), in the regulation of seed setting rate in rice (Oryza sativa) by examining its loss-of-function mutants generated via clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated9 (Cas9) technology. ESD1 was predominantly expressed at Stage 6 of panicle development, especially in the ovules. esd1 mutants displayed reduced seed setting rates with normal stamen development and pollen tube growth but abnormal pistil group. Investigation of embryo sacs revealed that during the mitosis of functional megaspores, some egg cells degraded during differentiation in esd1 mutants, thereby hindering subsequent fertilization process and reducing seed setting rate. In addition, the transcriptional level of O. sativa anaphase-promoting complex 6, a reported embryo sac developing gene, was significantly reduced in esd1 mutants. These results support that ESD1 is an important modulator of ESD and seed setting rate in rice. Together, this finding demonstrates that ESD1 positively regulates the seed setting rate by controlling ESD in rice and has implications for the improvement of rice yield.

Comparative transcriptome analysis reveals key insights into male sterility in Salvia miltiorrhiza Bunge

Background

Large-scale heterosis breeding depends upon stable, inherited male sterility lines. We accidentally discovered a male sterility line (SW-S) in the F₁progeny of a Salvia miltiorrhiza Bunge from Shandong, China (purple flowers) crossed with a S. miltiorrhiza f. alba from Sichuan, China (white flowers). We sought to provide insights into the pollen development for male sterility in S. miltiorrhiza.

Methods

The phenotypic and cytological features of the SW-S and fertile control SW-F were observed using scanning electron microscopy and paraffin sections to identify the key stage of male sterility. Transcriptome profiles were recorded for anthers at the tetrad stage of SW-S and SW-F using Illumina RNA-Seq.

Results

The paraffin sections showed that sterility mainly occurred at the tetrad stage of microspore development, during which the tapetum cells in the anther compartment completely fell off and gradually degraded in the sterile line. There was little-to-no callose deposited around the microspore cells. The tetrad microspore was shriveled and had abnormal morphology. Therefore, anthers at the tetrad stage of SW-S and fertile control SW-F were selected for comparative transcriptome analysis. In total, 266,722,270 clean reads were obtained from SW-S and SW-F, which contained 36,534 genes. There were 2,571 differentially expressed genes (DEGs) in SW-S and SW-F, of which 63.5% were downregulated. Gene Ontology (GO) enrichment analysis indicated that the differentially expressed genes were enriched in 56 functional groups (GO terms); of these, all DEGs involved in microgametogenesis and developmental maturation were downregulated in SW-S. These results were confirmed by quantitative RT-PCR. The two GO terms contained 18 DEGs, among which eight DEGs (namely: GPAT3, RHF1A, phosphatidylinositol, PFAS, MYB96, MYB78, Cals5, and LAT52) were related to gamete development. There were 10 DEGs related to development and maturation, among which three genes were directly related to pollen development (namely: ACT3, RPK2, and DRP1C). Therefore, we believe that these genes are directly or indirectly involved in the pollen abortion of SW-S. Our study provides insight into key genes related to sterility traits in S. miltiorrhiza, and the results can be further exploited in functional and mechanism studies.

The canonical α-SNAP is essential for gametophytic development in Arabidopsis

The development of male and female gametophytes is a pre-requisite for successful reproduction of angiosperms. Factors mediating vesicular trafficking are among the key regulators controlling gametophytic development. Fusion between vesicles and target membranes requires the assembly of a fusogenic soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs) complex, whose disassembly in turn ensures the recycle of individual SNARE components. The disassembly of post-fusion SNARE complexes is controlled by the AAA+ ATPase N-ethylmaleimide-sensitive factor (Sec18/NSF) and soluble NSF attachment protein (Sec17/α-SNAP) in yeast and metazoans. Although non-canonical α-SNAPs have been functionally characterized in soybeans, the biological function of canonical α-SNAPs has yet to be demonstrated in plants. We report here that the canonical α-SNAP in Arabidopsis is essential for male and female gametophytic development. Functional loss of the canonical α-SNAP in Arabidopsis results in gametophytic lethality by arresting the first mitosis during gametogenesis. We further show that Arabidopsis α-SNAP encodes two isoforms due to alternative splicing. Both isoforms interact with the Arabidopsis homolog of NSF whereas have distinct subcellular localizations. The presence of similar alternative splicing of human α-SNAP indicates that functional distinction of two α-SNAP isoforms is evolutionarily conserved.

