Expression of a pathogen-induced cysteine protease (AdCP) in tapetum results in male sterility in transgenic tobacco

Male sterility in plants: an overview of advancements from natural CMS to genetically manipulated systems for hybrid seed production

KEY MESSAGE

The male sterility system in plants has traditionally been utilized for hybrid seed production. In last three decades, genetic manipulation for male sterility has revolutionized this area of research related to hybrid seed production technology. Here, we have surveyed some of the natural cytoplasmic male sterility (CMS) systems that existed/ were developed in different crop plants for developing male sterility-fertility restoration systems used in hybrid seed production and highlighted some of the recent biotechnological advancements in the development of genetically engineered systems that occurred in this area. We have indicated the possible future directions toward the development of engineered male sterility systems. Cytoplasmic male sterility (CMS) is an important trait that is naturally prevalent in many plant species, which has been used in the development of hybrid varieties. This is associated with the use of appropriate genes for fertility restoration provided by the restorer line that restores fertility on the corresponding CMS line. The development of hybrids based on a CMS system has been demonstrated in several different crops. However, there are examples of species, which do not have usable cytoplasmic male sterility and fertility restoration systems (Cytoplasmic Genetic Male Sterility Systems-CGMS) for hybrid variety development. In such plants, it is necessary to develop usable male sterile lines through genetic engineering with the use of heterologous expression of suitable genes that control the development of male gametophyte and fertile male gamete formation. They can also be developed through gene editing using the recently developed CRISPR-Cas technology to knock out suitable genes that are responsible for the development of male gametes. The present review aims at providing an insight into the development of various technologies for successful production of hybrid varieties and is intended to provide only essential information on male sterility systems starting from naturally occurring ones to the genetically engineered systems obtained through different means.

Genotyping by Sequencing for SNP-Based Linkage Analysis and the Development of KASPar Markers for Male Sterility and Polyembryony in Citrus

Polyembryony and male sterility (MS) are essential characters for citrus breeding. MS, coupled with parthenocarpy, allows for addressing the diversification of diploid seedless mandarin varieties, and nucleocytoplasmic MS is the most prevalent system. Polyembryony limits the use of seed parents in scion breeding programs, and the recovery of monoembryonic hybrids to be used as female parents is a crucial pre-breeding component. The objectives of this work were the identification of SNPs closely linked with the genes implied in these traits for marker-assisted selection. Genotyping by sequencing was used to genotype 61 diploid hybrids from an F1 progeny recovered from crossing 'Kiyomi' and 'Murcott' tangors. A total of 6444 segregating markers were identified and used to establish the two parental genetic maps. They consisted of 1374 and 697 markers encompassing 1416.287 and 1339.735 cM for 'Kiyomi' and 'Murcott', respectively. Phenotyping for MS and polyembryony was performed. The genotype-trait association study identified a genomic region on LG8 which was significantly associated with MS, and a genomic region on LG1 which was significantly associated with polyembryony. Annotation of the identified region for MS revealed 19 candidate genes. One SNP KASPar marker was developed and fully validated for each trait.

MYB2 Is Important for Tapetal PCD and Pollen Development by Directly Activating Protease Expression in Arabidopsis

Tapetal programmed cell death (PCD) is a complex biological process that plays an important role in pollen formation and reproduction. Here, we identified the MYB2 transcription factor expressed in the tapetum from stage 5 to stage 11 that was essential for tapetal PCD and pollen development in Arabidopsis thaliana. Downregulation of MYB2 retarded tapetal degeneration, produced defective pollen, and decreased pollen vitality. EMSA and transcriptional activation analysis revealed that MYB2 acted as an upstream activator and directly regulated expression of the proteases CEP1 and βVPE. The expression of these proteases was lower in the buds of the myb2 mutant. Overexpression of either/both CEP1 or/and βVPE proteases partially recover pollen vitality in the myb2 background. Taken together, our results revealed that MYB2 regulates tapetal PCD and pollen development by directly activating expression of the proteases CEP1 and βVPE. Thus, a transcription factor/proteases regulatory and activated cascade was established for tapetal PCD during another development in Arabidopsis thaliana. Highlight: MYB2 is involved in tapetal PCD and pollen development by directly regulating expression of the protease CEP1 and βVPE and establishes a transcription factor/proteases regulatory and activated cascade.

Genome-wide association identifies candidate genes for drought tolerance in coast redwood and giant sequoia

Drought is a major limitation for survival and growth in plants. With more frequent and severe drought episodes occurring due to climate change, it is imperative to understand the genomic and physiological basis of drought tolerance to be able to predict how species will respond in the future. In this study, univariate and multitrait multivariate genome-wide association study methods were used to identify candidate genes in two iconic and ecosystem-dominating species of the western USA, coast redwood and giant sequoia, using 10 drought-related physiological and anatomical traits and genome-wide sequence-capture single nucleotide polymorphisms. Population-level phenotypic variation was found in carbon isotope discrimination, osmotic pressure at full turgor, xylem hydraulic diameter, and total area of transporting fibers in both species. Our study identified new 78 new marker × trait associations in coast redwood and six in giant sequoia, with genes involved in a range of metabolic, stress, and signaling pathways, among other functions. This study contributes to a better understanding of the genomic basis of drought tolerance in long-generation conifers and helps guide current and future conservation efforts in the species.

