A putative bHLH transcription factor is a candidate gene for male sterile 32, a locus affecting pollen and tapetum development in tomato

SlTDF1: A key regulator of tapetum degradation and pollen development in tomato

Pollen formation and development during the life cycle of flowering plant are crucial for maintaining reproductive and genetic diversity. In this study, an R2R3MYB family transcription factor, SlTDF1 (SlMYB35), was predominantly expressed in stamens. Repressed expression of SlTDF1 results in a delay in the degradation of the anther tapetum in tomatoes, which in turn leads to the formation of abnormal pollen, including a reduction in the number of single-fruit seeds and fertility when compared to wild-type plants. Analysis of paraffin sections demonstrated that SlTDF1 is a crucial factor in the maturation of tomato pollen. Further analysis of the transcriptomic data revealed that downregulation of the SlTDF1 gene significantly suppressed the expression of genes related to sugar metabolism and anther development. The findings of this study indicated that SlTDF1 plays a pivotal role in regulating tomato pollen development. Moreover, these findings provide a genetic resource for male sterility in tomato plants.

Molecular advances in research and applications of male sterility systems in tomato

Tomato, belonging to the nightshade family, is globally considered as a model system for classical and molecular genetics, genomics, and reproductive developmental studies. In the current scenario of climate change, hybrid development is among the crucial elements in the genetic improvement of crop plants. The phenomenon of male sterility is a viable approach for ensuring hybrid seed purity and reducing the cost of hybrid seed production. This review aims to shed light on the use of neoteric genomics and genome editing tools in understanding the genetics and molecular regulation of male sterility in tomato. Plant male gametophyte development is highly susceptible to environmental stress. Abnormalities at any stage of male reproductive development, such as premature or delayed tapetal cell degradation triggered by oxidative stress and programmed cell death (PCD) leads to male sterility in tomato. In tomato, more than 55 sporogenous, structural, and functional male sterile mutants, which are mainly under the control of recessive nuclear genes, have been reported. Recently, the role of open reading frames (ORFs) in governing cytoplasmic male sterility in tomato has also been documented. This review highlights the genetic and genomic progress in the investigation of underlying molecular pathways and practical application of potential male sterile mutants in tomato breeding. The applications and future prospects of genome engineering with CRISPR/Cas9 and mitoTALEN in the generation of novel male sterile systems to expedite tomato breeding is discussed.

Identification and Functional Analysis of the Ph-2 Gene Conferring Resistance to Late Blight (Phytophthora infestans) in Tomato

Late blight is a destructive disease affecting tomato production. The identification and characterization of resistance (R) genes are critical for the breeding of late blight-resistant cultivars. The incompletely dominant gene Ph-2 confers resistance against the race T1 of Phytophthora infestans in tomatoes. Herein, we identified Solyc10g085460 (RGA1) as a candidate gene for Ph-2 through the analysis of sequences and post-inoculation expression levels of genes located within the fine mapping interval. The RGA1 was subsequently validated to be a Ph-2 gene through targeted knockout and complementation analyses. It encodes a CC-NBS-LRR disease resistance protein, and transient expression assays conducted in the leaves of Nicotiana benthamiana indicate that Ph-2 is predominantly localized within the nucleus. In comparison to its susceptible allele (ph-2), the transient expression of Ph-2 can elicit hypersensitive responses (HR) in N. benthamiana, and subsequent investigations indicate that the structural integrity of the Ph-2 protein is likely a requirement for inducing HR in this species. Furthermore, ethylene and salicylic acid hormonal signaling pathways may mediate the transmission of the Ph-2 resistance signal, with PR1- and HR-related genes potentially involved in the Ph-2-mediated resistance. Our results could provide a theoretical foundation for the molecular breeding of tomato varieties resistant to late blight and offer valuable insights into elucidating the interaction mechanism between tomatoes and P. infestans.

SlUPA-like, a bHLH Transcription Factor in Tomato (Solanum lycopersicum), Serves as the Crosstalk of GA, JA and BR

The bHLH (basic Helix-Loop-Helix) transcription factor serves as pivotal controller in plant growth and development. In a previous study, the overexpression of SlUPA-like in Solanum lycopersicum L. Ailsa Craig (AC++) altered the JA (Jasmonic acid) response and endogenous GA (Gibberellic acid) content. However, the detailed regulation mechanism was not fully explored. In the present research, we found that the overexpression of SlUPA-like influenced the accumulation of GA, JA and BR (Brassinolide). RNA-Seq data illustrated that the expression levels of genes related to these plant hormones were significantly affected. Additionally, the interaction of SlUPA-like with SlMYB21, SlMYC2 and SlDELLA was characterized by employing Y2H (Yeast Two-Hybrid) and BiFC (Bimolecular Fluorescence Complementation) assay. Furthermore, Dual-LUC (Dual-Luciferase) assay and EMSA (Electrophoretic Mobility Shift Assay) identified that SlUPA-like directly targeted the E-box motif in the promoter of SlGID2 and activated the transcription of SlGID2. These results shed light on the potential role of SlUPA-like in mediating crosstalk among multiple plant hormones and established a robust theoretical framework for further unraveling the functions of SlUPA-like transcription factors in the context of plant growth and hormone signal transduction.