The Arabidopsis R-SNARE protein YKT61 is essential for gametophyte development

Gametophyte development is a pre-requisite for plant reproduction and seed yield; therefore, studies of gametophyte development help us understand fundamental biological questions and have potential applications in agriculture. The biogenesis and dynamics of endomembrane compartments are critical for cell survival, and their regulatory mechanisms are just beginning to be revealed. Here, we report that the Arabidopsis thaliana SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) protein YKT61 is essential for both male and female gametogenesis. By using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-based genome editing, we demonstrated that male and female gametophytes carrying YKT61 loss-of-function alleles do not survive. Specifically, loss of YKT61 function resulted in the arrest of male gametophytic development at pollen mitosis I and the degeneration of female gametophytes. A three-base-pair deletion in YKT61 in the ykt61-3 mutant resulted in a single-amino acid deletion in the longin domain of YKT61; the resulting mutant protein does not interact with multiple SNAREs and showed substantially reduced membrane association, suggesting that the N-terminal longin domain of YKT61 plays multiple roles in its function. This study demonstrates that Arabidopsis YKT61 is essential for male and female gametogenesis and sets an example for functional characterization of essential genes with the combination of Cas9-mediated editing and expression from a Cas9-resistant transgene.

Heterochromatic silencing is reinforced by ARID1-mediated small RNA movement in Arabidopsis pollen

In the plant male germline, transposable elements (TEs) are reactivated in the companion vegetative nucleus, resulting in siRNA production and the intercellular movement of these siRNAs to reinforce TE silencing in sperm. However, the mechanism by which siRNA movement is regulated remains unexplored. Here we show that ARID1, a transcription factor which is constitutively expressed in the vegetative nucleus but dynamically accumulates in the generative cell (the progenitor of sperm) to promote the second pollen mitosis, mediates siRNA movement to reinforce heterochromatic silencing in the male germline. We looked for regulators involved in the accumulation of ARID1 in the generative cell, and found that AGO9, a germline-specific AGO in Arabidopsis, is required for the accumulation of ARID1 in the generative cell. Mutations in either ARID1 or AGO9 lead to the interruption of not only the second pollen mitosis but also the movement of siRNA from the vegetative nucleus to the male germline, resulting in the release of heterochromatic silencing in the male germline. Moreover, conditional knockdown of ARID1 in the generative cell causes reduced heterochromatic silencing in both bicellular and mature pollen. This study provides insights into how a spatiotemporal transcription factor coordinates heterochromatic silencing and male germline maturation.

Identification and characterization of a female gametophyte defect in sdk1-7 +/- abi3-6 +/- heterozygotes of Arabidopsis thaliana

Successful reproduction in angiosperms is dependent on the highly synchronous development of their male and female gametophytes and the ensuing fusion of the gametes from these reproductive tissue types. When crossing a T-DNA insertion line sdk1-7-/-(Salk_024564), one of the S-domain receptor kinases involved in ABA responses with a fast neutron deletion line abi3-6-/-, the F1 heterozygotes (sdk1-7+/-abi3-6 +/-) displayed 50% ovule abortion suggesting a likely gametophytic defects. We identified and characterized an early stage female gametophyte developmental defect in the heterozygous mutant ovules. Recombination frequency analysis of the F2 progenies from selfed heterozygotes revealed a possible pseudo-linkage of sdk1-7 and abi3-6 suggesting a reciprocal translocation event in the heterozygote. Our study emphasizes the importance of robust analysis to distinguish gametophytic defect phenotypes caused by genetic interactions and that resulting from possible chromosomal translocation events.

Gametophyte-specific DEAD-box RNA helicase 29 is required for functional maturation of male and female gametophytes in Arabidopsis

The maturation of male and female gametophytes together with its impact on plant sexual reproduction has not received much attention, and the molecular mechanisms underlying the process are largely unknown. Here, we show that Arabidopsis DEAD-box RNA helicase 29 (RH29) is critical for the functional maturation of both male and female gametophytes. Homozygous rh29 mutants could not be obtained, and heterozygous mutant plants were semi-sterile. Progression of the cell cycle in rh29 female gametophytes was delayed. Delayed pollination experiments showed that rh29 female gametophytes underwent cell-fate specification but were unable to develop into functional gametophytes. Functional specification but not morphogenesis was also disrupted in rh29 male gametophytes, causing defective pollen tube growth in the pistil. RH29 was highly and specifically expressed in gametophytic cells. RH29 shares high amino acid sequence identity with yeast Dbp10p, which partially rescues the aborted-ovules phenotype of rh29/RH29 plants. RH29 is essential for the synthesis of REGULATORY PARTICLE TRIPLE A ATPase 5a (RPT5a), a subunit of the regulatory particle of the 26S proteasome. Our results suggest that gametophyte functional maturation is a necessary process for successful fertilization and that RH29 is essential for the functional maturation of both male and female gametophytes.

Arabidopsis RAD23B regulates pollen development by mediating degradation of KRP1

The ubiquitin (Ub)/26S proteasome system (UPS) plays a key role in plant growth, development, and survival by directing the turnover of numerous regulatory proteins. In the UPS, the ubiquitin-like (UBL) and ubiquitin-associated (UBA) domains function as hubs for ubiquitin-mediated protein degradation. Radiation sensitive 23 (RAD23), which has been identified as a UBL/UBA protein, contributes to the progression of the cell cycle, stress responses, ER proteolysis, and DNA repair. Here, we report that pollen development is arrested at the microspore stage in a rad23b null mutant. We demonstrate that RAD23B can directly interact with KIP-related protein 1 (KRP1) through its UBL-UBA domains. In addition, plants overexpressing KRP1 have defects in pollen development, which is a phenotype similar to the rad23b mutant. RAD23B promotes the degradation of KRP1 in vivo, which is accumulated following treatment with the proteasome inhibitor MG132. Our results indicate that RAD23B plays an important in pollen development by controlling the turnover of the key cell cycle protein, KRP1.

