A cellular mechanism underlying the restoration of thermo/photoperiod-sensitive genic male sterility

Cloning and functional verification of the male sterile gene BrQRT3 in Chinese cabbage

Chinese cabbage is a cross-pollinated crop with significant heterosis, and male sterile lines are an important way to produce hybrid seeds. In this study, a male sterile mutant msm0795 was identified in an EMS-mutagenized population of Chinese cabbage. Cytological observations revealed that the microspores failed to separate after the tetrad stage, and thus developed into abnormal pollen grains, resulting in anther abortion. MutMap combined with Kompetitive Allele Specific PCR genotyping showed that BraA01g011280.3.5 C was identified as the candidate gene, which encodes polygalacturonase QRT3 and plays a direct role in the degradation of pollen mother cell wall during microspore development, named BrQRT3. Subcellular localization and expression analyses demonstrated that BrQRT3 was localized in the cell membrane and was ubiquitously expressed in roots, stems, leaves, flower buds, and flowers, but the expression of BrQRT3 was gradually suppressed with the anther development. Ectopic expression confirmed that over-expression of BrQRT3 in qrt3 background Arabidopsis mutant can rescue the pollen defects caused by loss of AtQRT3 function. It is the first time to achieve a male sterile mutant caused by the mutation of BrQRT3 in Chinese cabbage. These findings contribute to elucidate the mechanism of BrQRT3 in regulating stamen development of Chinese cabbage.

Temperature and light reverse the fertility of rice P/TGMS line ostms19 via reactive oxygen species homeostasis

P/TGMS (Photo/thermo-sensitive genic male sterile) lines are crucial resources for two-line hybrid rice breeding. Previous studies revealed that slow development is a general mechanism for sterility-fertility conversion of P/TGMS in Arabidopsis. However, the difference in P/TGMS genes between rice and Arabidopsis suggests the presence of a distinct P/TGMS mechanism in rice. In this study, we isolated a novel P/TGMS line, ostms19, which shows sterility under high-temperature conditions and fertility under low-temperature conditions. OsTMS19 encodes a novel pentatricopeptide repeat (PPR) protein essential for pollen formation, in which a point mutation GTA(Val) to GCA(Ala) leads to ostms19 P/TGMS phenotype. It is highly expressed in the tapetum and localized to mitochondria. Under high temperature or long-day photoperiod conditions, excessive ROS accumulation in ostms19 anthers during pollen mitosis disrupts gene expression and intine formation, causing male sterility. Conversely, under low temperature or short-day photoperiod conditions, ROS can be effectively scavenged in anthers, resulting in fertility restoration. This indicates that ROS homeostasis is critical for fertility conversion. This relationship between ROS homeostasis and fertility conversion has also been observed in other tested rice P/TGMS lines. Therefore, we propose that ROS homeostasis is a general mechanism for the sterility-fertility conversion of rice P/TGMS lines.

Acyl carrier protein OsMTACP2 confers rice cold tolerance at the booting stage

Low temperatures occurring at the booting stage in rice (Oryza sativa L.) often result in yield loss by impeding male reproductive development. However, the underlying mechanisms by which rice responds to cold at this stage remain largely unknown. Here, we identified MITOCHONDRIAL ACYL CARRIER PROTEIN 2 (OsMTACP2), the encoded protein of which mediates lipid metabolism involved in the cold response at the booting stage. Loss of OsMTACP2 function compromised cold tolerance, hindering anther cuticle and pollen wall development, resulting in abnormal anther morphology, lower pollen fertility, and seed setting. OsMTACP2 was highly expressed in tapetal cells and microspores during anther development, with the encoded protein localizing to both mitochondria and the cytoplasm. Comparative transcriptomic analysis revealed differential expression of genes related to lipid metabolism between the wild type and the Osmtacp2-1 mutant in response to cold. Through a lipidomic analysis, we demonstrated that wax esters, which are the primary lipid components of the anther cuticle and pollen walls, function as cold-responsive lipids. Their levels increased dramatically in the wild type but not in Osmtacp2-1 when exposed to cold. Additionally, mutants of two cold-induced genes of wax ester biosynthesis, ECERIFERUM1 and WAX CRYSTAL-SPARSE LEAF2, showed decreased cold tolerance. These results suggest that OsMTACP2-mediated wax ester biosynthesis is essential for cold tolerance in rice at the booting stage.