The characteristic analysis of TaTDF1 reveals its function related to male sterility in wheat (Triticum aestivum L.)

BACKGROUND

The male sterile lines are an important foundation for heterosis utilization in wheat (Triticum aestivum L.). Thereinto, pollen development is one of the indispensable processes of wheat reproductive development, and its fertility plays an important role in wheat heterosis utilization, and are usually influencing by genes. However, these key genes and their regulatory networks during pollen abortion are poorly understood in wheat.

RESULTS

DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION 1 (TDF1) is a member of the R2R3-MYB family and has been shown to be essential for early tapetal layer development and pollen grain fertility in rice (Oryza sativa L.) and Arabidopsis thaliana. In order to clarify the function of TDF1 in wheat anthers development, we used OsTDF1 gene as a reference sequence and homologous cloned wheat TaTDF1 gene. TaTDF1 is localized in the nucleus. The average bolting time of Arabidopsis thaliana overexpressed strain (TaTDF1-OE) was 33 d, and its anther could be colored normally by Alexander staining solution, showing red. The dominant Mosaic suppression silence-line (TaTDF1-EAR) was blue-green in color, and the anthers were shrimpy and thin. The TaTDF1 interacting protein (TaMAP65) was confirmed using Yeast Two-Hybrid Assay (Y2H) and Bimolecular-Fluorescence Complementation (BiFC) experiments. The results showed that downregulated expression of TaTDF1 and TaMAP65 could cause anthers to be smaller and shrunken, leading to pollen abortion in TaTDF1 wheat plants induced by virus-induced gene-silencing technology. The expression pattern of TaTDF1 was influenced by TaMAP65.

CONCLUSIONS

Thus, systematically revealing the regulatory mechanism of wheat TaTDF1 during anther and pollen grain development may provide new information on the molecular mechanism of pollen abortion in wheat.

Acyl-CoA synthetase 1 plays an important role on pollen development and male fertility in tomato

The development of pollen is critical to male reproduction in flowering plants. Acyl-CoA synthetase (ACOS) genes play conserved functions in regulating pollen development in various plants. Our previous work found that knockout of the SlACOS1 gene in tomato might decrease fruit setting. The current study further revealed that SlACOS1 was important to pollen development and male fertility. The SlACOS1 gene was preferentially expressed in the stamen of the flower with the highest expression at the tetrad stage of anther development. Mutation of the SlACOS1 gene by the CRISPR/Cas9-editing system reduced pollen number and viability as well as fruit setting. The tapetum layer exhibited premature degradation and the pollen showed abnormal development appearing irregular, shriveled, or anucleate in Slacos1 mutants at the tetrad stage. The fatty acid metabolism in anthers was significantly impacted by mutation of the SlACOS1 gene. Furthermore, targeted fatty acids profiling using GC-MS found that contents of most fatty acids except C18:1 and C18:2 were reduced. Yeast complementation assay demonstrated that the substrate preferences of SlACOS1 were C16:0 and C18:0 fatty acids. Male fertility of Slacos1 mutant could be slightly restored by applying exogenous palmitic acid, a type of C16:0 fatty acid. Taken together, SlACOS1 played important roles on pollen development and male fertility by regulating the fatty acid metabolism and the development of tapetum and tetrad. Our findings will facilitate unraveling the mechanism of pollen development and male fertility in tomato.

A subclass II bHLH transcription factor in Marchantia polymorpha gives insight into the ancestral land plant trait of spore formation

Sporopollenin is often said to be one of the toughest biopolymers known to man. The shift in dormancy cell wall deposition from around the diploid zygotes of charophycean algae to sporopollenin around the haploid spores of land plants essentially imparted onto land plants the gift of passive motility, a key acquisition that contributed to their vast and successful colonization across terrestrial habitats.1,2 A putative transcription factor controlling the land plant mode of sporopollenin deposition is the subclass II bHLHs, which are conserved and novel to land plants, with mutants of genes in angiosperms and mosses divulging roles relating to tapetum degeneration and spore development.3,4,5,6,7 We demonstrate that a subclass II bHLH gene, MpbHLH37, regulates sporopollenin biosynthesis and deposition in the model liverwort Marchantia polymorpha. Mpbhlh37 sporophytes show a striking loss of secondary wall deposits of the capsule wall, the elaters, and the spore exine, all while maintaining spore viability, identifying MpbHLH37 as a master regulator of secondary wall deposits of the sporophyte. Localization of MpbHLH37 to the capsule wall and elaters of the sporophyte directly designates these tissue types as a bona fide tapetum in liverworts, giving support to the notion that the presence of a tapetum is an ancestral land plant trait. Finally, as early land plant spore walls exhibit evidence of tapetal deposition,8,9,10,11,12 a tapetal capsule wall could have provided these plants with a developmental mechanism for sporopollenin deposition.