Genome-Wide Association Analysis Identifies Candidate Genes Regulating Seed Number Per Silique in Arabidopsis thaliana

Seed weight and number ultimately determine seed yield. Arabidopsis seed number comprised of silique number and seed number per silique (SNS). Comparing seed development and weight, determinants of seed number remain largely uncharacterized. In this study, taking advantage of 107 available Arabidopsis accessions, genome-wide association analysis (GWAS) was employed to identify the candidate genes regulating SNS. GWAS-based genotype and phenotype association analysis identified 38 most significant SNPs marker sites that were mapped to specific chromosomal positions and allowed us to screen for dozens of candidate genes. One of them (PIN3) was selected for functional validation based on gene expression analysis. It is a positive regulator of Arabidopsis SNS. Although silique length of PIN3 loss of function mutant was not significantly changed, its SNS and seed density (SD) were significantly reduced as compared with the wild type. Notably, PIN3 overexpression lines driven by a placenta-specific promoter STK exhibited significantly shorter siliques, slightly reduced SNS, but significant increased SD compared with wild type, suggesting that PIN3 positively regulates SD through inducing ovule primordia initiation regardless of the placenta size. Ovule initiation determines the maximal possibility of SNS, and new genes and mechanism regulating SNS through modulating ovule initiation is worth further investigated.

Genetic and signalling pathways of dry fruit size: targets for genome editing-based crop improvement

Fruit is seed-bearing structures specific to angiosperm that form from the gynoecium after flowering. Fruit size is an important fitness character for plant evolution and an agronomical trait for crop domestication/improvement. Despite the functional and economic importance of fruit size, the underlying genes and mechanisms are poorly understood, especially for dry fruit types. Improving our understanding of the genomic basis for fruit size opens the potential to apply gene-editing technology such as CRISPR/Cas to modulate fruit size in a range of species. This review examines the genes involved in the regulation of fruit size and identifies their genetic/signalling pathways, including the phytohormones, transcription and elongation factors, ubiquitin-proteasome and microRNA pathways, G-protein and receptor kinases signalling, arabinogalactan and RNA-binding proteins. Interestingly, different plant taxa have conserved functions for various fruit size regulators, suggesting that common genome edits across species may have similar outcomes. Many fruit size regulators identified to date are pleiotropic and affect other organs such as seeds, flowers and leaves, indicating a coordinated regulation. The relationships between fruit size and fruit number/seed number per fruit/seed size, as well as future research questions, are also discussed.

Integrative transcriptomics and proteomics analysis constructs a new molecular model for ovule abortion in the female-sterile line of Pinus tabuliformis Carr

Ovule development is critical to plant reproduction and free nuclear mitosis of megagametophyte (FNMM) is vital for ovule development. However, most results of ovule development were based on the studies in angiosperms, and its molecular regulation remained largely unknown in gymnosperms, particularly, during FNMM. In this context, we studied the genome-wide difference between sterile line (SL) and fertile line (FL) ovules using transcriptomics and proteomics approaches in Pinus tabuliformis Carr. Comparative analyses revealed that genes involved in DNA replication, DNA damage repair, Cell cycle, Apoptosis and Energy metabolism were highlighted. Further results showed the low expressions of MCM 2-7, RRM1, etc. perhaps led to abnormal DNA replication and damage repair, and the significantly different expressions of PARP2, CCs1, CCs3, etc. implied that the accumulated DNA double-stranded breaks were failed to be repaired and the cell cycle was arrested at G2/M in SL ovules, potentially resulting in the occurrence of apoptosis. Moreover, the deficiency of ETF-QO might hinder FNMM. Consequently, FNMM stopped and ovule aborted in SL ovules. Our results suggested a selective regulatory mechanism led to FNMM half-stop and ovule abortion in P. tabuliformis and these insights could be exploited to investigate the molecular regulations of ovule development in woody gymnosperms.

Arabidopsis KETCH1 Is Critical for the Nuclear Accumulation of Ribosomal Proteins and Gametogenesis

Male and female gametophytes are generated from micro- or megaspore mother cells through consecutive meiotic and mitotic cell divisions. Defects in these divisions often result in gametophytic lethality. Gametophytic lethality was also reported when genes encoding ribosome-related proteins were mutated. Although numerous ribosomal proteins (RPs) have been identified in plants based on homology with their yeast and metazoan counterparts, how RPs are regulated, e.g., through dynamic subcellular targeting, is unknown. We report here that an Arabidopsis (Arabidopsis thaliana) importin β, KETCH1 (karyopherin enabling the transport of the cytoplasmic HYL1), is critical for gametogenesis. Karyopherins are molecular chaperones mediating nucleocytoplasmic protein transport. However, the role of KETCH1 during gametogenesis is independent of HYPONASTIC LEAVES 1 (HYL1), a previously reported KETCH1 cargo. Instead, KETCH1 interacts with several RPs and is critical for the nuclear accumulation of RPL27a, whose mutations caused similar gametophytic defects. We further showed that knocking down KETCH1 caused reduced ribosome biogenesis and translational capacity, which may trigger the arrest of mitotic cell cycle progression and lead to gametophytic lethality.

