Different Temporal and Spatial Gene Expression Patterns Occur during Anther Development

The transcription factors and pathways underpinning male reproductive development in Arabidopsis

As Arabidopsis flowers mature, specialized cells within the anthers undergo meiosis, leading to the production of haploid microspores that differentiate into mature pollen grains, each containing two sperm cells for double fertilization. During pollination, the pollen grains are dispersed from the anthers to the stigma for subsequent fertilization. Transcriptomic studies have identified a large number of genes expressed over the course of male reproductive development and subsequent functional characterization of some have revealed their involvement in floral meristem establishment, floral organ growth, sporogenesis, meiosis, microsporogenesis, and pollen maturation. These genes encode a plethora of proteins, ranging from transcriptional regulators to enzymes. This review will focus on the regulatory networks that control male reproductive development, starting from flower development and ending with anther dehiscence, with a focus on transcription factors and some of their notable target genes.

Introduction of barnase/barstar in soybean produces a rescuable male sterility system for hybrid breeding

Hybrid breeding for increased vigour has been used for over a century to boost agricultural outputs without requiring higher inputs. While this approach has led to some of the most substantial gains in crop productivity, breeding barriers have fundamentally limited soybean (Glycine max) from reaping the benefits of hybrid vigour. Soybean flowers self-pollinate prior to opening and thus are not readily amenable to outcrossing. In this study, we demonstrate that the barnase/barstar male sterility/rescue system can be used in soybean to produce hybrid seeds. By expressing the cytotoxic ribonuclease, barnase, under a tapetum-specific promoter in soybean anthers, we are able to completely block pollen maturation, creating male sterile plants. We show that fertility can be rescued in the F1 generation of these barnase-expressing lines when they are crossed with pollen from plants that express the barnase inhibitor, barstar. Importantly, we found that the successful rescue of male fertility is dependent on the relative dosage of barnase and barstar. When barnase and barstar were expressed under the same tapetum-specific promoter, the F1 offspring remained male sterile. When we expressed barstar under a relatively stronger promoter than barnase, we were able to achieve a successful rescue of male fertility in the F1 generation. This work demonstrates the successful implementation of a biotechnology approach to produce fertile hybrid offspring in soybean.

Comprehensive identification and functional characterization of GhpPLA gene family in reproductive organ development

BACKGROUND

Phospholipases As (PLAs) are acyl hydrolases that catalyze the release of free fatty acids in phospholipids and play multiple functions in plant growth and development. The three families of PLAs are: PLA1, PLA2 (sPLA), and patatin-related PLA (pPLA). The diverse functions that pPLAs play in the growth and development of a broad range of plants have been demonstrated by prior studies.

METHODS

Genome-wide analysis of the pPLA gene family and screening of genes for expression verification and gene silencing verification were conducted. Additionally, pollen vitality testing, analysis of the pollen expression pattern, and the detection of POD, SOD, CAT, MDA, and H2O2 were performed.

RESULT

In this study, 294 pPLAs were identified from 13 plant species, including 46 GhpPLAs that were divided into three subfamilies (I-III). Expression patterns showed that the majority of GhpPLAs were preferentially expressed in the petal, pistil, anther, and ovule, among other reproductive organs. Particularly, GhpPLA23 and GhpPLA44, were found to be potentially important for the reproductive development of G. hirsutum. Functional validation was demonstrated by VIGS which showed that reduced expression levels of GhpPLA23 and GhpPLA44 in the silenced plants were associated with a decrease in pollen activity. Moreover, a substantial shift in ROS and ROS scavengers and a considerable increase in POD, CAT, SOD, and other physiological parameters was found out in these silenced plants. Our results provide plausibility to the hypothesis that GhpPLA23 and GhpPLA44 had a major developmental impact on cotton reproductive systems. These results also suggest that pPLAs are important for G. hirsutum's reproductive development and suggest that they could be employed as potential genes for haploid induction.

CONCLUSIONS

The findings of the present research indicate that pPLA genes are essential for the development of floral organs and sperm cells in cotton. Consequently, this family might be important for the reproductive development of cotton and possibly for inducing the plant develop haploid progeny.

Synthetic developmental biology: molecular tools to re-design plant shoots and roots

Plant morphology and anatomy strongly influence agricultural yield. Crop domestication has strived for desirable growth and developmental traits, such as larger and more fruits and semi-dwarf architecture. Genetic engineering has accelerated rational, purpose-driven engineering of plant development, but it can be unpredictable. Developmental pathways are complex and riddled with environmental and hormonal inputs, as well as feedback and feedforward interactions, which occur at specific times and places in a growing multicellular organism. Rational modification of plant development would probably benefit from precision engineering based on synthetic biology approaches. This review outlines recently developed synthetic biology technologies for plant systems and highlights their potential for engineering plant growth and development. Streamlined and high-capacity genetic construction methods (Golden Gate DNA Assembly frameworks and toolkits) allow fast and variation-series cloning of multigene transgene constructs. This, together with a suite of gene regulation tools (e.g. cell type-specific promoters, logic gates, and multiplex regulation systems), is starting to enable developmental pathway engineering with predictable outcomes in model plant and crop species.

Evolution of sex in crops: recurrent scrap and rebuild

Sexuality is the main strategy for maintaining genetic diversity within a species. In flowering plants (angiosperms), sexuality is derived from ancestral hermaphroditism and multiple sexualities can be expressed in an individual. The mechanisms conferring chromosomal sex determination in plants (or dioecy) have been studied for over a century by both biologists and agricultural scientists, given the importance of this field for crop cultivation and breeding. Despite extensive research, the sex determining gene(s) in plants had not been identified until recently. In this review, we dissect plant sex evolution and determining systems, with a focus on crop species. We introduced classic studies with theoretical, genetic, and cytogenic approaches, as well as more recent research using advanced molecular and genomic techniques. Plants have undergone very frequent transitions into, and out of, dioecy. Although only a few sex determinants have been identified in plants, an integrative viewpoint on their evolutionary trends suggests that recurrent neofunctionalization events are potentially common, in a "scrap and (re)build" cycle. We also discuss the potential association between crop domestication and transitions in sexual systems. We focus on the contribution of duplication events, which are particularly frequent in plant taxa, as a trigger for the creation of new sexual systems.

Genic male and female sterility in vegetable crops

Vegetable crops are greatly appreciated for their beneficial nutritional and health components. Hybrid seeds are widely used in vegetable crops for advantages such as high yield and improved resistance, which require the participation of male (stamen) and female (pistil) reproductive organs. Male- or female-sterile plants are commonly used for production of hybrid seeds or seedless fruits in vegetables. In this review we will focus on the types of genic male sterility and factors affecting female fertility, summarize typical gene function and research progress related to reproductive organ identity and sporophyte and gametophyte development in vegetable crops [mainly tomato (Solanum lycopersicum) and cucumber (Cucumis sativus)], and discuss the research trends and application perspectives of the sterile trait in vegetable breeding and hybrid production, in order to provide a reference for fertility-related germplasm innovation.

bHLH010/089 Transcription Factors Control Pollen Wall Development via Specific Transcriptional and Metabolic Networks in Arabidopsis thaliana

The pollen wall is a specialized extracellular cell wall that protects male gametophytes from various environmental stresses and facilitates pollination. Here, we reported that bHLH010 and bHLH089 together are required for the development of the pollen wall by regulating their specific downstream transcriptional and metabolic networks. Both the exine and intine structures of bhlh010 bhlh089 pollen grains were severely defective. Further untargeted metabolomic and transcriptomic analyses revealed that the accumulation of pollen wall morphogenesis-related metabolites, including polysaccharides, glyceryl derivatives, and flavonols, were significantly changed, and the expression of such metabolic enzyme-encoding genes and transporter-encoding genes related to pollen wall morphogenesis was downregulated in bhlh010 bhlh089 mutants. Among these downstream target genes, CSLB03 is a novel target with no biological function being reported yet. We found that bHLH010 interacted with the two E-box sequences at the promoter of CSLB03 and directly activated the expression of CSLB03. The cslb03 mutant alleles showed bhlh010 bhlh089–like pollen developmental defects, with most of the pollen grains exhibiting defective pollen wall structures.