New Advances in the Study of Regulation of Tomato Flowering-Related Genes Using Biotechnological Approaches

The tomato is a convenient object for studying reproductive processes, which has become a classic. Such complex processes as flowering and fruit setting require an understanding of the fundamental principles of molecular interaction, the structures of genes and proteins, the construction of signaling pathways for transcription regulation, including the synchronous actions of cis-regulatory elements (promoter and enhancer), trans-regulatory elements (transcription factors and regulatory RNAs), and transposable elements and epigenetic regulators (DNA methylation and acetylation, chromatin structure). Here, we discuss the current state of research on tomatoes (2017-2023) devoted to studying the function of genes that regulate flowering and signal regulation systems using genome-editing technologies, RNA interference gene silencing, and gene overexpression, including heterologous expression. Although the central candidate genes for these regulatory components have been identified, a complete picture of their relationship has yet to be formed. Therefore, this review summarizes the latest achievements related to studying the processes of flowering and fruit set. This work attempts to display the gene interaction scheme to better understand the events under consideration.

Various tomato cultivars display contrasting morphological and molecular responses to a chronic heat stress

Climate change is one of the biggest threats that human society currently needs to face. Heat waves associated with global warming negatively affect plant growth and development and will increase in intensity and frequency in the coming years. Tomato is one of the most produced and consumed fruit in the world but remarkable yield losses occur every year due to the sensitivity of many cultivars to heat stress (HS). New insights into how tomato plants are responding to HS will contribute to the development of cultivars with high yields under harsh temperature conditions. In this study, the analysis of microsporogenesis and pollen germination rate of eleven tomato cultivars after exposure to a chronic HS revealed differences between genotypes. Pollen development was either delayed and/or desynchronized by HS depending on the cultivar considered. In addition, except for two, pollen germination was abolished by HS in all cultivars. The transcriptome of floral buds at two developmental stages (tetrad and pollen floral buds) of five cultivars revealed common and specific molecular responses implemented by tomato cultivars to cope with chronic HS. These data provide valuable insights into the diversity of the genetic response of floral buds from different cultivars to HS and may contribute to the development of future climate resilient tomato varieties.

Rapid generation of a tomato male sterility system and its feasible application in hybrid seed production

KEY MESSAGE

A practical approach for the rapid generation and feasible application of green hypocotyl male-sterile (GHMS) tm6 dfr lines in tomato hybrid breeding was established. Male sterility enables reduced cost and high seed purity during hybrid seed production. However, progress toward its commercial application has been slow in tomato due to the disadvantages of most natural male-sterile mutants. Here, we developed a practical method for efficient tomato hybrid seed production using a male-sterile system with visible marker, which was rapidly generated by CRISPR/Cas9-mediated gene editing. Two closely linked genes, TM6 and DFR, which were reported to be candidates of ms15 (male sterile-15) and aw (anthocyanin without) locus, respectively, were knocked out simultaneously in two elite tomato inbred lines. Mutagenesis of both genes generated green hypocotyl male-sterile (GHMS) lines. The GHMS lines exhibited male sterility across different genetic backgrounds and environmental conditions. They also showed green hypocotyl due to defective anthocyanin accumulation, which serves as a reliable visible marker for selecting male-sterile plants at the seedling stage. We further proposed a strategy for multiplying the GHMS system and verified its high efficiency in stable male sterility propagation. Moreover, elite hybrid seeds were produced using GHMS system for potential side effects evaluation, and no adverse influences were found on seed yield, seed quality as well as important agronomic traits. This study provides a practical approach for the rapid generation and feasible application of male sterility in tomato hybrid breeding.