Chloroplast-Localized Protoporphyrinogen IX Oxidase1 Is Involved in the Mitotic Cell Cycle in Arabidopsis

Protoporphyrinogen IX oxidase1 (PPO1) catalyzes the oxidation of protoporphyrinogen IX to form protoporphyrin IX in the plastid tetrapyrrole biosynthesis pathway and is also essential for plastid RNA editing in Arabidopsis thaliana. The Arabidopsis ppo1-1 mutation was previously shown to be seedling lethal; however, in this study, we showed that the heterozygous ppo1-1/+ mutant exhibited reproductive growth defects characterized by reduced silique length and seed set, as well as aborted pollen development. In this mutant, the second mitotic division was blocked during male gametogenesis, whereas female gametogenesis was impaired at the one-nucleate stage. Before perishing at the seedling stage, the homozygous ppo1-1 mutant displayed reduced hypocotyl and root length, increased levels of reactive oxygen species accumulation and elevated cell death, especially under light conditions. Wild-type seedlings treated with acifluorfen, a PPO1 inhibitor, showed similar phenotypes to the ppo1-1 mutants, and both plants possessed a high proportion of 2C nuclei and a low proportion of 8C nuclei compared with the untreated wild type. Genome-wide RNA-seq analysis showed that a number of genes, including cell cycle-related genes, were differentially regulated by PPO1. Consistently, PPO1 was highly expressed in the pollen, anther, pistil and root apical meristem cells actively undergoing cell division. Our study reveals a role for PPO1 involved in the mitotic cell cycle during gametogenesis and seedling development.

Global transcriptome and gene co-expression network analyses on the development of distyly in Primula oreodoxa

Distyly is a genetically controlled flower polymorphism that has intrigued both botanists and evolutionary biologists ever since Darwin's time. Despite extensive reports on the pollination and evolution of distylous systems, the genetic basis and mechanism of molecular regulation remain unclear. In the present study, comparative transcriptome profiling was conducted in primrose (Primula oreodoxa), the prime research model for heterostyly. Thirty-six transcriptomes were sequenced for styles at different stages and corolla tube in the three morphs of P. oreodoxa. Large numbers of differentially expressed genes (DEGs) were detected in the transcriptomes of styles across different morphs. Several transcription factors (TFs) and phytohormone metabolism-related genes were highlighted in S-morphs. A growing number of genes showed differential expression patterns along with the development of styles, suggesting that the genetic control of distyly may be more complicated than ever expected. Analysis of co-expression networks and module-trait relationships identified modules significantly associated with style development. CYP734A50, a key S-locus gene whose products degrade brassinosteroids, was co-expressed with many genes in the module and showed significant negative association with style length. In addition, crucial TFs involved in phytohormone signaling pathways were found to be connected with CYP734A50 in the co-expression module. Our global transcriptomic analysis has identified DEGs that are potentially involved in regulation of style length in P. oreodoxa, and may shed light on the evolution and broad biological processes of heterostyly.

BICELLULAR POLLEN 1 is a modulator of DNA replication and pollen development in Arabidopsis

During male gametogenesis in Arabidopsis, the haploid microspore undergoes an asymmetric division to produce a vegetative and a generative cell, the latter of which continues to divide symmetrically to form two sperms. This simple system couples cell cycle with cell fate specification. Here we addressed the role of DNA replication in male gametogenesis using a mutant bicellular pollen 1 (bice1), which produces bicellular, rather than tricellular, pollen grains as in the wild-type plant at anthesis. The mutation prolonged DNA synthesis of the generative cell, which resulted in c. 40% of pollen grains arrested at the two-nucleate stage. The extended S phase did not impact the cell fate of the generative cell as shown by cell-specific markers. BICE1 encodes a plant homolog of human D123 protein that is required for G1 progression, but the underlying mechanism is unknown. Here we showed that BICE1 interacts with MCM4 and MCM7 of the pre-replication complex. Consistently, double mutations in BICE1 and MCM4, or MCM7, also led to bicellular pollen and condensed chromosomes. These suggest that BICE1 plays a role in modulating DNA replication via interaction with MCM4 and MCM7.

Genetic and transcriptomic dissection of the fiber length trait from a cotton (Gossypium hirsutum L.) MAGIC population

BACKGROUND

Improving cotton fiber length without reducing yield is one of the major goals of cotton breeding. However, genetic improvement of cotton fiber length by breeding has been a challenge due to the narrow genetic diversity of modern cotton cultivars and negative correlations between fiber quality and yield traits. A multi-parent advanced generation inter-cross (MAGIC) population developed through random mating provides an excellent genetic resource that allows quantitative trait loci (QTL) and causal genes to be identified.