SWOLLEN TAPETUM AND STERILITY 1 is required for tapetum degeneration and pollen wall formation in rice

The pollen wall is important for protecting the male gametophyte and for fertilization. The lipid components of the pollen wall are mainly synthesized and transported from the sporophytic tapetum. Although several factors related to lipid biosynthesis have been characterized, the molecular mechanisms underlying lipid biosynthesis during pollen development in rice (Oryza sativa L.) remain elusive. Here, we showed that mutation in the SWOLLEN TAPETUM AND STERILITY 1 (STS1) gene causes delayed tapetum degradation and aborted pollen wall formation in rice. STS1 encodes an endoplasmic reticulum (ER)-localized protein that contains domain of unknown function (DUF) 726 and exhibits lipase activity. Lipidomic and transcriptomic analyses showed that STS1 is involved in anther lipid homeostasis. Moreover, STS1 interacts with Polyketide Synthase 2 (OsPKS2) and Acyl-CoA Synthetase 12 (OsACOS12), two enzymes crucial in lipidic sporopollenin biosynthesis in pollen wall formation, suggesting a potentially lipidic metabolon for sporopollenin biosynthesis in rice. Collectively, our results indicate that STS1 is an important factor for lipid biosynthesis in reproduction, providing a target for the artificial control of male fertility in hybrid rice breeding and insight into the function of DUF726-containing protein in plants.

Gene activation via Cre/lox-mediated excision in cowpea (Vigna unguiculata)

Expression of Cre recombinase by AtRps5a_pro or AtDD45_pro enabled Cre/lox-mediated recombination at an early embryonic developmental stage upon crossing, activating transgenes in the hybrid cowpea and tobacco. Genetic engineering ideally results in precise spatiotemporal control of transgene expression. To activate transgenes exclusively in a hybrid upon fertilization, we evaluated a Cre/lox-mediated gene activation system with the Cre recombinase expressed by either AtRps5a or AtDD45 promoters that showed activity in egg cells and young embryos. In crosses between Cre recombinase lines and transgenic lines harboring a lox-excision reporter cassette with ZsGreen driven by the AtUbq3 promoter after Cre/lox-mediated recombination, we observed complete excision of the lox-flanked intervening DNA sequence between the AtUbq3_pro and the ZsGreen coding sequence in F₁ progeny upon genotyping but no ZsGreen expression in F₁ seeds or seedlings. The incapability to observe ZsGreen fluorescence was attributed to the activity of the AtUbq3_pro. Strong ZsGreen expression in F₁ seeds was observed after recombination when ZsGreen was driven by the AtUbq10 promoter. Using the AtDD45_pro to express Cre resulted in more variation in recombination frequencies between transgenic lines and crosses. Regardless of the promoter used to regulate Cre, mosaic F₁ progeny were rare, suggesting gene activation at an early embryo-developmental stage. Observation of ZsGreen-expressing tobacco embryos at the globular stage from crosses with the AtRps5a_proCre lines pollinated by the AtUbq3_prolox line supported the early activation mode.

Harnessing tissue-specific genome editing in plants through CRISPR/Cas system: current state and future prospects

In a nutshell, tissue-specific CRISPR/Cas genome editing is the most promising approach for crop improvement which can bypass the hurdle associated with constitutive GE such as off target and pleotropic effects for targeted crop improvement. CRISPR/Cas is a powerful genome-editing tool with a wide range of applications for the genetic improvement of crops. However, the constitutive genome editing of vital genes is often associated with pleiotropic effects on other genes, needless metabolic burden, or interference in the cellular machinery. Tissue-specific genome editing (TSGE), on the other hand, enables researchers to study those genes in specific cells, tissues, or organs without disturbing neighboring groups of cells. Until recently, there was only limited proof of the TSGE concept, where the CRISPR-TSKO tool was successfully used in Arabidopsis, tomato, and cotton, laying a solid foundation for crop improvement. In this review, we have laid out valuable insights into the concept and application of TSGE on relatively unexplored areas such as grain trait improvement under favorable or unfavorable conditions. We also enlisted some of the prominent tissue-specific promoters and described the procedure of their isolation with several TSGE promoter expression systems in detail. Moreover, we highlighted potential negative regulatory genes that could be targeted through TSGE using tissue-specific promoters. In a nutshell, tissue-specific CRISPR/Cas genome editing is the most promising approach for crop improvement which can bypass the hurdle associated with constitutive GE such as off target and pleotropic effects for targeted crop improvement.

The Evolution and Expression Profiles of EC1 Gene Family during Development in Cotton

Fertilization is essential to sexual reproduction of flowering plants. EC1 (EGG CELL 1) proteins have a conserved cysteine spacer characteristic and play a crucial role in double fertilization process in many plant species. However, to date, the role of EC1 gene family in cotton is fully unknown. Hence, detailed bioinformatics analysis was explored to elucidate the biological mechanisms of EC1 gene family in cotton. In this study, we identified 66 genes in 10 plant species in which a total of 39 EC1 genes were detected from cotton genome. Phylogenetic analysis clustered the identified EC1 genes into three families (I-III) and all of them contain Prolamin-like domains. A good collinearity was observed in the synteny analysis of the orthologs from cotton genomes. Whole-genome duplication was determined to be one of the major impetuses for the expansion of the EC1 gene family during the process of evolution. qRT-PCR analysis showed that EC1 genes were highly expressed in reproductive tissues under multiple stresses, signifying their potential role in enhancing stress tolerance or responses. Additionally, gene interaction networks showed that EC1 genes may be involved in cell stress and response transcriptional regulator in the synergid cells and activate the expression of genes required for pollen tube guidance. Our results provide novel functional insights into the evolution and functional elucidation of EC1 gene family in cotton.

Alternative splicing dynamics and evolutionary divergence during embryogenesis in wheat species

Among polyploid species with complex genomic architecture, variations in the regulation of alternative splicing (AS) provide opportunities for transcriptional and proteomic plasticity and the potential for generating trait diversities. However, the evolution of AS and its influence on grain development in diploid grass and valuable polyploid wheat crops are poorly understood. To address this knowledge gap, we developed a pipeline for the analysis of alternatively spliced transcript isoforms, which takes the high sequence similarity among polyploid wheat subgenomes into account. Through analysis of synteny and detection of collinearity of homoeologous subgenomes, conserved and specific AS events across five wheat and grass species were identified. A global analysis of the regulation of AS in diploid grass and polyploid wheat grains revealed diversity in AS events not only between the endosperm, pericarp and embryo overdevelopment, but also between subgenomes. Analysis of AS in homoeologous triads of polyploid wheats revealed evolutionary divergence between gene-level and transcript-level regulation of embryogenesis. Evolutionary age analysis indicated that the generation of novel transcript isoforms has occurred in young genes at a more rapid rate than in ancient genes. These findings, together with the development of comprehensive AS resources for wheat and grass species, advance understanding of the evolution of regulatory features of AS during embryogenesis and grain development in wheat.

A Rapid Pipeline for Pollen- and Anther-Specific Gene Discovery Based on Transcriptome Profiling Analysis of Maize Tissues

Recently, crop breeders have widely adopted a new biotechnology-based process, termed Seed Production Technology (SPT), to produce hybrid varieties. The SPT does not produce nuclear male-sterile lines, and instead utilizes transgenic SPT maintainer lines to pollinate male-sterile plants for propagation of nuclear-recessive male-sterile lines. A late-stage pollen-specific promoter is an essential component of the pollen-inactivating cassette used by the SPT maintainers. While a number of plant pollen-specific promoters have been reported so far, their usefulness in SPT has remained limited. To increase the repertoire of pollen-specific promoters for the maize community, we conducted a comprehensive comparative analysis of transcriptome profiles of mature pollen and mature anthers against other tissue types. We found that maize pollen has much less expressed genes (>1 FPKM) than other tissue types, but the pollen grain has a large set of distinct genes, called pollen-specific genes, which are exclusively or much higher (100 folds) expressed in pollen than other tissue types. Utilizing transcript abundance and correlation coefficient analysis, 1215 mature pollen-specific (MPS) genes and 1009 mature anther-specific (MAS) genes were identified in B73 transcriptome. These two gene sets had similar GO term and KEGG pathway enrichment patterns, indicating that their members share similar functions in the maize reproductive process. Of the genes, 623 were shared between the two sets, called mature anther- and pollen-specific (MAPS) genes, which represent the late-stage pollen-specific genes of the maize genome. Functional annotation analysis of MAPS showed that 447 MAPS genes (71.7% of MAPS) belonged to genes encoding pollen allergen protein. Their 2-kb promoters were analyzed for cis-element enrichment and six well-known pollen-specific cis-elements (AGAAA, TCCACCA, TGTGGTT, [TA]AAAG, AAATGA, and TTTCT) were found highly enriched in the promoters of MAPS. Interestingly, JA-responsive cis-element GCC box (GCCGCC) and ABA-responsive cis-element-coupling element1 (ABRE-CE1, CCACC) were also found enriched in the MAPS promoters, indicating that JA and ABA signaling likely regulate pollen-specific MAPS expression. This study describes a robust and straightforward pipeline to discover pollen-specific promotes from publicly available data while providing maize breeders and the maize industry a number of late-stage (mature) pollen-specific promoters for use in SPT for hybrid breeding and seed production.