Editing of ORF138 restores fertility of Ogura cytoplasmic male sterile broccoli via mitoTALENs

Cytoplasmic male sterility (CMS), encoded by the mitochondrial open reading frames (ORFs), has long been used to economically produce crop hybrids. However, the utilization of CMS also hinders the exploitation of sterility and fertility variation in the absence of a restorer line, which in turn narrows the genetic background and reduces biodiversity. Here, we used a mitochondrial targeted transcription activator-like effector nuclease (mitoTALENs) to knock out ORF138 from the Ogura CMS broccoli hybrid. The knockout was confirmed by the amplification and re-sequencing read mapping to the mitochondrial genome. As a result, knockout of ORF138 restored the fertility of the CMS hybrid, and simultaneously manifested a cold-sensitive male sterility. ORF138 depletion is stably inherited to the next generation, allowing for direct use in the breeding process. In addition, we proposed a highly reliable and cost-effective toolkit to accelerate the life cycle of fertile lines from CMS-derived broccoli hybrids. By applying the k-mean clustering and interaction network analysis, we identified the central gene networks involved in the fertility restoration and cold-sensitive male sterility. Our study enables mitochondrial genome editing via mitoTALENs in Brassicaceae vegetable crops and provides evidence that the sex production machinery and its temperature-responsive ability are regulated by the mitochondria.

Non-coding RNAs-mediated environmental surveillance determines male fertility in plants

Plants are continuously exposed to environmental stresses leading to significant yield losses. With the changing climatic conditions, the intensity and duration of these stresses are expected to increase, posing a severe threat to crop productivity worldwide. Male gametogenesis is one of the most sensitive developmental stages. Exposure to environmental stresses during this stage leads to male sterility and yield loss. Elucidating the underlying molecular mechanism of environment-affected male sterility is essential to address this challenge. High-throughput RNA sequencing studies, loss-of-function phenotypes of sRNA biogenesis genes and functional genomics studies with non-coding RNAs have started to unveil the roles of small RNAs, long non-coding RNAs and the complex regulatory interactions between them in regulating male fertility under different growth regimes. Here, we discuss the current understanding of the non-coding RNA-mediated environmental stress surveillance and regulation of male fertility in plants. The candidate ncRNAs emerging from these studies can be leveraged to generate environment-sensitive male sterile lines for hybrid breeding or mitigate the impact of climate change on male fertility, as the situation demands.

Biosynthesis and transport of pollen coat precursors in angiosperms

The pollen coat is a hydrophobic mixture on the pollen grain surface, which plays an important role in protecting male gametes from various environmental stresses and microorganism attacks, and in pollen-stigma interactions during pollination in angiosperms. An abnormal pollen coat can result in humidity-sensitive genic male sterility (HGMS), which can be used in two-line hybrid crop breeding. Despite the crucial functions of the pollen coat and the application prospect of its mutants, few studies have focused on pollen coat formation. In this Review, the morphology, composition and function of different types of pollen coat are assessed. On the basis of the ultrastructure and development process of the anther wall and exine found in rice and Arabidopsis, the genes and proteins involved in the biosynthesis of pollen coat precursors and the possible transport and regulation process are sorted. Additionally, current challenges and future perspectives, including potential strategies utilizing HGMS genes in heterosis and plant molecular breeding, are highlighted.