RESULTS

An Upland cotton MAGIC population, consisting of 550 recombinant inbred lines (RILs) derived from eleven different cultivars, was used to identify fiber length QTLs and potential genes that contribute to longer fibers. A genome wide association study (GWAS) identified a cluster of single nucleotide polymorphisms (SNPs) on chromosome (Chr.) D11 that is significantly associated with fiber length. Further evaluation of the Chr. D11 genomic region among lines of the MAGIC population detected that 90% of RILs have a D11 haplotype similar to the reference TM-1 genome (D11-ref), whereas 10% of RILs inherited an alternative haplotype from one of the parents (D11-alt). The average length of fibers of D11-alt RILs was significantly shorter compared to D11-ref RILs, suggesting that alleles in the D11-alt haplotype contributed to the inferior fiber quality. RNAseq analysis of the longest and shortest fiber length RILs from D11-ref and D11-alt populations identified 949 significantly differentially expressed genes (DEGs). Gene set enrichment analysis revealed that different functional categories of genes were over-represented during fiber elongation between the four selected RILs. We found 12 genes possessing non-synonymous SNPs (nsSNPs) significantly associated with the fiber length, and three that were highly significant and were clustered at D11:24-Mb, including D11G1928, D11G1929 and D11G1931.

CONCLUSION

The results of this study provide insights into molecular aspects of genetic variation in fiber length and suggests candidate genes for genetic manipulation for cotton improvement.

Genetic and transcriptomic dissection of the fiber length trait from a cotton (Gossypium hirsutum L.) MAGIC population

Background

Improving cotton fiber length without reducing yield is one of the major goals of cotton breeding. However, genetic improvement of cotton fiber length by breeding has been a challenge due to the narrow genetic diversity of modern cotton cultivars and negative correlations between fiber quality and yield traits. A multi-parent advanced generation inter-cross (MAGIC) population developed through random mating provides an excellent genetic resource that allows quantitative trait loci (QTL) and causal genes to be identified.

Results

An Upland cotton MAGIC population, consisting of 550 recombinant inbred lines (RILs) derived from eleven different cultivars, was used to identify fiber length QTLs and potential genes that contribute to longer fibers. A genome wide association study (GWAS) identified a cluster of single nucleotide polymorphisms (SNPs) on chromosome (Chr.) D11 that is significantly associated with fiber length. Further evaluation of the Chr. D11 genomic region among lines of the MAGIC population detected that 90% of RILs have a D11 haplotype similar to the reference TM-1 genome (D11-ref), whereas 10% of RILs inherited an alternative haplotype from one of the parents (D11-alt). The average length of fibers of D11-alt RILs was significantly shorter compared to D11-ref RILs, suggesting that alleles in the D11-alt haplotype contributed to the inferior fiber quality. RNAseq analysis of the longest and shortest fiber length RILs from D11-ref and D11-alt populations identified 949 significantly differentially expressed genes (DEGs). Gene set enrichment analysis revealed that different functional categories of genes were over-represented during fiber elongation between the four selected RILs. We found 12 genes possessing non-synonymous SNPs (nsSNPs) significantly associated with the fiber length, and three that were highly significant and were clustered at D11:24-Mb, including D11G1928, D11G1929 and D11G1931.

Conclusion

The results of this study provide insights into molecular aspects of genetic variation in fiber length and suggests candidate genes for genetic manipulation for cotton improvement.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5427-5) contains supplementary material, which is available to authorized users.

Multiple mechanisms explain how reduced KRP expression increases leaf size of Arabidopsis thaliana

Although cell number generally correlates with organ size, the role of cell cycle control in growth regulation is still largely unsolved. We studied kip related protein (krp) 4, 6 and 7 single, double and triple mutants of Arabidopsis thaliana to understand the role of cell cycle inhibitory proteins in leaf development. We performed leaf growth and seed size analysis, kinematic analysis, flow cytometery, transcriptome analysis and mathematical modeling of G1/S and G2/M checkpoint progression of the mitotic and endoreplication cycle. Double and triple mutants progressively increased mature leaf size, because of elevated expression of cell cycle and DNA replication genes stimulating progression through the division and endoreplication cycle. However, cell number was also already increased before leaf emergence, as a result of an increased cell number in the embryo. We show that increased embryo and seed size in krp4/6/7 results from seed abortion, presumably reducing resource competition, and that seed size differences contribute to the phenotype of several large-leaf mutants. Our results provide a new mechanistic understanding of the role of cell cycle regulation in leaf development and highlight the contribution of the embryo to the development of leaves after germination in general.