Cytological and morphology characteristics of natural microsporogenesis within Camellia oleifera

Camellia oleifera is believed to exhibit a complex intraspecific polyploidy phenomenon. Abnormal microsporogenesis can promote the formation of unreduced gametes in plants and lead to sexual polyploidy, so it is hypothesized that improper meiosis probably results in the formation of natural polyploidy in Camellia oleifera. In this study, based on the cytological observation of meiosis in pollen mother cells (PMCs), we found natural 2n pollen for the first time in Camellia oleifera, which may lead to the formation of natural polyploids by sexual polyploidization. Additionally, abnormal cytological behaviour during meiosis, including univalent chromosomes, extraequatorial chromosomes, early segregation, laggard chromosomes, chromosome stickiness, asynchronous meiosis and deviant cytokinesis (monad, dyads, triads), was observed, which could be the cause of 2n pollen formation. Moreover, we confirmed a relationship among the length-width ratio of flower buds, stylet length and microsporogenesis. This result suggested that we can immediately determine the microsporogenesis stages by phenotypic characteristics, which may be applicable to breeding advanced germplasm in Camellia oleifera.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01002-5.

Pollen PCP-B peptides unlock a stigma peptide-receptor kinase gating mechanism for pollination

Sexual reproduction in angiosperms relies on precise communications between the pollen and pistil. The molecular mechanisms underlying these communications remain elusive. We established that in Arabidopsis, a stigmatic gatekeeper, the ANJEA-FERONIA (ANJ-FER) receptor kinase complex, perceives the RAPID ALKALINIZATION FACTOR peptides RALF23 and RALF33 to induce reactive oxygen species (ROS) production in the stigma papillae, whereas pollination reduces stigmatic ROS, allowing pollen hydration. Upon pollination, the POLLEN COAT PROTEIN B-class peptides (PCP-Bs) compete with RALF23/33 for binding to the ANJ-FER complex, leading to a decline of stigmatic ROS that facilitates pollen hydration. Our results elucidate a molecular gating mechanism in which distinct peptide classes from pollen compete with stigma peptides for interaction with a stigmatic receptor kinase complex, allowing the pollen to hydrate and germinate.

Dynamics of cell wall structure and related genomic resources for drought tolerance in rice

Cell wall plasticity plays a very crucial role in vegetative and reproductive development of rice under drought and is a highly potential trait for improving rice yield under drought. Drought is a major constraint in rice (Oryza sativa L.) cultivation severely affecting all developmental stages, with the reproductive stage being the most sensitive. Rice plants employ multiple strategies to cope with drought, in which modification in cell wall dynamics plays a crucial role. Over the years, significant progress has been made in discovering the cell wall-specific genomic resources related to drought tolerance at vegetative and reproductive stages of rice. However, questions remain about how the drought-induced changes in cell wall made by these genomic resources potentially influence the vegetative and reproductive development of rice. The possibly major candidate genes underlying the function of quantitative trait loci directly or indirectly associated with the cell wall plasticization-mediated drought tolerance of rice might have a huge promise in dissecting the putative genomic regions associated with cell wall plasticity under drought. Furthermore, engineering the drought tolerance of rice using cell wall-related genes from resurrection plants may have huge prospects for rice yield improvement. Here, we review the comprehensive multidisciplinary analyses to unravel different components and mechanisms involved in drought-induced cell wall plasticity at vegetative and reproductive stages that could be targeted for improving rice yield under drought.

Genome-wide identification of GH3 genes in Brassica oleracea and identification of a promoter region for anther-specific expression of a GH3 gene

Background

The Gretchen Hagen 3 (GH3) genes encode acyl acid amido synthetases, many of which have been shown to modulate the amount of active plant hormones or their precursors. GH3 genes, especially Group III subgroup 6 GH3 genes, and their expression patterns in economically important B. oleracea var. oleracea have not been systematically identified.

Results

As a first step to understand regulation and molecular functions of Group III subgroup 6 GH3 genes, 34 GH3 genes including four subgroup 6 genes were identified in B. oleracea var. oleracea. Synteny found around subgroup 6 GH3 genes in B. oleracea var. oleracea and Arabidopsis thaliana indicated that these genes are evolutionarily related. Although expression of four subgroup 6 GH3 genes in B. oleracea var. oleracea is not induced by auxin, gibberellic acid, or jasmonic acid, the genes show different organ-dependent expression patterns. Among subgroup 6 GH3 genes in B. oleracea var. oleracea, only BoGH3.13–1 is expressed in anthers when microspores, polarized microspores, and bicellular pollens are present, similar to two out of four syntenic A. thaliana subgroup 6 GH3 genes. Detailed analyses of promoter activities further showed that BoGH3.13–1 is expressed in tapetal cells and pollens in anther, and also expressed in leaf primordia and floral abscission zones.

Conclusions

Sixty-two base pairs (bp) region (− 340 ~ − 279 bp upstream from start codon) and about 450 bp region (− 1489 to − 1017 bp) in BoGH3.13–1 promoter are important for expressions in anther and expressions in leaf primordia and floral abscission zones, respectively. The identified anther-specific promoter region can be used to develop male sterile transgenic Brassica plants.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07345-9.

Two-step method for isolation of high-quality RNA from stored seeds of maize rich in starch

A modified SDS-Trizol method was optimized for isolation of total RNA from the stored maize seeds at regular interval of one month for 4 months. Use of SDS extraction buffer before the use of Trizol reduced the co-precipitation problem associated with high carbohydrate content in the seed. Recorded mean RNA yield from seeds across the storage intervals was 978.6 ± 65.46 ng/µl. Average spectrophotometric values (A _260/280) of isolated RNA varied from 1.974 ± 0.033 to 1.998 ± 0.022. Attempts to isolate RNA from green leaves using Trizol method also ensured comparable quality and quantity of the isolated RNA. RNA yield from fresh leaves was recorded 1008.2 ± 77.088 ng/µl which is slightly higher than the mean RNA yield from seeds across months. Observed mean A _260/280 values of isolated RNA were 1.984 ± 0.030. DNase treatment further improved the A _260/280 ratio in both seeds (2.003 ± 0.006) and leaves (2.012 ± 0.037). High quality and quantity along with integrity of the isolated RNA was ensured through downstream analysis after RNA extraction such as first-strand cDNA synthesis and normal PCR. Extraction of RNA from the stored seeds using modified SDS-based Trizol method and from fresh leaves using Trizol method opened new possibility of understanding role of key genes involving developmental steps especially in the stored seeds.

Transcriptomic Study for Identification of Major Nitrogen Stress Responsive Genes in Australian Bread Wheat Cultivars

High nitrogen use efficiency (NUE) in bread wheat is pivotal to sustain high productivity. Knowledge about the physiological and transcriptomic changes that regulate NUE, in particular how plants cope with nitrogen (N) stress during flowering and the grain filling period, is crucial in achieving high NUE. Nitrogen response is differentially manifested in different tissues and shows significant genetic variability. A comparative transcriptome study was carried out using RNA-seq analysis to investigate the effect of nitrogen levels on gene expression at 0 days post anthesis (0 DPA) and 10 DPA in second leaf and grain tissues of three Australian wheat (Triticum aestivum) varieties that were known to have varying NUEs. A total of 12,344 differentially expressed genes (DEGs) were identified under nitrogen stress where down-regulated DEGs were predominantly associated with carbohydrate metabolic process, photosynthesis, light-harvesting, and defense response, whereas the up-regulated DEGs were associated with nucleotide metabolism, proteolysis, and transmembrane transport under nitrogen stress. Protein–protein interaction and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways analysis further revealed that highly interacted down-regulated DEGs were involved in light-harvesting and photosynthesis, and up-regulated DEGs were mostly involved in steroid biosynthesis under N stress. The common down-regulated genes across the cultivars included photosystem II 10 kDa polypeptide family proteins, plant protein 1589 of uncharacterized protein function, etc., whereas common up-regulated genes included glutamate carboxypeptidase 2, placenta-specific8 (PLAC8) family protein, and a sulfate transporter. On the other hand, high NUE cultivar Mace responded to nitrogen stress by down-regulation of a stress-related gene annotated as beta-1,3-endoglucanase and pathogenesis-related protein (PR-4, PR-1) and up-regulation of MYB/SANT domain-containing RADIALIS (RAD)-like transcription factors. The medium NUE cultivar Spitfire and low NUE cultivar Volcani demonstrated strong down-regulation of Photosystem II 10 kDa polypeptide family protein and predominant up-regulation of 11S globulin seed storage protein 2 and protein transport protein Sec61 subunit gamma. In grain tissue, most of the DEGs were related to nitrogen metabolism and proteolysis. The DEGs with high abundance in high NUE cultivar can be good candidates to develop nitrogen stress-tolerant variety with improved NUE.