Genome-wide comparative transcriptome analysis of the A4-CMS line ICPA 2043 and its maintainer ICPB 2043 during the floral bud development of pigeonpea

Cytoplasmic male sterility (CMS) offers a unique system to understand cytoplasmic nuclear crosstalk, and is also employed for exploitation of hybrid vigor in various crops. Pigeonpea A4-CMS, a predominant source of male sterility, is being used for efficient hybrid seed production. The molecular mechanisms of CMS trait remain poorly studied in pigeonpea. We performed genome-wide transcriptome profiling of A4-CMS line ICPA 2043 and its isogenic maintainer ICPB 2043 at two different stages of floral bud development (stage S1 and stage S2). Consistent with the evidences from some other crops, we also observed significant difference in the expression levels of genes in the later stage, i.e., stage S2. Differential expression was observed for 143 and 55 genes within the two stages of ICPA 2043 and ICPB 2043, respectively. We obtained only 10 differentially expressed genes (DEGs) between the stage S1 of the two genotypes, whereas expression change was significant for 582 genes in the case of stage S2. The qRT-PCR assay of randomly selected six genes supported the differential expression of genes between ICPA 2043 and ICPB 2043. Further, GO and KEGG pathway mapping suggested a possible compromise in key bioprocesses during flower and pollen development. Besides providing novel insights into the functional genomics of CMS trait, our results were in strong agreement with the gene expression atlas of pigeonpea that implicated various candidate genes like sucrose-proton symporter 2 and an uncharacterized protein along with pectate lyase, pectinesterase inhibitors, L-ascorbate oxidase homolog, ATPase, β-galactosidase, polygalacturonase, and aldose 1-epimerase for pollen development of pigeonpea. The dataset presented here provides a rich genomic resource to improve understanding of CMS trait and its deployment in heterosis breeding in pigeonpea.

BcAP3, a MADS box gene, controls stamen development and male sterility in Pak-choi (Brassica rapa ssp. chinensis)

Stamen development is an important developmental process controlled by multiple internal and external factors. Developmental abnormalities of stamens can disrupt the structure and function of anthers, and then result in male sterility. As well known, APETELA 3 (AP3) has a clear function in regulating stamen development, which may impact in male sterility. However, the mechanisms of stamen development and male sterility controlled by AP3 are still not very clear, particular in Pak-choi (Brassica rapa ssp. chinensis). In this work, BcAP3 encoded a protein containing a MADS-box domain, which was a homolog of AtAP3, was identified in Pak-choi. Sequence alignments and phylogenetic analysis indicated that BcAP3 was highly similar to AtAP3. BcAP3 was shown to be localized to the nucleus and exhibited the potential of transcription factor. The transcript of BcAP3 was only expressed in flowers of Pak-choi, indicating that it may act in flower development. Overexpression of BcAP3 in Arabidopsis resulted in developmental abnormalities of anther wall and low vigor pollen, which were associated with the phenotype of male sterility. Expression levels of NST1 and NST2, involved in secondary wall thickening in anther walls, were significantly higher in the BcAP3-transgenic plants than in control plants, suggesting that BcAP3 may affect anther wall development by regulating NST1 and NST2. Taken together, our study demonstrated that BcAP3 could play an essential role in stamen development and male sterility.

A proteomic study of cysteine protease induced cell death in anthers of male sterile tobacco transgenic plants

Manifestation of male sterility in plants is an important requirement for hybrid seed production. Tapetum cell layer of anther is a primary target for genetic manipulation for male sterility. In our previous report, the targeted expression of Arachis cysteine protease in tapetum led to premature degeneration of tapetal layer that resulted in complete male sterility in transgenic tobacco plants. To correlate cysteine protease mediated cell death of tapetum, transmission electron microscopy (TEM) and proteomic pattern of anthers of cysteine protease induced male sterile plant were compared with the untransformed control plant. TEM study revealed the abnormal growth of tapetal cells exhibiting excessive vacuolization that synchronized with irregular exine wall formation of the microspores. In anther proteome, a total 250 protein spots were detected that were reproducible and exhibited similar distribution pattern. Further, anther proteome of male sterile plant showed the significant upregulation (≥ 1.5) of 56 protein spots. Using Mass spectroscopy (MALDI TOF/TOF), we have identified 14 protein spots that were involved in several processes such as energy metabolism, protein synthesis, plastid protein, lipid metabolism, and cell wall assembly. Upregulation of patatin-like protein-2 homolog, carboxylesterase 17 and dicer like protein-4 in male sterile anthers that have been demonstrated to induce cell death, suggesting that cysteine protease mediated premature tapetal cell death might involve the lipid peroxidation pathway in coordination with gene silencing mechanism.

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).