Environment-sensitive genic male sterility in rice and other plants

Environment-sensitive genic male sterility is a type of male sterility that is affected by both genetic and environmental factors. Environment-sensitive genic male sterile lines are not only used in two-line hybrid breeding but are also good materials for studying plant-environment interactions. In this study we review the research progress on environment-sensitive genic male sterility in rice from the perspectives of epigenetic, transcriptional, posttranscriptional, posttranslational and metabolic mechanisms as well as signal transduction processes. While significant progress has been made in the genetics, gene cloning and understanding of the molecular mechanisms of environment-sensitive genic male sterility in recent years, the relevant regulatory network is still poorly understood in rice. We therefore also review studies of environment-sensitive genic male sterility in Arabidopsis and other crops, hoping to promote research in this field in rice. Finally, we analyse the challenges posed by environment-sensitive genic male sterility and provide corresponding suggestions. This review will contribute towards an understanding the molecular genetics of environment-sensitive genic male sterility and its application in two-line hybrid breeding in rice and other species.

Molecular mapping of genomic regions and identification of possible candidate genes associated with gynoecious sex expression in bitter gourd

Bitter gourd is an important vegetable crop grown throughout the tropics mainly because of its high nutritional value. Sex expression and identification of gynoecious trait in cucurbitaceous vegetable crops has facilitated the hybrid breeding programme in a great way to improve productivity. In bitter gourd, gynoecious sex expression is poorly reported and detailed molecular pathways involve yet to be studied. The present experiment was conducted to study the inheritance, identify the genomic regions associated with gynoecious sex expression and to reveal possible candidate genes through QTL-seq. Segregation for the gynoecious and monoecious sex forms in the F₂ progenies indicated single recessive gene controlling gynoecious sex expression in the genotype, PVGy-201. Gynoecious parent, PVGy-201, Monoecious parent, Pusa Do Mausami (PDM), and two contrasting bulks were constituted for deep-sequencing. A total of 10.56, 23.11, 15.07, and 19.38 Gb of clean reads from PVGy-201, PDM, gynoecious bulk and monoecious bulks were generated. Based on the ΔSNP index, 1.31 Mb regions on the chromosome 1 was identified to be associated with gynoecious sex expression in bitter gourd. In the QTL region 293,467 PVGy-201 unique variants, including SNPs and indels, were identified. In the identified QTL region, a total of 1019 homozygous variants were identified between PVGy1 and PDM genomes and 71 among them were non-synonymous variants (SNPS and INDELs), out of which 11 variants (7 INDELs, 4 SNPs) were classified as high impact variants with frame shift/stop gain effect. In total twelve genes associated with male and female gametophyte development were identified in the QTL-region. Ethylene-responsive transcription factor 12, Auxin response factor 6, Copper-transporting ATPase RAN1, CBL-interacting serine/threonine-protein kinase 23, ABC transporter C family member 2, DEAD-box ATP-dependent RNA helicase 1 isoform X2, Polygalacturonase QRT3-like isoform X2, Protein CHROMATIN REMODELING 4 were identified with possible role in gynoecious sex expression. Promoter region variation in 8 among the 12 genes indicated their role in determining gynoecious sex expression in bitter gourd genotype, DBGy-1. The findings in the study provides insight about sex expression in bitter gourd and will facilitate fine mapping and more precise identification of candidate genes through their functional validation.

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.

A 163-bp insertion in the Capana10g000198 encoding a MYB transcription factor causes male sterility in pepper (Capsicum annuum L.)