Haploidy in Tobacco Induced by PsASGR-BBML Transgenes via Parthenogenesis

BACKGROUND

Engineering apomixis in sexually reproducing plants has been long desired because of the potential to fix hybrid vigor. Validating the functionality of genes originated from apomictic species that contribute to apomixis upon transfer to sexually reproducing species is an important step. The PsASGR-BABYBOOM-like (PsASGR-BBML) gene from Pennisetum squamulatum confers parthenogenesis in this apomict, and its functionality was demonstrated in several sexually reproducing monocots but not in any dicots.

METHODS

We introduced the PsASGR-BBML gene regulated by egg cell-specific promoters, either AtDD45 or AtRKD2, into tobacco, and analyzed progeny of the transgenic lines resulting from self-pollination and crossing by flow cytometry.

RESULTS

We identified haploid progeny at a frequency lower than 1% in the AtDD45pro lines, while at a frequency of 9.3% for an octoploid (2n = 8x) AtRKD2pro line. Haploid production in the T2 generation, derived from the tetraploid T1 offspring of this original octoploid AtRKD2pro line, was also observed. Pollinated by homozygous transgenic tobacco carrying a DsRed marker gene, 4x progeny of the AtRKD2pro line yielded parthenogenetic embryos identified as DsRed negative. We verified that the DsRed negative seedlings recovered were haploid (2x).

CONCLUSION

The PsASGR-BBML gene regulated by egg cell-specific promoters could enable parthenogenesis in tobacco, a dicotyledon species.

Metabolomic Profiling of Nicotiana Spp. Nectars Indicate That Pollinator Feeding Preference Is a Stronger Determinant Than Plant Phylogenetics in Shaping Nectar Diversity

Floral nectar is a rich secretion produced by the nectary gland and is offered as reward to attract pollinators leading to improved seed set. Nectars are composed of a complex mixture of sugars, amino acids, proteins, vitamins, lipids, organic and inorganic acids. This composition is influenced by several factors, including floral morphology, mechanism of nectar secretion, time of flowering, and visitation by pollinators. The objective of this study was to determine the contributions of flowering time, plant phylogeny, and pollinator selection on nectar composition in Nicotiana. The main classes of nectar metabolites (sugars and amino acids) were quantified using gas chromatography/mass spectrometric analytical platforms to identify differences among fifteen Nicotiana species representing day- and night-flowering plants from ten sections of the genus that are visited by five different primary pollinators. The nectar metabolomes of different Nicotiana species can predict the feeding preferences of the target pollinator(s) of each species, and the nectar sugars (i.e., glucose, fructose, and sucrose) are a distinguishing feature of Nicotiana species phylogeny. Moreover, comparative statistical analysis indicate that pollinators are a stronger determinant of nectar composition than plant phylogeny.

Temporal Distinction between Male and Female Floral Organ Development in Nicotiana tabacum cv. Xanthi (Solanaceae)

Early floral developmental investigations provide crucial evidence for phylogenetic and molecular studies of plants. The developmental and evolutionary mechanisms underlying the variations in floral organs are critical for a thorough understanding of the diversification of flowers. Ontogenetic comparisons between anthers and pistil within single flowers were characterized over time in Nicotiana tabacum cv. Xanthi. The ages of 42 tobacco flower or flower primordia were estimated using corolla growth analysis. Results showed that the protodermal layer in carpel primordia contributes to carpel development by both anticlinal and periclinal divisions. Periclinal divisions in the hypodermal layer of the placenta were observed around 4.8 ± 1.3 days after the formation of early carpel primordia (ECP) and ovule initiation occurred 10.0 ± 0.5 days after ECP. Meiosis in anthers and ovules began about 8.9 ± 1.1 days and 14.4 ± 1.3 days after ECP, respectively. Results showed an evident temporal distinction between megasporogenesis and microsporogenesis. Flower ages spanned a 17-day interval, starting with flower primordia containing the ECP and anther primordia to the tetrad stage of meiosis in megasporocytes and the bicellular stage in pollen grains. These results establish a solid foundation for future studies in order to identify the developmental and molecular mechanisms responsible for the mating system in tobacco.

Nicotiana tabacum pollen-pistil interactions show unexpected spatial and temporal differences in pollen tube growth among genotypes

KEY MESSAGE

This research revealed diverse PTG rates among intraspecific pollen-pistil interactions that showed variable dependency on the stigma and mature TT. Pollen-pistil interactions regulate pollen tube growth (PTG) rates and are determinants of fertilization and seed set. This research focuses on the diversity of intraspecific PTG rates and the spatial and temporal regulation of PTG among Nicotiana tabacum genotypes. Nonrandom mating within self-compatible species has been noted, but little is known on the mechanisms involved. To begin research on nonrandom mating, we took advantage of the model reproductive system of N. tabacum and used seventeen diverse N. tabacum genotypes in a complete pollination diallel to measure the diversity of intraspecific pollen-pistil interactions. The 289 intraspecific interactions showed surprisingly large differences in PTG rates. The interaction between specific males and females resulted in 18 specific combining abilities that were significantly different, indicating the importance of the specific genotype interaction in regulating intraspecific PTG. No single female or male genotype exerted overall control of PTG rates, as determined by a general combining ability analysis. Slow and fast pollen-pistil interactions showed spatial differences in growth rates along the style. Slower interactions had a slower initial PTG rate while fast interactions had faster consistent rates of growth indicating spatial regulation of PTG in the pistil. Removal of the stigma or the mature transmitting tissue (TT) showed the tissue-specific component of PTG regulation. Stigma removal resulted in slower or no change in PTG rate depending on the pollen and pistil genotypes. Removal of the TT, which necessitated removal of the stigma, showed no change, slower or unexpectedly, increased growth rates relative to growth rates without a stigma. These data show the diverse nature of pollen-pistil interactions in N. tabacum genotypes providing a system to further investigate the regulation of PTG.

Effects on Plant Growth and Reproduction of a Peach R2R3-MYB Transcription Factor Overexpressed in Tobacco

In plants, anthocyanin production is controlled by MYB and bHLH transcription factors. In peach, among the members of these families, MYB10.1 and bHLH3 have been shown to be the most important genes for production of these pigments during fruit ripening. Anthocyanins are valuable molecules, and the overexpression of regulatory genes in annual fast-growing plants has been explored for their biotechnological production. The overexpression of peach MYB10.1 in tobacco plants induced anthocyanin pigmentation, which was particularly strong in the reproductive parts. Pigment production was the result of an up-regulation of the expression level of key genes of the flavonoid biosynthetic pathway, such as NtCHS, NtCHI, NtF3H, NtDFR, NtANS, and NtUFGT, as well as of the proanthocyanidin biosynthetic pathway such as NtLAR. Nevertheless, phenotypic alterations in transgenic tobacco lines were not only limited to anthocyanin production. Lines showing a strong phenotype (type I) exhibited irregular leaf shape and size and reduced plant height. Moreover, flowers had reduced length of anther's filament, nondehiscent anthers, reduced pistil length, aborted nectary glands, and impaired capsule development, but the reproductive parts including androecium, gynoecium, and petals were more pigmented that in wild type. Surprisingly, overexpression of peach MYB10.1 led to suppression of NtMYB305, which is required for floral development and, of one of its target genes, NECTARIN1 (NtNCE1), involved in the nectary gland formation. MYB10.1 overexpression up-regulated JA biosynthetic (NtAOS) and signaling (NtJAZd) genes, as well as 1-aminocyclopropane-1-carboxylate oxidase (NtACO) in flowers. The alteration of these hormonal pathways might be among the causes of the observed floral abnormalities with defects in both male and female gametophyte development. In particular, approximately only 30% of pollen grains of type I lines were viable, while during megaspore formation, there was a block during FG1 (St3-II). This block seemed to be associated to an excessive accumulation of callose. It can be concluded that the overexpression of peach MYB10.1 in tobacco not only regulates flavonoid biosynthesis (anthocyanin and proanthocyanidin) in the reproductive parts but also plays a role in other processes such as vegetative and reproductive development.