REM34 and REM35 Control Female and Male Gametophyte Development in Arabidopsis thaliana

The REproductive Meristem (REM) gene family encodes for transcription factors belonging to the B3 DNA binding domain superfamily. In Arabidopsis thaliana, the REM gene family is composed of 45 members, preferentially expressed during flower, ovule, and seed developments. Only a few members of this family have been functionally characterized: VERNALIZATION1 (VRN1) and, most recently, TARGET OF FLC AND SVP1 (TFS1) regulate flowering time and VERDANDI (VDD), together with VALKYRIE (VAL) that control the death of the receptive synergid cell in the female gametophyte. We investigated the role of REM34, REM35, and REM36, three closely related and linked genes similarly expressed in both female and male gametophytes. Simultaneous silencing by RNA interference (RNAi) caused about 50% of the ovules to remain unfertilized. Careful evaluation of both ovule and pollen developments showed that this partial sterility of the transgenic RNAi lines was due to a postmeiotic block in both female and male gametophytes. Furthermore, protein interaction assays revealed that REM34 and REM35 interact, which suggests that they work together during the first stages of gametogenesis.

Repertoire of plant RING E3 ubiquitin ligases revisited: New groups counting gene families and single genes

E3 ubiquitin ligases of the ubiquitin proteasome system (UPS) mediate recognition of substrates and later transfer the ubiquitin (Ub). They are the most expanded components of the system. The Really Interesting New Gene (RING) domain contains 40-60 residues that are highly represented among E3 ubiquitin ligases. The Arabidopsis thaliana E3 ubiquitin ligases with a RING finger primarily contain RING-HC or RING-H2 type domains or less frequently RING-v, RING-C2, RING-D, RING-S/T and RING-G type domains. Our previous work on three E3 ubiquitin ligase families with a RING-H2 type domain, ATL, BTL, and CTL, suggested that a phylogenetic distribution based on the RING domain allowed for the creation a catalog of known domains or unknown conserved motifs. This work provided a useful and comprehensive view of particular families of RING E3 ubiquitin ligases. We updated the annotation of A. thaliana RING proteins and surveyed RING proteins from 30 species across eukaryotes. Based on domain architecture profile of the A. thaliana proteins, we catalogued 4711 RING finger proteins into 107 groups, including 66 previously described gene families or single genes and 36 novel families or undescribed genes. Forty-four groups were specific to a plant lineage while 41 groups consisted of proteins found in all eukaryotic species. Our present study updates the current classification of plant RING finger proteins and reiterates the importance of these proteins in plant growth and adaptation.

RNA-seq Analysis Reveals Gene Expression Profiling of Female Fertile and Sterile Ovules of PinusTabulaeformis Carr. during Free Nuclear Mitosis of the Female Gametophyte

The development of the female gametophyte (FG) is one of the key processes of life cycle alteration between the haploid gametophyte and the diploid sporophytes in plants and it is required for successful seed development after fertilization. It is well demonstrated that free nuclear mitosis (FNM) of FG is crucial for the development of the ovule. However, studies of the molecular mechanism of ovule and FG development focused mainly on angiosperms, such as Arabidopsis thaliana and further investigation of gymnosperms remains to be completed. Here, Illumina sequencing of six transcriptomic libraries obtained from developing and abortive ovules at different stages during free nuclear mitosis of magagametophyte (FNMM) was used to acquire transcriptome data and gene expression profiles of Pinus tabulaeformis. Six cDNA libraries generated a total of 71.0 million high-quality clean reads that aligned with 63,449 unigenes and the comparison between developing and abortive ovules identified 7174 differentially expressed genes (DEGs). From the functional annotation results, DEGs involved in the cell cycle and phytohormone regulation were highlighted to reveal their biological importance in ovule development. Furthermore, validation of DEGs from the phytohormone signal transduction pathway was performed using quantitative real-time PCR analysis, revealing the dynamics of transcriptional networks and potential key components in the regulation of FG development in P. tabulaeformis were identified. These findings provide new insights into the regulatory mechanisms of ovule development in woody gymnosperms.

Auxin production in diploid microsporocytes is necessary and sufficient for early stages of pollen development

Gametophytic development in Arabidopsis depends on nutrients and cell wall materials from sporophytic cells. However, it is not clear whether hormones and signaling molecules from sporophytic tissues are also required for gametophytic development. Herein, we show that auxin produced by the flavin monooxygenases YUC2 and YUC6 in the sporophytic microsporocytes is essential for early stages of pollen development. The first asymmetric mitotic division (PMI) of haploid microspores is the earliest event in male gametophyte development. Microspore development in yuc2yuc6 double mutants arrests before PMI and consequently yuc2yuc6 fail to produce viable pollens. Our genetic analyses reveal that YUC2 and YUC6 act as sporophytic genes for pollen formation. We further show that ectopic production of auxin in tapetum, which provides nutrients for pollen development, fails to rescue the sterile phenotypes of yuc2yuc6. In contrast, production of auxin in either microsporocytes or microspores rescued the defects of pollen development in yuc2yuc6 double mutants. Our results demonstrate that local auxin biosynthesis in sporophytic microsporocytic cells and microspore controls male gametophyte development during the generation transition from sporophyte to male gametophyte.