Current insights and advances into plant male sterility: new precision breeding technology based on genome editing applications

Plant male sterility (MS) represents the inability of the plant to generate functional anthers, pollen, or male gametes. Developing MS lines represents one of the most important challenges in plant breeding programs, since the establishment of MS lines is a major goal in F1 hybrid production. For these reasons, MS lines have been developed in several species of economic interest, particularly in horticultural crops and ornamental plants. Over the years, MS has been accomplished through many different techniques ranging from approaches based on cross-mediated conventional breeding methods, to advanced devices based on knowledge of genetics and genomics to the most advanced molecular technologies based on genome editing (GE). GE methods, in particular gene knockout mediated by CRISPR/Cas-related tools, have resulted in flexible and successful strategic ideas used to alter the function of key genes, regulating numerous biological processes including MS. These precision breeding technologies are less time-consuming and can accelerate the creation of new genetic variability with the accumulation of favorable alleles, able to dramatically change the biological process and resulting in a potential efficiency of cultivar development bypassing sexual crosses. The main goal of this manuscript is to provide a general overview of insights and advances into plant male sterility, focusing the attention on the recent new breeding GE-based applications capable of inducing MS by targeting specific nuclear genic loci. A summary of the mechanisms underlying the recent CRISPR technology and relative success applications are described for the main crop and ornamental species. The future challenges and new potential applications of CRISPR/Cas systems in MS mutant production and other potential opportunities will be discussed, as generating CRISPR-edited DNA-free by transient transformation system and transgenerational gene editing for introducing desirable alleles and for precision breeding strategies.

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.

SlMYB72 affects pollen development by regulating autophagy in tomato

The formation and development of pollen are among the most critical processes for reproduction and genetic diversity in the life cycle of flowering plants. The present study found that SlMYB72 was highly expressed in the pollen and tapetum of tomato flowers. Downregulation of SlMYB72 led to a decrease in the amounts of seeds due to abnormal pollen development compared with wild-type plants. Downregulation of SlMYB72 delayed tapetum degradation and inhibited autophagy in tomato anther. Overexpression of SlMYB72 led to abnormal pollen development and delayed tapetum degradation. Expression levels of some autophagy-related genes (ATGs) were decreased in SlMYB72 downregulated plants and increased in overexpression plants. SlMYB72 was directly bound to ACCAAC/ACCAAA motif of the SlATG7 promoter and activated its expression. Downregulation of SlATG7 inhibited the autophagy process and tapetum degradation, resulting in abnormal pollen development in tomatoes. These results indicated SlMYB72 affects the tapetum degradation and pollen development by transcriptional activation of SlATG7 and autophagy in tomato anther. The study expands the understanding of the regulation of autophagy by SlMYB72, uncovers the critical role that autophagy plays in pollen development, and provides potential candidate genes for the production of male-sterility in plants.

Elucidation of the GAUT gene family in eight Rosaceae species and function analysis of PbrGAUT22 in pear pollen tube growth

The phylogenetic relationship and evolutionary history of the GAUT gene family were identified in 8 Rosaseae species. PbrGAUT22 was involved in controlling pollen tube growth by regulating the content of pectins. In plants, galacturonosyltransferases (GAUTs) were involved in homogalacturonan biosynthesis and functioned in maintaining pollen tube cell wall integrity. However, the feature and evolutionary history of the GAUT gene family in Rosaceae species and candidates in pear pollen tube growth remain unclear. Here, we identified 190 GAUT genes in 8 Rosaceae species, including Chinese white pear (Pyrus bretschneideri), European pear (Pyrus communis), apple (Malus × domestica), peach (Prunus persica), Japanese apricot (Prunus mume), sweet cherry (Prunus avium), woodland strawberry (Fragaria vesca) and black raspberry (Rubus occidentalis). Members in GAUT gene family were divided into 4 subfamilies according to the phylogenetic and structural analysis. Whole-genome duplication events and dispersed duplicates drove the expansion of the GAUT gene family. Among 23 pollen-expressed PbrGAUT genes in pear, PbrGAUT22 showed increased expression level during 1-6 h post-cultured pollen tubes. PbrGAUT22 was localized to the cytoplasm and plasma membrane. Knockdown of PbrGAUT22 expression in pollen tubes caused the decrease of pectin content and inhibited pear pollen tubes growth. Taken together, we investigated the identification and evolution of the GAUT gene family in Rosaceae species, and found that PbrGAUT22 played an essential role in the synthesis of pectin and the growth of pear pollen tubes.

Basic Helix-Loop-Helix Transcription Factors: Regulators for Plant Growth Development and Abiotic Stress Responses

Plant basic helix-loop-helix (bHLH) transcription factors are involved in many physiological processes, and they play important roles in the abiotic stress responses. The literature related to genome sequences has increased, with genome-wide studies on the bHLH transcription factors in plants. Researchers have detailed the functionally characterized bHLH transcription factors from different aspects in the model plant Arabidopsis thaliana, such as iron homeostasis and abiotic stresses; however, other important economic crops, such as rice, have not been summarized and highlighted. The bHLH members in the same subfamily have similar functions; therefore, unraveling their regulatory mechanisms will help us to identify and understand the roles of some of the unknown bHLH transcription factors in the same subfamily. In this review, we summarize the available knowledge on functionally characterized bHLH transcription factors according to four categories: plant growth and development; metabolism synthesis; plant signaling, and abiotic stress responses. We also highlight the roles of the bHLH transcription factors in some economic crops, especially in rice, and discuss future research directions for possible genetic applications in crop breeding.