Engineered Male Sterility by Early Anther Ablation Using the Pea Anther-Specific Promoter PsEND1

Genetic engineered male sterility has different applications, ranging from hybrid seed production to bioconfinement of transgenes in genetic modified crops. The impact of this technology is currently patent in a wide range of crops, including legumes, which has helped to deal with the challenges of global food security. Production of engineered male sterile plants by expression of a ribonuclease gene under the control of an anther- or pollen-specific promoter has proven to be an efficient way to generate pollen-free elite cultivars. In the last years, we have been studying the genetic control of flower development in legumes and several genes that are specifically expressed in a determinate floral organ were identified. Pisum sativum ENDOTHECIUM 1 (PsEND1) is a pea anther-specific gene displaying very early expression in the anther primordium cells. This expression pattern has been assessed in both model plants and crops (tomato, tobacco, oilseed rape, rice, wheat) using genetic constructs carrying the PsEND1 promoter fused to the uidA reporter gene. This promoter fused to the barnase gene produces full anther ablation at early developmental stages, preventing the production of mature pollen grains in all plant species tested. Additional effects produced by the early anther ablation in the PsEND1::barnase-barstar plants, with interesting biotechnological applications, have also been described, such as redirection of resources to increase vegetative growth, reduction of the need for deadheading to extend the flowering period, or elimination of pollen allergens in ornamental plants (Kalanchoe, Pelargonium). Moreover, early anther ablation in transgenic PsEND1::barnase-barstar tomato plants promotes the developing of the ovaries into parthenocarpic fruits due to the absence of signals generated during the fertilization process and can be considered an efficient tool to promote fruit set and to produce seedless fruits. In legumes, the production of new hybrid cultivars will contribute to enhance yield and productivity by exploiting the hybrid vigor generated. The PsEND1::barnase-barstar construct could be also useful to generate parental lines in hybrid breeding approaches to produce new cultivars in different legume species.

Anther and pollen development in sweet cherry (Prunus avium L.) in relation to winter dormancy

Anther and pollen development is a highly conserved process in angiosperms, but while pollen formation in annual plants occurs in a few days, in temperate woody perennials, it requires several months. How anther and pollen development is framed in terms of seasonality plays a clear part in reproductive success. In this study, seasonal anther and pollen development is characterized in two sweet cherry cultivars over 2 years, paying special attention to the period of dormancy and unveiling the role of starch in this process. We evaluated starch content from the autumn until bud burst with the help of an image analysis system fitted to a light microscope. Microscope observations allowed the temporal relationship of pollen development to the phenological stages of flower and bud development to be determined. In both cultivars and years, anther and pollen development followed the same pattern. Development was halted by dormancy, when the anthers showed no morphological changes until several weeks after chilling fulfillment, until the milder temperatures reactivated development. After dormancy, starch was accumulated in the connective tissue until tracheary element differentiation. Quantification of starch in the connective tissue of anthers revealed its importance in supporting pollen meiosis and subsequent anther growth.

Targeted expression of a cysteine protease (AdCP) in tapetum induces male sterility in Indian mustard, Brassica juncea

The development of male sterile plants is a prerequisite to developing hybrid varieties to harness the benefits of hybrid vigor in crops and enhancing crop productivity for sustainable agriculture. In plants, cysteine proteases have been known for their multifaceted roles during programmed cell death, and in ubiquitin- and proteasome-mediated proteolysis. Here, we showed that Arachis diogoi cysteine protease (AdCP) expressed under the TA-29 promoter induced complete male sterility in Indian mustard, Brassica juncea. The herbicide resistance gene bar was used for the selection of transgenic plants. Mustard transgenic plants exhibited male sterile phenotype and failed to produce functional pollen grains. Irregularly shaped aborted pollen grains with groove-like structures were observed in male sterile plants during scanning electron microscopy analysis. The T₁ progeny plants obtained from the seed of primary transgenic male sterile plants crossed with the wild-type plants exhibited segregation of the progeny into male sterile and fertile plants with normal seed development. Further, male sterile plants exhibited higher transcript levels of AdCP in anther tissues, which is consistent with its expression under the tapetum-specific promoter. Our results clearly suggest that the targeted expression of AdCP provides a potential tool for developing male sterile lines in crop plants by the malfunction of tapetal cells leading to male sterility as shown earlier in tobacco transgenic plants (Shukla et al. 2014, Funct Integr Genomics 14:307-317).

Enhanced Nicotiana benthamiana immune responses caused by heterologous plant genes from Pinellia ternata

BACKGROUND

Pinellia ternata is a Chinese traditional medicinal herb, used to cure diseases including insomnia, eclampsia and cervical carcinoma, for hundreds of years. Non-self-recognition in multicellular organisms can initiate the innate immunity to avoid the invasion of pathogens. A design for pathogen independent, heterosis based, fresh resistance can be generated in F1 hybrid was proposed.

RESULTS

By library functional screening, we found that P. ternata genes, named as ptHR375 and ptHR941, were identified with the potential to trigger a hypersensitive response in Nicotiana benthamiana. Significant induction of ROS and Callose deposition in N. benthamiana leaves along with activation of pathogenesis-related genes viz.; PR-1a, PR-5, PDF1.2, NPR1, PAL, RBOHB and ERF1 and antioxidant enzymes was observed. After transformation into N. benthamiana, expression of pathogenesis related genes was significantly up-regulated to generate high level of resistance against Phytophthora capsici without affecting the normal seed germination and morphological characters of the transformed N. benthamiana. UPLC-QTOF-MS analysis of ptHR375 transformed N. benthamiana revealed the induction of Oxytetracycline, Cuelure, Allantoin, Diethylstilbestrol and 1,2-Benzisothiazol-3(2H)-one as bioactive compounds. Here we also proved that F1 hybrids, produced by crossing of the ptHR375 and ptHR941 transformed and non-transformed N. benthamiana, show significant high levels of PR-gene expressions and pathogen resistance.

CONCLUSIONS

Heterologous plant genes can activate disease resistance in another plant species and furthermore, by generating F1 hybrids, fresh pathogen independent plant immunity can be obtained. It is also concluded that ptHR375 and ptHR941 play their role in SA and JA/ET defense pathways to activate the resistance against invading pathogens.

A novel cysteine-rich peptide regulates cell expansion in the tobacco pistil and influences its final size

Plant morphogenesis is dependent on cell proliferation and cell expansion, which are responsible for establishing final organ size and shape during development. Several genes have been described as encoding components of the plant cell development machinery, among which are the plant peptides. Here we describe a novel cysteine-rich plant peptide (68 amino acids), encoded by a small open reading frame gene (sORF). It is specifically expressed in the reproductive organs of Nicotiana tabacum and is developmentally regulated. N- and C-terminal translational fusions with GFP in protoplasts have demonstrated that the peptide is not secreted. Knockdown transgenic plants produced by RNAi exhibited enlarged pistils due to cell expansion and the gene was named Small Peptide Inhibitor of Cell Expansion (SPICE). Estimation of nuclear DNA content using flow cytometry has shown that cell expansion in pistils was not correlated with endoreduplication. Decreased SPICE expression also affected anther growth and pollen formation, resulting in male sterility in at least one transgenic plant. Our results revealed that SPICE is a novel reproductive organ specific gene that controls cell expansion, probably as a component of a signal transduction pathway.

Nectar Analysis Throughout the Genus Nicotiana Suggests Conserved Mechanisms of Nectar Production and Biochemical Action

We have evaluated the floral nectars of nine species from different sections of the genus Nicotiana. These nine species effectively cover the genus. We found that the nectary glands from these different species showed similar developmental regulation with swelling of nectaries during the first half of development and a distinct color change in the nectary gland as development approaches anthesis. When we examined the composition of the nectar from these nine different species we found that they were similar in content. Carbohydrate compositions of these various nectars varied between these species with N. bonariensis showing the highest and N. sylvestris lowest level of sugars. Based upon the amount of carbohydrates, the nectars fell into two groups. We found that hydrogen peroxide accumulated in the nectars of each of these species. While all species showed the presence of hydrogen peroxide in nectar, the quantitative amounts of hydrogen peroxide which was very high in N. rustica and N. bonariensis, suggesting be a common characteristic in short flower Nicotiana species. We further found that the antioxidant ascorbate accumulated in nectar and β-carotene accumulated in nectaries. β-carotene was most high in nectaries of N. bonariensis. We also examined the presence of proteins in the nectars of these species. The protein profile and quantities varied significantly between species, although all species have showed the presence of proteins in their nectars. We performed a limited proteomic analysis of several proteins from these nectars and determined that each of the five abundant proteins examined were identified as Nectarin 1, Nectarin 3, or Nectarin 5. Thus, based upon the results found in numerous species across the genus Nicotiana, we conclude that the mechanisms identified are similar to those mechanisms found in previous studies on ornamental tobacco nectars. Further, these similarities are remarkably conserved, throughout the genus Nicotiana.