Plant life cycle alternates between the diploid sporophyte generation and the haploid gametophyte generation. Understanding the molecular mechanisms governing the generation alternation impacts fundamental plant biology and plant breeding. It is known that the development of haploid generation in vascular plants requires the diploid tapetum cells to supply nutrients. Here we show that the male gametophyte (haploid) development in Arabidopsis requires auxin produced in the diploid microsporocytic cells. Moreover, we show that auxin produced in microsporocytic cells and microspore is also sufficient to support normal development of the haploid microspores. This work demonstrates that Arabidopsis uses two different diploid cell types to supply growth hormone and nutrients for the growth of the haploid generation.

SIAMESE-RELATED1 Is Regulated Posttranslationally and Participates in Repression of Leaf Growth under Moderate Drought

The plant cell cycle is tightly regulated by factors that integrate endogenous cues and environmental signals to adapt plant growth to changing conditions. Under drought, cell division in young leaves is blocked by an active mechanism, reducing the evaporative surface and conserving energy resources. The molecular function of cyclin-dependent kinase-inhibitory proteins (CKIs) in regulating the cell cycle has already been well studied, but little is known about their involvement in cell cycle regulation under adverse growth conditions. In this study, we show that the transcript of the CKI gene SIAMESE-RELATED1 (SMR1) is quickly induced under moderate drought in young Arabidopsis (Arabidopsis thaliana) leaves. Functional characterization further revealed that SMR1 inhibits cell division and affects meristem activity, thereby restricting the growth of leaves and roots. Moreover, we demonstrate that SMR1 is a short-lived protein that is degraded by the 26S proteasome after being ubiquitinated by a Cullin-RING E3 ubiquitin ligase. Consequently, overexpression of a more stable variant of the SMR1 protein leads to a much stronger phenotype than overexpression of the native SMR1. Under moderate drought, both the SMR1 transcript and SMR1 protein accumulate. Despite this induction, smr1 mutants do not show overall tolerance to drought stress but do show less growth inhibition of young leaves under drought. Surprisingly, the growth-repressive hormone ethylene promotes SMR1 induction, but the classical drought hormone abscisic acid does not.

Arabidopsis ICK/KRP cyclin-dependent kinase inhibitors function to ensure the formation of one megaspore mother cell and one functional megaspore per ovule

In most plants, the female germline starts with the differentiation of one megaspore mother cell (MMC) in each ovule that produces four megaspores through meiosis, one of which survives to become the functional megaspore (FM). The FM further develops into an embryo sac. Little is known regarding the control of MMC formation to one per ovule and the selective survival of the FM. The ICK/KRPs (interactor/inhibitor of cyclin-dependent kinase (CDK)/Kip-related proteins) are plant CDK inhibitors and cell cycle regulators. Here we report that in the ovules of Arabidopsis mutant with all seven ICK/KRP genes inactivated, supernumerary MMCs, FMs and embryo sacs were formed and the two embryo sacs could be fertilized to form two embryos with separate endosperm compartments. Twin seedlings were observed in about 2% seeds. Further, in the mutant ovules the number and position of surviving megaspores from one MMC were variable, indicating that the positional signal for determining the survival of megaspore was affected. Strikingly, ICK4 fusion protein with yellow fluorescence protein was strongly present in the degenerative megaspores but absent in the FM, suggesting an important role of ICKs in the degeneration of non-functional megaspores. The absence of or much weaker phenotypes in lower orders of mutants and complementation of the septuple mutant by ICK4 or ICK7 indicate that multiple ICK/KRPs function redundantly in restricting the formation of more than one MMC and in the selective survival of FM, which are critical to ensure the development of one embryo sac and one embryo per ovule.

In most plants, the female germline starts with the differentiation of one megaspore mother cell (MMC) in each ovule that produces multiple megaspores through meiosis. One of the megaspores in a fixed position survives to become the functional megaspore (FM) while the other megaspores undergo degeneration. The FM further develops into an embryo sac. We have been working on the functions and regulation of a family of plant cyclin-dependent kinase inhibitors called ICKs or KRPs. We observed that in the ovules of Arabidopsis mutant with all seven ICK/KRP genes inactivated, multiple MMCs, FMs and embryo sacs were formed, and the embryo sacs could be fertilized to produce two embryos with separate endosperm compartments. Further, in mutant ovules the number and position of surviving megaspores from one MMC were variable and ICK4-YFP (yellow fluorescence protein) fusion protein was strongly expressed in the degenerative megaspores but absent in the FM. Those findings together with other results in our study indicate that multiple ICK/KRPs function redundantly in controlling the formation of one MMC per ovule and also in the degeneration of non-functional megaspores, which are critical for the subsequent development of one embryo sac per ovule and one embryo per seed.