Tomato POLLEN DEFICIENT 2 encodes a G-type lectin receptor kinase required for viable pollen grain formation

Pollen development is a crucial biological process indispensable for seed set in flowering plants and for successful crop breeding. However, little is known about the molecular mechanisms regulating pollen development in crop species. This study reports a novel male-sterile tomato mutant, pollen deficient 2 (pod2), characterized by the production of non-viable pollen grains and resulting in the development of small parthenocarpic fruits. A combined strategy of mapping-by-sequencing and RNA interference-mediated gene silencing was used to prove that the pod2 phenotype is caused by the loss of Solanum lycopersicum G-type lectin receptor kinase II.9 (SlG-LecRK-II.9) activity. In situ hybridization of floral buds showed that POD2/SlG-LecRK-II.9 is specifically expressed in tapetal cells and microspores at the late tetrad stage. Accordingly, abnormalities in meiosis and tapetum programmed cell death in pod2 occurred during microsporogenesis, resulting in the formation of four dysfunctional microspores leading to an aberrant microgametogenesis process. RNA-seq analyses supported the existence of alterations at the final stage of microsporogenesis, since we found tomato deregulated genes whose counterparts in Arabidopsis are essential for the normal progression of male meiosis and cytokinesis. Collectively, our results revealed the essential role of POD2/SlG-LecRK-II.9 in regulating tomato pollen development.

The tapetal tissue is essential for the maintenance of redox homeostasis during microgametogenesis in tomato

The tapetum is a specialized layer of cells within the anther, adjacent to the sporogenous tissue. During its short life, it provides nutrients, molecules and materials to the pollen mother cells and microsporocytes, being essential during callose degradation and pollen wall formation. The interaction between the tapetum and sporogenous cells in Solanum lycopersicum (tomato) plants, despite its importance for breeding purposes, is poorly understood. To investigate this process, gene editing was used to generate loss-of-function mutants that showed the complete and specific absence of tapetal cells. These plants were obtained targeting the previously uncharacterized Solyc03g097530 (SlTPD1) gene, essential for tapetum specification in tomato plants. In the absence of tapetum, sporogenous cells developed and callose deposition was observed. However, sporocytes failed to undergo the process of meiosis and finally degenerated, leading to male sterility. Transcriptomic analysis conducted in mutant anthers lacking tapetum revealed the downregulation of a set of genes related to redox homeostasis. Indeed, mutant anthers showed a reduction in the accumulation of reactive oxygen species (ROS) at early stages and altered activity of ROS-scavenging enzymes. The results obtained highlight the importance of the tapetal tissue in maintaining redox homeostasis during male gametogenesis in tomato plants.

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.

The SlHB8 acts as a negative regulator in tapetum development and pollen wall formation in Tomato

Pollen development is crucial for the fruit setting process of tomatoes, but the underlying regulatory mechanism remains to be elucidated. Here, we report the isolation of one HD-Zip III family transcription factor, SlHB8, whose expression levels decreased as pollen development progressed. SlHB8 knockout using CRISPR/Cas9 increased pollen activity, subsequently inducing fruit setting, whereas overexpression displayed opposite phenotypes. Overexpression lines under control of the 35 s and p2A11 promoters revealed that SlHB8 reduced pollen activity by affecting early pollen development. Transmission electron microscopy and TUNEL analyses showed that SlHB8 accelerated tapetum degradation, leading to collapsed and infertile pollen without an intine and an abnormal exine. RNA-seq analysis of tomato anthers at the tetrad stage showed that SlHB8 positively regulates SPL/NZZ expression and the tapetum programmed cell death conserved genetic pathway DYT1-TDF1-AMS-MYB80 as well as other genes related to tapetum and pollen wall development. In addition, DNA affinity purification sequencing, electrophoretic mobility shift assay, yeast one-hybrid assay and dual-luciferase assay revealed SlHB8 directly activated the expression of genes related to pollen wall development. The study findings demonstrate that SlHB8 is involved in tapetum development and degradation and plays an important role in anther development.

Gene silencing, knockout and over-expression of a transcription factor ABORTED MICROSPORES (SlAMS) strongly affects pollen viability in tomato (Solanum lycopersicum)

BACKGROUND

The tomato (Solanum lycopersicum L.) is an economically valuable crop grown worldwide. Because the use of sterile males reduces the cost of F1 seed production, the innovation of male sterility is of great significance for tomato breeding. The ABORTED MICROSPORES gene (AMS), which encodes for a basic helix-loop-helix (bHLH) transcription factor, has been previously indicated as an essential gene for tapetum development in Arabidopsis and rice. To determine the function of the SlAMS gene (AMS gene from S. lycopersicum) and verify whether it is a potential candidate gene for generating the male sterility in tomato, we used virus-induced gene silencing (VIGS), CRISPR/Cas9-mediated genome editing and over-expression technology to transform tomato via Agrobacterium infection.