A shared cis-regulatory module activates transcription in the suspensor of plant embryos

The mechanisms controlling the transcription of gene sets in specific regions of a plant embryo shortly after fertilization remain unknown. Previously, we showed that G564 mRNA, encoding a protein of unknown function, accumulates to high levels in the giant suspensor of both Scarlet Runner Bean (SRB) and Common Bean embryos, and a cis-regulatory module containing three unique DNA sequences, designated as the 10-bp, Region 2, and Fifth motifs, is required for G564 suspensor-specific transcription [Henry KF, et al. (2015) Plant Mol Biol 88:207-217; Kawashima T, et al. (2009) Proc Natl Acad Sci USA 106:3627-3632]. We tested the hypothesis that these motifs are also required for transcription of the SRB GA 20-oxidase gene, which encodes a gibberellic acid hormone biosynthesis enzyme and is coexpressed with G564 at a high level in giant bean suspensors. We used deletion and gain-of-function experiments in transgenic tobacco embryos to show that two GA 20-oxidase DNA regions are required for suspensor-specific transcription, one in the 5' UTR (+119 to +205) and another in the 5' upstream region (-341 to -316). Mutagenesis of sequences in these two regions determined that the cis-regulatory motifs required for G564 suspensor transcription are also required for GA 20-oxidase transcription within the suspensor, although the motif arrangement differs. Our results demonstrate the flexibility of motif positioning within a cis-regulatory module that activates gene transcription within giant bean suspensors and suggest that G564 and GA 20-oxidase comprise part of a suspensor gene regulatory network.

Transcriptome de novo assembly and analysis of differentially expressed genes related to cytoplasmic male sterility in onion

Onion (Allium cepa L.) is one of the major vegetables in China and accounts for a large proportion of China's vegetable exports. Onion cytoplasmic male sterility, which is often used in onion breeding, is caused by the interaction between the nuclear genes and the cytoplasm. However, the underlying molecular mechanism of onion cytoplasmic male sterility remains unclear. In this study, we analysed the anther microstructure of the onion cytoplasmic male sterile line SA2 and the onion maintainer line SB2. We found that the pollen abortion in SA2 occurred at the tetrad stage during the microspore development, which was very different from that in SB2. We used the Illumina HiSeq platform to sequence RNA from anthers at the tetrad stage collected from the SA2 and SB2 lines. The RNA sequencing and transcriptome assembly produced 146,413 All-Unigenes. Based on an analysis of the differentially expressed genes, we identified two cytoplasmic control genes, atp9 and cox1, and three nuclear-related genes, SERK1, AG and AMS. These transcriptomic results were also verified by fluorescence quantitative PCR. Our study provides important information about genes related to onion cytoplasmic male sterility, and it will help improve the understanding of the molecular mechanism of onion cytoplasmic male sterility.

Isolation and Characterization of the Flavonol Regulator CcMYB12 From the Globe Artichoke [Cynara cardunculus var. scolymus (L.) Fiori]

Flavonoids are a well-studied group of secondary metabolites, belonging to the phenylpropanoid pathway. Flavonoids are known to exhibit health promoting effects such as antioxidant capacities, anti-cancer and anti-inflammatory activity. Globe artichoke is an important source of bioactive phenolic compounds, including flavonoids. To study the regulation of their biosynthesis, a R2R3-MYB transcription factor, CcMYB12, was isolated from artichoke leaves. Phylogenetic analysis showed that this protein belongs to the MYB subgroup 7 (flavonol-specific MYB), which includes Arabidopsis AtMYB12, grapevine VvMYBF1, and tomato SlMYB12. CcMYB12 transcripts were detected specifically in artichoke immature inflorescence and young leaves and overlapped with the profiles of flavonol biosynthetic genes. Electrophoretic mobility shift assays (EMSAs) revealed that recombinant CcMYB12 protein is able to bind to ACII element, a DNA binding site ubiquitously present in the promoters of genes encoding flavonol biosynthetic enzymes. In transgenic Arabidopsis plants, the overexpression of CcMYB12 activated the expression of endogenous flavonol biosynthesis genes, leading to an increase of flavonol accumulation and a decrease of anthocyanins in leaves. Likewise, in transgenic tobacco petals and leaves, the overexpression of CcMYB12 decreased anthocyanin levels and increased flavonols.

Evolvement of transgenic male-sterility and fertility-restoration system in rice for production of hybrid varieties

We have developed a unique male-sterility and fertility-restoration system in rice by combining Brassica napus cysteine-protease gene (BnCysP1) with anther-specific P12 promoter of rice for facilitating production of hybrid varieties. In diverse crop plants, male-sterility has been exploited as a useful approach for production of hybrid varieties to harness the benefits of hybrid vigour. The promoter region of Os12bglu38 gene of rice has been isolated from the developing panicles and was designated as P12. The promoter was fused with gusA reporter gene and was expressed in Arabidopsis and rice systems. Transgenic plants exhibited GUS activity in tapetal cells and pollen of the developing anthers indicating anther/pollen-specific expression of the promoter. For engineering nuclear male sterility, the coding region of Brassica napus cysteine protease1 (BnCysP1) was isolated from developing seeds and fused to P12 promoter. Transgenic rice plants obtained with P12-BnCysP1 failed to produce functional pollen grains. The F₁ seeds obtained from BnCysP1 male-sterile plants and untransformed controls showed 1:1 (tolerant:sensitive) ratio when germinated on the MS medium supplemented with phosphinothricin (5 mg/l), confirming that the male sterility has been successfully engineered in rice. For male fertility restoration, transgenic rice plants carrying BnCysP1Si silencing system were developed. The pollination of BnCysP1 male-sterile (female-fertile) plants with BnCysP1Si pollen resulted in normal grain filling. The F₁ seeds of BnCysP1 × BnCysP1Si when germinated on the MS basal medium containing PPT (5 mg/l) and hygromycin (70 mg/l) exhibited 1:1 (tolerant:sensitive) ratio and the tolerant plants invariably showed normal grain filling. The overall results clearly suggest that the customized male-sterility & fertility-restoration system can be exploited for quality hybrid seed production in various crops.

Regulation of tapetum-specific A9 promoter by transcription factors AtMYB80, AtMYB1 and AtMYB4 in Arabidopsis thaliana and Nicotiana tabacum

Tapetum-specific promoters have been successfully used for developing transgenic-based pollination control systems. Although several tapetum-specific promoters have been identified, in-depth studies on regulation of such promoters are scarce. The present study analyzes the regulation of the A9 promoter, one of the first tapetum-specific promoter identified in Arabidopsis thaliana. Transcription factors (TFs) AtMYB80, AtMYB1 (positive regulators) identified by in silico analysis were found to upregulate A9 promoter activity following the over-expression of the TFs in transient and stable (transgenic) expression assays in both A. thaliana and tobacco. Furthermore, mutations of binding sites of these TFs in the A9 promoter led to loss of its activity. The role of a negative regulator AtMYB4 was also studied by analyzing the activity of A9 promoter following transient expression of RNAi against the TF and by mutating binding sites for AtMYB4 in the A9 promoter. While no changes were observed in case of A. thaliana, the A9 promoter was activated in the roots of transgenic tobacco plants, highlighting the role of these cis-elements in keeping the A9 promoter repressed in the roots of tobacco.

The R2R3MYB VvMYBPA1 from grape reprograms the phenylpropanoid pathway in tobacco flowers

MAIN CONCLUSION

This work shows that, in tobacco, the ectopic expression of VvMYBPA1 , a grape regulator of proanthocyanidin biosynthesis, up- or down-regulates different branches of the phenylproanoid pathway, in a structure-specific fashion. Proanthocyanidins are flavonoids of paramount importance for animal and human diet. Research interest increasingly tilts towards generating crops enriched with these health-promoting compounds. Flavonoids synthesis is regulated by the MBW transcriptional complex, made of R2R3MYB, bHLH and WD40 proteins, with the MYB components liable for channeling the complex towards specific branches of the pathway. Hence, using tobacco as a model, here, we tested if the ectopic expression of the proanthocyanidin regulator VvMYBPA1 from grape induces the biosynthesis of these compounds in not-naturally committed cells. Here, we show, via targeted transcriptomic and metabolic analyses of primary transgenic lines and their progeny, that VvMYBPA1 alters the phenylpropanoid pathway in tobacco floral organs, in a structure-specific fashion. We also report that a modest VvMYBPA1 expression is sufficient to induce the expression of both proanthocyanidin-specific and early genes of the phenylpropanoid pathway. Consequently, proanthocyanidins and chlorogenic acids are induced or de novo synthetised in floral limbs, tubes and stamens. Other phenylpropanoid branches are conversely induced or depleted according to the floral structure. Our study documents a novel and distinct function of VvMYBPA1 with respect to other MYBs regulating proanthocyanidins. Present findings may have major implications in designing strategies for enriching crops with health-promoting compounds.

Reactive oxygen species mediate tapetal programmed cell death in tobacco and tomato

BACKGROUND

Hybrid vigor is highly valued in the agricultural industry. Male sterility is an important trait for crop breeding. Pollen development is under strict control of both gametophytic and sporophytic factors, and defects in this process can result in male sterility. Both in the dicot Arabidopsis and in the moncot rice, proper timing of programmed cell death (PCD) in the tapetum ensures pollen development. Dynamic ROS levels have been reported to control tapetal PCD, and thus pollen development, in Arabidopsis and rice. However, it was unclear whether it is evolutionarily conserved, as only those two distantly related species were studied.