Male sterility provides an efficient approach for commercial exploitation of heterosis. Despite more than 20 genic male sterile (GMS) mutants documented in pepper (Capsicum annuum L.), only two causal genes have been successfully identified. Here, a novel spontaneous recessive GMS mutant, designated msc-3, is identified and characterized at both phenotypic and histological levels. Pollen abortion of msc-3 mutant may be due to the delayed tapetum degradation, leading to the non-degeneration of tetrads callosic wall. Then, a modified MutMap method and molecular marker linkage analysis were employed to fine mapping the msc-3 locus, which was delimited to the ~139.91-kb region harboring 10 annotated genes. Gene expression and structure variation analyses indicate the Capana10g000198, encoding a R2R3-MYB transcription factor, is the best candidate gene for the msc-3 locus. Expression profiling analysis shows the Capana10g000198 is an anther-specific gene, and a 163-bp insertion in the Capana10g000198 is highly correlated with the male sterile (MS) phenotype. Additionally, downregulation of Capana10g000198 in male fertile plants through virus-induced gene silencing resulted in male sterility. Finally, possible regulatory relationships of the msc-3 gene with the other two reported pepper GMS genes, msc-1 and msc-2, have been studied, and comparative transcriptome analysis reveals the expression of 16 GMS homologs are significantly downregulated in the MS anthers. Overall, our results reveal that Capana10g000198 is the causal gene underlying the msc-3 locus, providing important theoretical clues and basis for further in-depth study on the regulatory mechanisms of pollen development in pepper.

Mutation of glucose-methanol-choline oxidoreductase leads to thermosensitive genic male sterility in rice and Arabidopsis

Thermosensitive genic male sterility (TGMS) lines serve as the major genetic resource for two-line hybrid breeding in rice. However, their unstable sterility under occasional low temperatures in summer highly limits their application. In this study, we identified a novel rice TGMS line, ostms18, of cultivar ZH11 (Oryza sativa ssp. japonica). ostms18 sterility is more stable in summer than the TGMS line carrying the widely used locus tms5 in the ZH11 genetic background, suggesting its potential application for rice breeding. The ostms18 TGMS trait is caused by the point mutation from Gly to Ser in a glucose-methanol-choline (GMC) oxidoreductase; knockout of the oxidoreductase was previously reported to cause complete male sterility. Cellular analysis revealed the pollen wall of ostms18 to be defective, leading to aborted pollen under high temperature. Further analysis showed that the tapetal transcription factor OsMS188 directly regulates OsTMS18 for pollen wall formation. Under low temperature, the flawed pollen wall in ostms18 is sufficient to protect its microspore, allowing for development of functional pollen and restoring fertility. We identified the orthologous gene in Arabidopsis. Although mutants for the gene were fertile under normal conditions (24°C), fertility was significantly reduced under high temperature (28°C), exhibiting a TGMS trait. A cellular mechanism integrated with genetic mutations and different plant species for fertility restoration of TGMS lines is proposed.

Degradation of de-esterified pctin/homogalacturonan by the polygalacturonase GhNSP is necessary for pollen exine formation and male fertility in cotton

The pollen wall exine provides a protective layer for the male gametophyte and is largely composed of sporopollenin, which comprises fatty acid derivatives and phenolics. However, the biochemical nature of the external exine is poorly understood. Here, we show that the male sterile line 1355A of cotton mutated in NO SPINE POLLEN (GhNSP) leads to defective exine formation. The GhNSP locus was identified through map-based cloning and confirmed by genetic analysis (co-segregation test and allele prediction using the CRISPR/Cas9 system). In situ hybridization showed that GhNSP is highly expressed in tapetum. GhNSP encodes a polygalacturonase protein homologous to AtQRT3, which suggests a function for polygalacturonase in pollen exine formation. These results indicate that GhNSP is functionally different from AtQRT3, the latter has the function of microspore separation. Biochemical analysis showed that the percentage of de-esterified pectin was significantly increased in the 1355A anthers at developmental stage 8. Furthermore, immunofluorescence studies using antibodies to the de-esterified and esterified homogalacturonan (JIM5 and JIM7) showed that the Ghnsp mutant exhibits abundant of de-esterified homogalacturonan in the tapetum and exine, coupled with defective exine formation. The characterization of GhNSP provides new understanding of the role of polygalacturonase and de-esterified homogalacturonan in pollen exine formation.