RESULTS

Here, the full-length SlAMS gene with 1806 bp from S. lycopersicum (Accession No. MK591950.1) was cloned from pollen cDNA. The results of pollen grains staining showed that, the non-viable pollen proportions of SlAMS-silenced (75%), -knockouted (89%) and -overexpressed plants (60%) were significantly higher than the wild type plants (less than 10%; P < 0.01). In three cases, the morphology of non-viable pollen grains appeared tetragonal, circular, atrophic, shriveled, or otherwise abnormally shaped, while those of wild type appeared oval and plump. Furthermore, the qRT-PCR analysis indicated that SlAMS in anthers of SlAMS-silenced and -knockouted plants had remarkably lower expression than in that of wild type (P < 0.01), and yet it had higher expression in SlAMS-overexpressed plants (P < 0.01).

CONCLUSION

In this paper, Our research suggested alternative approaches to generating male sterility in tomato, among which CRISPR/Cas9-mediated editing of SlAMS implied the best performance. We also demonstrated that the downregulation and upregulation of SlAMS both affected the pollen formation and notably led to reduction of pollen viability, suggesting SlAMS might be essential for regulating pollen development in tomato. These findings may facilitate studies on clarifying the SlAMS-associated molecular regulatory mechanism of pollen development in tomato.

Generating Novel Male Sterile Tomatoes by Editing Respiratory Burst Oxidase Homolog Genes

Hybrid breeding of tomatoes (Solanum lycopersicum), an important vegetable crop, is an effective way to improve yield and enhance disease and stress resistance. However, the efficiency of tomato hybridization is hindered by self-fertilization, which can be overcome using male sterile lines. It has been reported that reactive oxygen species (ROS) act as a key regulator for anther development, mediated by RBOH (Respiratory Burst Oxidase Homolog) genes. Here, two tomato anther-expressed genes, LeRBOH (Solyc01g099620) and LeRBOHE (Solyc07g042460), were selected to cultivate novel tomato male sterile strains. By using a CRISPR/Cas9 system with a two-sgRNA module, the lerboh, lerbohe, and lerboh lerbohe mutant lines were generated, among which the lerbohe and lerboh lerbohe mutants displayed complete male sterility but could accept wild-type pollens and produce fruits normally. Further analysis uncovered significantly decreased ROS levels and abnormal programmed cell death in lerboh lerbohe anthers, indicating a key role of ROS metabolism in tomato pollen development. Taken together, our work demonstrates a successful application of gene editing via CRISPR/Cas9 in generating male sterile tomatoes and afforded helpful information for understanding how RBOH genes regulating tomato reproduction process.

Disruption of the bHLH transcription factor Abnormal Tapetum 1 causes male sterility in watermelon

Although male sterility has been identified as a useful trait for hybrid vigor utilization and hybrid seed production, its underlying molecular mechanisms in Cucurbitaceae species are still largely unclear. Here, a spontaneous male-sterile watermelon mutant, Se18, was reported to have abnormal tapetum development, which resulted in completely aborted pollen grains. Map-based cloning demonstrated that the causal gene Citrullus lanatus Abnormal Tapetum 1 (ClATM1) encodes a basic helix-loop-helix (bHLH) transcription factor with a 10-bp deletion and produces a truncated protein without the bHLH interaction and functional (BIF) domain in Se18 plants. qRT-PCR and RNA in situ hybridization showed that ClATM1 is specifically expressed in the tapetum layer and in microsporocytes during stages 6-8a of anther development. The genetic function of ClATM1 in regulating anther development was verified by CRISPR/Cas9-mediated mutagenesis. Moreover, ClATM1 was significantly downregulated in the Se18 mutant, displaying a clear dose effect at the transcriptional level. Subsequent dual-luciferase reporter, β-glucuronidase (GUS) activity, and yeast one-hybrid assays indicated that ClATM1 could activate its own transcriptional expression through promoter binding. Collectively, ClATM1 is the first male sterility gene cloned from watermelon, and its self-regulatory activity provides new insights into the molecular mechanism underlying anther development in plants.