RESULTS

Here, we performed histological analyses of anther development of two economically important dicot species, tobacco and tomato. We identified the same ROS amplitude during anther development in these two species and found that dynamic ROS levels correlate with the initiation and progression of tapetal PCD. We further showed that manipulating ROS levels during anther development severely impaired pollen development, resulting in partial male sterility. Finally, real-time quantitative PCR showed that several tobacco and tomato RBOHs, encoding NADPH oxidases, are preferentially expressed in anthers.

CONCLUSION

This study demonstrated evolutionarily conserved ROS amplitude during anther development by examining two commercially important crop species in the Solanaceae. Manipulating ROS amplitude through genetic interference of RBOHs therefore may provide a practical way to generate male sterile plants.

Function Identification of the Nucleotides in Key cis-Element of DYSFUNCTIONAL TAPETUM1 (DYT1) Promoter

As a core regulatory gene of the anther development, DYSFUNCTIONAL TAPETUM1 (DYT1) was expressed in tapetum preferentially. Previous study had confirmed that a "CTCC" sequence within DYT1 promoter was indispensable for correct DYT1 expression. However, precise analysis on the function of each nucleotide of this sequence still lacks. Here we employed site mutation assay to identify the function roles of the nucleotides. As a result, the "T" and final "C" of "CTCC" were found essential for the temporal and spatial specificity of DYT1 expression, whereas the other two "C" nucleotides exhibited substitutable somewhat. The substitutes of two flanking nucleotides of "CTCC," however, hardly affected the normal promoter function, suggesting that the "CTCC" sequence as a whole did meet the standard of a canonical cis-element by definition. In addition, it was found that as short as 497 bp DYT1 promoter was sufficient for tissue-specific expression, while longer 505 bp DYT1 promoter sequence was sufficient for species-specific expression.

Transcriptional regulation of male-sterility in 7B-1 male-sterile tomato mutant

The 7B-1 tomato (Solanum lycopersicum L. cv Rutgers) is a male-sterile mutant with enhanced tolerance to abiotic stress, which makes it a potential candidate for hybrid seed breeding and stress engineering. To underline the molecular mechanism regulating the male-sterility in 7B-1, transcriptomic profiles of the 7B-1 male-sterile and wild type (WT) anthers were studied using mRNA sequencing (RNA-Seq). In total, 768 differentially expressed genes (DEGs) were identified, including 132 up-regulated and 636 down-regulated transcripts. Gene ontology (GO) enrichment analysis of DEGs suggested a general impact of the 7B-1 mutation on metabolic processes, such as proteolysis and carbohydrate catabolic process. Sixteen candidates with key roles in regulation of anther development were subjected to further analysis using qRT-PCR and in situ hybridization. Cytological studies showed several defects associated with anther development in the 7B-1 mutant, including unsynchronized anther maturation, dysfunctional meiosis, arrested microspores, defect in callose degradation and abnormal tapetum development. TUNEL assay showed a defect in programmed cell death (PCD) of tapetal cells in 7B-1 anthers. The present study provides insights into the transcriptome of the 7B-1 mutant. We identified several genes with altered expression level in 7B-1 (including beta-1,3 glucanase, GA2oxs, cystatin, cysteine protease, pectinesterase, TA29, and actin) that could potentially regulate anther developmental processes, such as meiosis, tapetum development, and cell-wall formation/degradation.

Escherichia colimazEF Toxin-Antitoxin System as a Tool to Target Cell Ablation in Plants

BACKGROUND/AIMS

The Escherichia coli MazF is an endoribonuclease that cleaves mRNA at ACA sequences, thereby triggering inhibition of protein synthesis. The aim of this study is to evaluate the efficiency of the mazEF toxin-antitoxin system in plants to develop biotechnological tools for targeted cell ablation.

METHODS

A double transformation strategy, combining expression of the mazE antitoxin gene under the control of the CaMV 35S promoter, reported to drive expression in all plant cells except within the tapetum, together with the expression of the mazF gene under the control of the TA29 tapetum-specific promoter in transgenic tobacco, was applied.

RESULTS

No transgenic TA29-mazF line could be regenerated, suggesting that the TA29 promoter is not strictly tapetum specific and that MazF is toxic for plant cells. The regenerated 35S-mazE/TA29-mazF double-transformed lines gave a unique phenotype where the tapetal cell layer was necrosed resulting in the absence of pollen.

CONCLUSION

These results show that the E. colimazEF system can be used to induce death of specific plant cell types and can provide a new tool to plant cell ablation.

Isolation and Functional Validation of Salinity and Osmotic Stress Inducible Promoter from the Maize Type-II H+-Pyrophosphatase Gene by Deletion Analysis in Transgenic Tobacco Plants

Salinity and drought severely affect both plant growth and productivity, making the isolation and characterization of salinity- or drought-inducible promoters suitable for genetic improvement of crop resistance highly desirable. In this study, a 1468-bp sequence upstream of the translation initiation codon ATG of the promoter for ZmGAPP (maize Type-II H+-pyrophosphatase gene) was cloned. Nine 5´ deletion fragments (D1-D9) of different lengths of the ZmGAPP promoter were fused with the GUS reporter and translocated into tobacco. The deletion analysis showed that fragments D1-D8 responded well to NaCl and PEG stresses, whereas fragment D9 and CaMV 35S did not. The D8 segment (219 bp; -219 to -1 bp) exhibited the highest promoter activity of all tissues, with the exception of petals among the D1-D9 transgenic tobacco, which corresponds to about 10% and 25% of CaMV 35S under normal and NaCl or PEG stress conditions, respectively. As such, the D8 segment may confer strong gene expression in a salinity and osmotic stress inducible manner. A 71-bp segment (-219 to -148 bp) was considered as the key region regulating ZmGAPP response to NaCl or PEG stress, as transient transformation assays demonstrated that the 71-bp sequence was sufficient for the salinity or osmotic stress response. These results enhance our understanding of the molecular mechanisms regulating ZmGAPP expression, and that the D8 promoter would be an ideal candidate for moderating expression of drought and salinity response genes in transgenic plants.

The VviMYB80 Gene is Abnormally Expressed in Vitis vinifera L. cv. 'Zhong Shan Hong' and its Expression in Tobacco Driven by the 35S Promoter Causes Male Sterility

Anther development is a very precise and complicated process. In Arabidopsis, the AtMYB80 transcription factor regulates genes involved in pollen development and controls the timing of tapetal programmed cell death (PCD). In this study, we isolated and characterized cDNA for VviMYB80 expressed in flower buds of male-sterile Vitis vinifera L. cv. 'Zhong Shan Hong', a late-maturing cultivar derived from self-progeny of cv. 'Wink'. VviMYB80 belongs to the MYB80 subfamily and clusters with AtMYB35/TDF1 in a distinct clade. We found that in flower buds, expression of the VviMYB80 gene in cv. 'Zhong Shan Hong' sharply increased at the tetrad stage, resulting in a higher and earlier transcript level than that found in cv. 'Wink'. Expression of the VviMYB80 gene, driven by the 35S promoter, caused pleiotropic effects on the stamens, including smaller and shriveled anthers, delayed dehiscence, fewer seeds, shorter anther filaments, distorted pollen shape and a lack of cytoplasm, with the tapetum exhibiting hypertrophy in transformed tobacco. These results suggest that VviMYB80 may play an important role in stamen development and that expression of VviMYB80 driven by the 35S promoter in tobacco induces male sterility.

Characterization of a spermidine hydroxycinnamoyltransferase in Malus domestica highlights the evolutionary conservation of trihydroxycinnamoyl spermidines in pollen coat of core Eudicotyledons

Phenolamides, so called hydroxycinnamic acid amides, are specialized metabolites produced in higher plants, involved in development, reproduction and serve as defence compounds in biotic interactions. Among them, trihydroxycinnamoyl spermidine derivatives were initially found to be synthetized by a spermidine hydroxycinnamoyltransferase (AtSHT) in Arabidopsis thaliana and to accumulate in the pollen coat. This study reports the identification, in Malus domestica, of an acyltransferase able to complement the sht mutant of Arabidopsis. The quantitative RT-PCR expression profile of MdSHT reveals a specific expression in flowers coordinated with anther development and tapetum cell activities. Three phenolamides including N (1),N (5),N (10)-tricoumaroyl spermidine and N (1),N (5)-dicoumaroyl-N (10)-caffeoyl spermidine identified by LC/MS, were shown to accumulate specifically in pollen grain coat of apple tree. Moreover, in vitro biochemical characterization confirmed MdSHT capacity to synthesize tri-substituted spermidine derivatives with a substrate specificity restricted to p-coumaroyl-CoA and caffeoyl-CoA as an acyl donor. Further investigations of the presence of tri-substituted hydroxycinnamoyl spermidine conjugates in higher plants were performed by targeted metabolic analyses in pollens coupled with bioinformatic analyses of putative SHT orthologues in a wide range of available plant genomes. This work highlights a probable early evolutionary appearance in the common ancestral core Eudicotyledons of a novel enzyme from the BAHD acyltransferase superfamily, dedicated to the synthesis of trihydroxycinnamoyl spermidines in pollen coat. This pathway was maintained in most species; however, recent evolutionary divergences have appeared among Eudicotyledons, such as an organ reallocation of SHT gene expression in Fabales and a loss of SHT in Malvales and Cucurbitales.