MALE STERILITY 3 encodes a plant homeodomain-finger protein for male fertility in soybean

Male-sterile plants are used in hybrid breeding to improve yield in soybean (Glycine max (L.) Merr.). Developing the capability to alter fertility under different environmental conditions could broaden germplasm resources and simplify hybrid production. However, molecular mechanisms potentially underlying such a system in soybean were unclear. Here, using positional cloning, we identified a gene, MALE STERILITY 3 (MS3), which encodes a nuclear-localized protein containing a plant homeodomain (PHD)-finger domain. A spontaneous mutation in ms3 causing premature termination of MS3 translation and partial loss of the PHD-finger. Transgenetic analysis indicated that MS3 knockout resulted in nonfunctional pollen and no self-pollinated pods, and RNA-seq analysis revealed that MS3 affects the expression of genes associated with carbohydrate metabolism. Strikingly, the fertility of mutant ms3 can restore under long-d conditions. The mutant could thus be used to create a new, more stable photoperiod-sensitive genic male sterility line for two-line hybrid seed production, with significant impact on hybrid breeding and production.

Plant elicitor peptide signalling confers rice resistance to piercing-sucking insect herbivores and pathogens

Rice is a staple food crop worldwide, and its production is severely threatened by phloem-feeding insect herbivores, particularly the brown planthopper (BPH, Nilaparvata lugens), and destructive pathogens. Despite the identification of many BPH resistance genes, the molecular basis of rice resistance to BPH remains largely unclear. Here, we report that the plant elicitor peptide (Pep) signalling confers rice resistance to BPH. Both rice PEP RECEPTORs (PEPRs) and PRECURSORs of PEP (PROPEPs), particularly OsPROPEP3, were transcriptionally induced in leaf sheaths upon BPH infestation. Knockout of OsPEPRs impaired rice resistance to BPH, whereas exogenous application of OsPep3 improved the resistance. Hormone measurement and co-profiling of transcriptomics and metabolomics in OsPep3-treated rice leaf sheaths suggested potential contributions of jasmonic acid biosynthesis, lipid metabolism and phenylpropanoid metabolism to OsPep3-induced rice immunity. Moreover, OsPep3 elicitation also strengthened rice resistance to the fungal pathogen Magnaporthe oryzae and bacterial pathogen Xanthamonas oryzae pv. oryzae and provoked immune responses in wheat. Collectively, this work demonstrates a previously unappreciated importance of the Pep signalling in plants for combating piercing-sucking insect herbivores and promises exogenous application of OsPep3 as an eco-friendly immune stimulator in agriculture for crop protection against a broad spectrum of insect pests and pathogens.

Slow development allows redundant genes to restore the fertility of rpg1, a TGMS line in Arabidopsis

Slow development has been shown to be a general mechanism to restore the fertility of thermo-sensitive and photoperiod-sensitive genic male sterile (TGMS and PGMS) lines in Arabidopsis. rpg1 is a TGMS line defective in primexine, which is essential for pollen wall pattern formation. Here, we showed that RPG1-GFP was highly expressed in microsporocytes, microspores, and pollen grains but not in the tapetum in the complemented transgenic line, suggesting that microsporocytes are the main sporophytic cells for primexine formation. Further cytological observations showed that primexine formation in rpg1 was partially restored under slow growth conditions, leading to its fertility restoration. RPG2 is the homolog of RPG1 in Arabidopsis. We revealed that the fertility recovery of rpg1 rpg2 was significantly reduced compared with that of rpg1 under low temperature. The RPG2-GFP protein was also expressed in microsporocytes in the RPG2-GFP (WT) transgenic line. These results suggest that RPG2 plays a redundant role in rpg1 fertility restoration. rpg1 plants were male sterile at the early growth stage, while their fertility was partially restored at the late developmental stage. The fertility of the rpg1 lateral branches was also partially restored. Further growth analysis showed that slow growth at the late reproductive stage or on the lateral branches led to fertility restoration. This work reveals the importance of gene redundancy in fertility restoration for TGMS lines and provides further insight into pollen wall pattern formation.