An integrated transcriptomic and proteomic approach to dynamically study the mechanism of pollen-pistil interactions during jasmine crossing

Jasmine (Jasminum sambac Aiton, Oleaceae) flowers are widely consumed in many countries for their tea-making, medicinal and ornamental properties. To improve the quality and yield of flowers, it is very important to carry out cross-breeding between different petal types of jasmine. However, because of the difficulty of sexual reproduction, there is no report on the success of jasmine crosses. In this paper, single- and double-petal jasmine plants were crossed artificially. The stigmas of single-petal plants post pollination, including those at 0 h after pollination (CK), 1 h after pollination (T1) and 6 h after pollination (T2), were sequenced by transcriptomic combined with proteomic analyses. A total of 178,098 gene products were assembled. Simultaneously, a total of 2337 protein species were identified. Some regulatory gene products and functional protein species were identified that may be involved in the process of pollen-pistil interactions. These findings suggest that the identified differentially expressed gene products and differentially accumulated protein species may play vital roles in jasmine plants in response to pollen-pistil interactions, providing important genetic resources for further functional dissection of the molecular mechanisms of these interactions. SIGNIFICANCE: These results have important scientific significance to take effective measures to overcome pre-fertilization barriers and to guide the cross breeding of jasmine. Further, they can also be used for reference in other plant breeding with the same fertilization barriers.

Comprehensive analysis of SSRs and database construction using all complete gene-coding sequences in major horticultural and representative plants

Simple sequence repeats (SSRs) are one of the most important genetic markers and widely exist in most species. Here, we identified 249,822 SSRs from 3,951,919 genes in 112 plants. Then, we conducted a comprehensive analysis of these SSRs and constructed a plant SSR database (PSSRD). Interestingly, more SSRs were found in lower plants than in higher plants, showing that lower plants needed to adapt to early extreme environments. Four specific enriched functional terms in the lower plant Chlamydomonas reinhardtii were detected when it was compared with seven other higher plants. In addition, Guanylate_cyc existed in more genes of lower plants than of higher plants. In our PSSRD, we constructed an interactive plotting function in the chart interface, and users can easily view the detailed information of SSRs. All SSR information, including sequences, primers, and annotations, can be downloaded from our database. Moreover, we developed Web SSR Finder and Batch SSR Finder tools, which can be easily used for identifying SSRs. Our database was developed using PHP, HTML, JavaScript, and MySQL, which are freely available at http://www.pssrd.info/ . We conducted an analysis of the Myb gene families and flowering genes as two applications of the PSSRD. Further analysis indicated that whole-genome duplication and whole-genome triplication played a major role in the expansion of the Myb gene families. These SSR markers in our database will greatly facilitate comparative genomics and functional genomics studies in the future.

The impaired biosynthetic networks in defective tapetum lead to male sterility in watermelon

Heterosis has been widely applied in watermelon breeding, because of the higher resistance and yield of hybrid. As the basis of heterosis utilization, genic male sterility (GMS) is an important tool for facilitating hybrid seed production, while the detailed mechanism in watermelon is still largely unknown. Here, we report a spontaneous mutant Se18 exhibited complete male sterility due to the uniquely multilayered tapetum and the un-meiotic pollen mother cells during pollen development. Using TMT based quantitative proteomic analyses, a total of 348 differentially abundant proteins (DAPs) were detected with the overwhelming majority down-regulated in mutant Se18. By analyzing the putative orthologs/homologs of Arabidopsis GMS related genes, the biosynthesis and transport of sporopollenin and tryphine precursors were predictably altered in mutant compared to its sibling wild type. Moreover, the general phenylpropanoid pathway as well as its related metabolisms was also expectably impaired in mutant, coincident with the pale yellow petals. Notably, some key transcriptional factors regulating tapetum development, together with their down-regulated targets, offered potentially valuable candidates regarding of male sterility. Collectively, the disrupted regulatory networks underlying male sterility of watermelon was proposed, which provide novel insights into genetic mechanism of male reproductive process and rich gene resources for future research. SIGNIFICANCE: Watermelon is an importantly economical cucurbit crop worldwide, with high nutritional value. Although several male sterile mutants have been identified in watermelon, the underlying molecular mechanism is poorly elucidated. Comparative cytological analysis revealed that the defective development of tapetum was responsible for male sterility in mutant Se18. Combined with the morphological comparison, male floral buds at 2.0-2.5 mm in diameter were confirmed with no obvious phenotypic differences but distinct cytological defects, which were in turn sampled for TMT based proteomic analyses. Referring to functionally characterized GMS related genes, the genetic pathway DYT1-TDF1-AMS-MS188-MS1 regulating tapetum development, together with some downstream targets, were considerably altered in mutant Se18. Moreover, enrichment analyses illustrated the general phenylpropanoid related metabolisms, as well as the biosynthesis and transport of sporopollenin and tryphine precursors, were significantly disrupted in defective anther development. Collectively, the proposed regulatory networks in watermelon not only contribute to a better understanding of molecular mechanisms underlying male sterility, but also provide valuable GMS related candidates for future researches.