Comprehensive Tissue-Specific Transcriptome Analysis Reveals Distinct Regulatory Programs during Early Tomato Fruit Development

Fruit formation and early development involve a range of physiological and morphological transformations of the various constituent tissues of the ovary. These developmental changes vary considerably according to tissue type, but molecular analyses at an organ-wide level inevitably obscure many tissue-specific phenomena. We used laser-capture microdissection coupled to high-throughput RNA sequencing to analyze the transcriptome of ovaries and fruit tissues of the wild tomato species Solanum pimpinellifolium. This laser-capture microdissection-high-throughput RNA sequencing approach allowed quantitative global profiling of gene expression at previously unobtainable levels of spatial resolution, revealing numerous contrasting transcriptome profiles and uncovering rare and cell type-specific transcripts. Coexpressed gene clusters linked specific tissues and stages to major transcriptional changes underlying the ovary-to-fruit transition and provided evidence of regulatory modules related to cell division, photosynthesis, and auxin transport in internal fruit tissues, together with parallel specialization of the pericarp transcriptome in stress responses and secondary metabolism. Analysis of transcription factor expression and regulatory motifs indicated putative gene regulatory modules that may regulate the development of different tissues and hormonal processes. Major alterations in the expression of hormone metabolic and signaling components illustrate the complex hormonal control underpinning fruit formation, with intricate spatiotemporal variations suggesting separate regulatory programs.

A Gene Expression Profiling of Early Rice Stamen Development that Reveals Inhibition of Photosynthetic Genes by OsMADS58

Stamen is a unique plant organ wherein germ cells or microsporocytes that commit to meiosis are initiated from somatic cells during its early developmental process. While genes determining stamen identity are known according to the ABC model of floral development, little information is available on how these genes affect germ cell initiation. By using the Affymetrix GeneChip Rice Genome Array to assess 51 279 transcripts, we established a dynamic gene expression profile (GEP) of the early developmental process of rice (Oryza sativa) stamen. Systematic analysis of the GEP data revealed novel expression patterns of some developmentally important genes including meiosis-, tapetum-, and phytohormone-related genes. Following the finding that a substantial amount of nuclear genes encoding photosynthetic proteins are expressed at the low levels in early rice stamen, through the ChIP-seq analysis we found that a C-class MADS box protein, OsMADS58, binds many nuclear-encoded genes participated in photosystem and light reactions and the expression levels of most of them are increased when expression of OsMADS58 is downregulated in the osmads58 mutant. Furthermore, more pro-chloroplasts are observed and increased signals of reactive oxygen species are detected in the osmads58 mutant anthers. These findings implicate a novel link between stamen identity determination and hypoxia status establishment.

A novel male sterility-fertility restoration system in plants for hybrid seed production

Hybrid seeds are used for stimulated crop production, as they harness heterosis. The achievement of complete male-sterility in the female-parent and the restored-fertility in F1-hybrids are the major bottlenecks in the commercial hybrid seed production. Here, we report a male sterility-fertility restoration system by engineering the in most nutritive anther wall layer tapetum of female and male parents. In the female parent, high-level, and stringent expression of Arabidopsis autophagy-related gene BECLIN1 was achieved in the tapetum, which altered the tapetal degeneration program, leading to male sterility. This works on our previously demonstrated expression cassette based on functional complementation of TATA-box mutant (TGTA) promoter and TATA-binding protein mutant3 (TBPm3), with modification by conjugating Long Hypocotyle in Far-Red1 fragment (HFR1(NT131)) with TBPm3 (HFR1(NT131)-TBPm3) to exercise regulatory control over it. In the male parent, tapetum-specific Constitutive photo-morphogenesis1 (COP1) was expressed. The F1 obtained by crossing these engineered parents showed decreased BECLIN1 expression, which was further completely abolished when COP1-mutant (COP1(L105A)) was used as a male parent, leading to normal tapetal development and restored fertility. The system works on COP1-HFR1 interaction and COP1-mediated degradation of TBPm3 pool (HFR1(NT131)-TBPm3). The system can be deployed for hybrid seed production in agricultural crops.

A cis-regulatory module activating transcription in the suspensor contains five cis-regulatory elements

Little is known about the molecular mechanisms by which the embryo proper and suspensor of plant embryos activate specific gene sets shortly after fertilization. We analyzed the upstream region of the Scarlet Runner Bean (Phaseolus coccineus) G564 gene in order to understand how genes are activated specifically in the suspensor during early embryo development. Previously, we showed that a 54-bp fragment of the G564 upstream region is sufficient for suspensor transcription and contains at least three required cis-regulatory sequences, including the 10-bp motif (5'-GAAAAGCGAA-3'), the 10 bp-like motif (5'-GAAAAACGAA-3'), and Region 2 motif (partial sequence 5'-TTGGT-3'). Here, we use site-directed mutagenesis experiments in transgenic tobacco globular-stage embryos to identify two additional cis-regulatory elements within the 54-bp cis-regulatory module that are required for G564 suspensor transcription: the Fifth motif (5'-GAGTTA-3') and a third 10-bp-related sequence (5'-GAAAACCACA-3'). Further deletion of the 54-bp fragment revealed that a 47-bp fragment containing the five motifs (the 10-bp, 10-bp-like, 10-bp-related, Region 2 and Fifth motifs) is sufficient for suspensor transcription, and represents a cis-regulatory module. A consensus sequence for each type of motif was determined by comparing motif sequences shown to activate suspensor transcription. Phylogenetic analyses suggest that the regulation of G564 is evolutionarily conserved. A homologous cis-regulatory module was found upstream of the G564 ortholog in the Common Bean (Phaseolus vulgaris), indicating that the regulation of G564 is evolutionarily conserved in closely related bean species.

Tapetum-specific expression of a cytoplasmic orf507 gene causes semi-male sterility in transgenic peppers

Though cytoplasmic male sterility (CMS) in peppers is associated with the orf507 gene, definitive and direct evidence that it directly causes male sterility is still lacking. In this study, differences in histochemical localization of anther cytochrome c oxidase between the pepper CMS line and maintainer line were observed mainly in the tapetal cells and tapetal membrane. Inducible and specific expression of the orf507 gene in the pepper maintainer line found that transformants were morphologically similar to untransformed and transformed control plants, but had shrunken anthers that showed little dehiscence and fewer pollen grains with lower germination rate and higher naturally damaged rate. These characters were different from those of CMS line which does not produce any pollen grains. Meanwhile a pollination test using transformants as the male parent set few fruit and there were few seeds in the limited number of fruits. At the tetrad stage, ablation of the tapetal cell induced by premature programmed cell death (PCD) occurred in the transformants and the microspores were distorted and degraded at the mononuclear stage. Stable transmission of induced semi-male sterility was confirmed by a test cross. In addition, expression of orf507 in the maintainer lines seemed to inhibit expression of atp6-2 to a certain extent, and lead to the increase of the activity of cytochrome c oxidase and the ATP hydrolysis of the mitochondrial F1Fo-ATP synthase. These results introduce the premature PCD caused by orf507 gene in tapetal cells and semi-male sterility, but not complete male sterility.

Plant Sterol Diversity in Pollen from Angiosperms

Here we have examined the composition of free sterols and steryl esters of pollen from selected angiosperm species, as a first step towards a comprehensive analysis of sterol biogenesis in the male gametophyte. We detected four major sterol structural groups: cycloartenol derivatives bearing a 9β,19-cyclopropyl group, sterols with a double bond at C-7(8), sterols with a double bond at C-5(6), and stanols. All these groups were unequally distributed among species. However, the distribution of sterols as free sterols or as steryl esters in pollen grains indicated that free sterols were mostly Δ(5)-sterols and that steryl esters were predominantly 9β,19-cyclopropyl sterols. In order to link the sterol composition of a pollen grain at anthesis with the requirement for membrane lipid constituents of the pollen tube, we germinated pollen grains from Nicotiana tabacum, a model plant in reproductive biology. In the presence of radiolabelled mevalonic acid and in a time course series of measurements, we showed that cycloeucalenol was identified as the major neosynthesized sterol. Furthermore, the inhibition of cycloeucalenol neosynthesis by squalestatin was in full agreement with a de novo biogenesis and an apparent truncated pathway in the pollen tube.