Delayed callose degradation restores the fertility of multiple P/TGMS lines in Arabidopsis

Photoperiod/temperature-sensitive genic male sterility (P/TGMS) is widely applied for improving crop production. Previous investigations using the reversible male sterile (rvms) mutant showed that slow development is a general mechanism for restoring fertility to P/TGMS lines in Arabidopsis. In this work, we isolated a restorer of rvms-2 (res3), as the male sterility of rvms-2 was rescued by res3. Phenotype analysis and molecular cloning show that a point mutation in UPEX1 l in res3 leads to delayed secretion of callase A6 from the tapetum to the locule and tetrad callose wall degradation. Electrophoretic mobility shift assay and chromatin immunoprecipitation analysis demonstrated that the tapetal transcription factor ABORTED MICROSPORES directly regulates UPEX1 expression, revealing a pathway for tapetum secretory function. Early degradation of the callose wall in the transgenic line eliminated the fertility restoration effect of res3. The fertility of multiple known P/TGMS lines with pollen wall defects was also restored by res3. We propose that the remnant callose wall may broadly compensate for the pollen wall defects of P/TGMS lines by providing protection for pollen formation. A cellular mechanism is proposed to explain how slow development restores the fertility of P/TGMS lines in Arabidopsis.

The GA-DELLA-OsMS188 module controls male reproductive development in rice

Pollen protects male sperm and allows flowering plants to adapt to diverse terrestrial environments, thereby leading to the rapid expansion of plants into new regions. The process of anther/pollen development is coordinately regulated by internal and external factors including hormones. Currently, the molecular mechanisms underlying gibberellin (GA)-mediated male reproductive development in plants remain unknown. We show here that rice DELLA/SLR1, which encodes the central negative regulator of GA signaling, is essential for rice anther development. The slr1-5 mutant exhibits premature programmed cell death of the tapetum, lacks Ubisch bodies, and has no exine and no mature pollen. SLR1 is mainly expressed in tapetal cells and tetrads, and is required for the appropriate expression of genes encoding key factors of pollen development, which are suggested to be OsMS188-targeted genes. OsMS188 is the main component in the essential genetic program of tapetum and pollen development. Further, we demonstrate that SLR1 interacts with OsMS188 to cooperatively activate the expression of the sporopollenin biosynthesis and transport-related genes CYP703A3, DPW, ABCG15 and PKS1 for rapid formation of pollen walls. Overall, the results of this study suggest that the GA hormonal signal is integrated into the anther genetic program and regulates rice anther development through the GA-DELLA-OsMS188 module.

Genome-Wide Identification and Characterization of Polygalacturonase Gene Family in Maize (Zea mays L.)

Polygalacturonase (PG, EC 3.2.1.15) is a crucial enzyme for pectin degradation and is involved in various developmental processes such as fruit ripening, pollen development, cell expansion, and organ abscission. However, information on the PG gene family in the maize (Zea mays L.) genome and the specific members involved in maize anther development are still lacking. In this study, we identified 55 PG family genes from the maize genome and further characterized their evolutionary relationship and expression patterns. Phylogenetic analysis revealed that ZmPGs are grouped into six Clades, and gene structures of the same Clade are highly conserved, suggesting their functional conservation. The ZmPGs are randomly distributed across maize chromosomes, and collinearity analysis showed that many ZmPGs might be derived from tandem duplications and segmental duplications, and these genes are under purifying selection. Furthermore, gene expression analysis provided insights into possible functional divergence among ZmPGs. Based on the RNA-seq data analysis, we found that many ZmPGs are expressed in various tissues while 18 ZmPGs are highly expressed in maize anther, and their detailed expression profiles in different anther developmental stages were further investigated by using RT-qPCR analysis. These results provide valuable information for further functional characterization and application of the ZmPGs in maize.