Rapid generation of tomato male-sterile lines with a marker use for hybrid seed production by CRISPR/Cas9 system

Owing to their superior agronomic performance, the hybrids of vegetable crops are currently applied extensively. However, effective hybrid production requires a laborious manual emasculation to ensure the purity of hybrid seeds in tomato because of the lack of an effective male sterility system. Here, we created two types of tomato nuclear male-sterile lines with different screening markers in a clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system. Co-knockouts of male sterile 1035 (Ms1035) and glutathione S-transferase (GSTAA) created a male-sterile line marked by a green hypocotyl. The Ms1035 biallelic mutation was introduced into the woolly tomato background, resulting in the linkage of male sterility and a non-woolly phenotype. Two types of male-sterile lines were easily selected at the seedling stage by hypocotyl color or trichome density and further showed high seed purity during hybrid seed production. Our work established the procedure for a rapid transfer of the male-sterile phenotype to the parents of hybrids without extra-modification by the CRISPR/Cas9 system that can be practically applied to hybrid seed production in tomato. This method will be the basis and example for sterile parent creation of multiple crops for hybrid production with the CRISPR/Cas9 system.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-021-01215-2.

A study of male fertility control in Medicago truncatula uncovers an evolutionarily conserved recruitment of two tapetal bHLH subfamilies in plant sexual reproduction

Male sterility is an important tool for plant breeding and hybrid seed production. Male-sterile mutants are largely due to an abnormal development of either the sporophytic or gametophytic anther tissues. Tapetum, a key sporophytic tissue, provides nutrients for pollen development, and its delayed degeneration induces pollen abortion. Numerous bHLH proteins have been documented to participate in the degeneration of the tapetum in angiosperms, but relatively little attention has been given to the evolution of the involved developmental pathways across the phylogeny of land plants. A combination of cellular, molecular, biochemical and evolutionary analyses was used to investigate the male fertility control in Medicago truncatula. We characterized the male-sterile mutant empty anther1 (ean1) and identified EAN1 as a tapetum-specific bHLH transcription factor necessary for tapetum degeneration. Our study uncovered an evolutionarily conserved recruitment of bHLH subfamily II and III(a + c)1 in the regulation of tapetum degeneration. EAN1 belongs to the subfamily II and specifically forms heterodimers with the subfamily III(a + c)1 members, which suggests a heterodimerization mechanism conserved in angiosperms. Our work suggested that the pathway of two tapetal-bHLH subfamilies is conserved in all land plants, and likely was established before the divergence of the spore-producing land plants.

Knockout of SlMS10 Gene (Solyc02g079810) Encoding bHLH Transcription Factor Using CRISPR/Cas9 System Confers Male Sterility Phenotype in Tomato

The utilization of male sterility into hybrid seed production reduces its cost and ensures high purity of tomato varieties because it does not produce pollen and has exserted stigmas. Here, we report on the generation of gene edited lines into male sterility phenotype by knockout of SlMS10 gene (Solyc02g079810) encoding the bHLH transcription factor that regulates meiosis and cell death of the tapetum during microsporogenesis in the tomato. Twenty-eight gene edited lines out of 60 transgenic plants were selected. Of these, eleven different mutation types at the target site of the SlMS10 gene were selected through deep sequencing analysis. These mutations were confirmed to be transmitted to subsequent generations. The null lines without the transferred DNA (T-DNA) were obtained by segregation in the T₁ and T₂ generations. In addition, we showed that the cr-ms10-1-4 mutant line exhibited dysfunctional meiosis and abnormal tapetum during flower development, resulting in no pollen production. RT-PCR analysis showed that the most genes associated with pollen and tapetum development in tomatoes had lower expression in the cr-ms10-1-4 mutant line compared to wild type. We demonstrate that modification of the SlMS10 gene via CRISPR/Cas9-mediated genome editing results in male sterility of tomato plants. Our results suggest an alternative approach to generating male sterility in crops.

A biotechnology-based male-sterility system for hybrid seed production in tomato

Incorporating male sterility into hybrid seed production reduces its cost and ensures high varietal purity. Despite these advantages, male-sterile lines have not been widely used to produce tomato (Solanum lycopersicum) hybrid seeds. We describe the development of a biotechnology-based breeding platform that utilized genic male sterility to produce hybrid seeds. In this platform, we generated a novel male-sterile tomato line by clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9)-mediated mutagenesis of a stamen-specific gene SlSTR1 and devised a transgenic maintainer by transforming male-sterile plants with a fertility-restoration gene linked to a seedling-colour gene. Offspring of crosses between a hemizygous maintainer and the homozygous male-sterile plant segregated into 50% non-transgenic male-sterile plants and 50% male-fertile maintainer plants, which could be easily distinguished by seedling colour. This system has great practical potential for hybrid seed breeding and production as it overcomes the problems intrinsic to other male-sterility systems and can be easily adapted for a range of tomato cultivars and diverse vegetable crops.