Two SUMO Proteases SUMO PROTEASE RELATED TO FERTILITY1 and 2 Are Required for Fertility in Arabidopsis

In plants, the posttranslational modification small ubiquitin-like modifier (SUMO) is involved in regulating several important developmental and cellular processes, including flowering time control and responses to biotic and abiotic stresses. Here, we report two proteases, SUMO PROTEASE RELATED TO FERTILITY1 (SPF1) and SPF2, that regulate male and female gamete and embryo development and remove SUMO from proteins in vitro and in vivo. spf1 mutants exhibit abnormal floral structures and embryo development, while spf2 mutants exhibit largely a wild-type phenotype. However, spf1 spf2 double mutants exhibit severe abnormalities in microgametogenesis, megagametogenesis, and embryo development, suggesting that the two genes are functionally redundant. Mutation of SPF1 and SPF2 genes also results in misexpression of generative- and embryo-specific genes. In vitro, SPF1 and SPF2 process SUMO1 precursors into a mature form, and as expected in vivo, spf1 and spf2 mutants accumulate SUMO conjugates. Using a yeast two-hybrid screen, we identified EMBRYO SAC DEVELOPMENT ARREST9 (EDA9) as an SPF1-interacting protein. In vivo, we demonstrate that EDA9 is sumolyated and that, in spf1 mutants, EDA9-SUMO conjugates increase in abundance, demonstrating that EDA9 is a substrate of SPF1. Together, our results demonstrate that SPF1 and SPF2 are two SUMO proteases important for plant development in Arabidopsis (Arabidopsis thaliana).

SNAIL1 is essential for female gametogenesis in Arabidopsis thaliana

Two yeast Brix family members Ssf1 and Ssf2, involved in large ribosomal subunit synthesis, are essential for yeast cell viability and mating efficiency. Their putative homologs exist in the Arabidopsis genome; however, their role in plant development is unknown. Here, we show that Arabidopsis thaliana SNAIL1 (AtSNAIL1), a protein sharing high sequence identity with yeast Ssf1 and Ssf2, is critical to mitosis progression of female gametophyte development. The snail1 homozygous mutant was nonviable and its heterozygous mutant was semi-sterile with shorter siliques. The mutation in SNAIL1 led to absence of female transmission and reduced male transmission. Further phenotypic analysis showed that the synchronic development of female gametophyte in the snail1 heterozygous mutant was greatly impaired and the snail1 pollen tube growth, in vivo, was also compromised. Furthermore, SNAIL1 was a nucleolar-localized protein with a putative role in protein synthesis. Our data suggest that SNAIL1 may function in ribosome biogenesis like Ssf1 and Ssf2 and plays an important role during megagametogenesis in Arabidopsis.

Exploiting cell cycle inhibitor genes of the KRP family to control root-knot nematode induced feeding sites in plants

Cell cycle control in galls provoked by root-knot nematodes involves the activity of inhibitor genes like the Arabidopsis ICK/KRP members. Ectopic KRP1, KRP2 and KRP4 expression resulted in decreased gall size by inhibiting mitotic activity, whereas KRP6 induces mitosis in galls. Herein, we investigate the role of KRP3, KRP5 and KRP7 during gall development and compared their role with previously studied members of this class of cell cycle inhibitors. Overexpression of KRP3 and KRP7 culminated in undersized giant cells, with KRP3^OE galls presenting peculiar elongated giant cells. Nuclei in KRP3^OE and KRP5^OE lines presented a convoluted and apparently connected phenotype. This appearance may be associated with the punctuated protein nuclear localization driven by specific common motifs. As well, ectopic expression of KRP3^OE and KRP5^OE affected nematode development and offspring. Decreased mitotic activity in galls of KRP3^OE and KRP7^OE lines led to a reduced gall size which presented distinct shapes - from more elongated like in the KRP3^OE line to small rounded like in the KRP7^OE line. Results presented strongly support the idea that induced expression of cell cycle inhibitors such as KRP3 and KRP7 in galls can be envisaged as a conceivable strategy for nematode feeding site control in crop species attacked by phytopathogenic nematodes.

Anther-preferential expressing gene PMR is essential for the mitosis of pollen development in rice

Phenotype identification, expression examination, and function prediction declared that the anther-preferential expressing gene PMR may participate in regulation of male gametophyte development in rice. Male germline development in flowering plants produces the pair of sperm cells for double fertilization and the pollen mitosis is a key process of it. Although the structural features of male gametophyte have been defined, the molecular mechanisms regulating the mitotic cell cycle are not well elucidated in rice. Here, we reported an anther-preferential expressing gene in rice, PMR (Pollen Mitosis Relative), playing an essential role in male gametogenesis. When PMR gene was suppressed via RNAi, the mitosis of microspore was severely damaged, and the plants formed unmatured pollens containing only one or two nucleuses at the anthesis, ultimately leading to serious reduction of pollen fertility and seed-setting. The CRISPR mutants, pmr-1 and pmr-2, both showed the similar defects as the PMR-RNAi lines. Further analysis revealed that PMR together with its co-expressing genes were liable to participate in the regulation of DNA metabolism in the nucleus, and affected the activities of some enzymes related to the cell cycle. We finally discussed that unknown protein PMR contained the PHD, SWIB and Plus-3 domains and they might have coordinating functions in regulation pathway of the pollen mitosis in rice.