Endogenous tassel-specific small RNAs-mediated RNA interference enables a novel glyphosate-inducible male sterility system for commercial production of hybrid seed in Zea mays L

Hybrid crops produce higher yields than their inbred parents due to heterosis. For high purity of hybrid seeds, it is critical to eliminate self-pollination. Manual or mechanical removal of male parts (such as detasseling in maize) is labor-intensive, fuel and time-consuming, and can cause physical damage to female plants, resulting in significant seed yield reductions. Many male-sterility systems either require a maintainer for male-sterile line propagation or are often affected by environmental factors. Roundup® Hybridization System (RHS) utilizes glyphosate to induce male sterility, which effectively eliminates the need for maintainer lines and removal of male parts for commercial hybrid seed production. The first-generation RHS (RHS1) is based on low expression of a glyphosate-insensitive 5-enolpyruvylshikimate-3-phosphate synthase (CP4 EPSPS) in pollen. This report presents the second-generation RHS (RHS2) technology built on RNA interference (RNAi) combined with CP4 EPSPS. It utilizes maize endogenous male tissue-specific small interfering RNAs (mts-siRNAs) to trigger cleavage of the CP4 EPSPS mRNA specifically in tassels, resulting in glyphosate-sensitive male cells due to lack of the CP4 EPSPS protein. Male sterility is then induced by glyphosate application at the stages critical for pollen development, and the male-sterile plants are used as the female parent to produce hybrid seed. The endogenous mts-siRNAs are conserved across maize germplasms, and the inducible male sterility was replicated in representative germplasms through introgression of a CP4 EPSPS transgene containing the mts-siRNA target sequence. This technology combines the relative simplicity and convenience of a systemic herbicide spray methodology with targeted protein expression to create an inducible male sterility system for industrial production of row crop hybrid seeds in an environmentally-independent manner.

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.

Molecular Approaches for Manipulating Male Sterility and Strategies for Fertility Restoration in Plants

Usable pollination control systems have proven to be effective system for the development of hybrid crop varieties, which are important for optimal performance over varied environments and years. They also act as a biocontainment to check horizontal transgene flow. In the last two decades, many genetic manipulations involving genes controlling the production of cytotoxic products, conditional male sterility, altering metabolic processes, post-transcriptional gene silencing, RNA editing and chloroplast engineering methods have been used to develop a proper pollination control system. In this review article, we outline the approaches used for generating male sterile plants using an effective pollination control system to highlight the recent progress that occurred in this area. Furthermore, we propose possible future directions for biotechnological improvements that will allow the farmers to buy hybrid seed once for many generations in a cost-effective manner.

The dead seed coat functions as a long-term storage for active hydrolytic enzymes

Seed development culminates in programmed cell death (PCD) and hardening of organs enclosing the embryo (e.g., pericarp, seed coat) providing essentially a physical shield for protection during storage in the soil. We examined the proposal that dead organs enclosing embryos are unique entities that store and release upon hydration active proteins that might increase seed persistence in soil, germination and seedling establishment. Proteome analyses of dead seed coats of Brassicaceae species revealed hundreds of proteins being stored in the seed coat and released upon hydration, many are stress-associated proteins such as nucleases, proteases and chitinases. Functional analysis revealed that dead seed coats function as long-term storage for multiple active hydrolytic enzymes (e.g., nucleases) that can persist in active forms for decades. Substances released from the dead seed coat of the annual desert plant Anastatica hierochuntica displayed strong antimicrobial activity. Our data highlighted a previously unrecognized feature of dead organs enclosing embryos (e.g., seed coat) functioning not only as a physical shield for embryo protection but also as a long-term storage for active proteins and other substances that are released upon hydration to the “seedsphere” and could contribute to seed persistence in the soil, germination and seedling establishment.

Development of an inducible male-sterility system in rice through pollen-specific expression of l-ornithinase (argE) gene of E. coli

In the present investigation, an inducible male-sterility system has been developed in the rice. In order to introduce inducible male-sterility, the coding region of l-ornithinase (argE) gene of E. coli was fused to the Oryza sativa indica pollen allergen (OSIPA) promoter sequence which is known to function specifically in the pollen grains. Transgenic plants were obtained with argE gene and the transgenic status of plants was confirmed by PCR and Southern blot analyses. RT-PCR analysis confirmed the tissue-specific expression of argE in the anthers of transgenic rice plants. Transgenic rice plants expressing argE, after application of N-acetyl-phosphinothricin (N-ac-PPT), became completely male-sterile owing to the pollen-specific expression of argE. However, argE-transgenic plants were found to be self fertile when N-ac-PPT was not applied. Normal fertile seeds were obtained from the cross pollination between male-sterile argE transgenics and untransformed control plants, indicating that the female fertility is not affected by the N-ac-PPT treatment. These results clearly suggest that the expression of argE gene affects only the male gametophyte but not the gynoecium development. Induction of complete male-sterility in the rice is a first of its kind, and moreover this male- sterility system does not require the deployment of any specific restorer line.

Targeted expression of cystatin restores fertility in cysteine protease induced male sterile tobacco plants

Fertility restoration in male sterile plants is an essential requirement for their utilization in hybrid seed production. In an earlier investigation, we have demonstrated that the targeted expression of a cysteine protease in tapetal cell layer resulted in complete male sterility in tobacco transgenic plants. In the present investigation, we have used a cystatin gene, which encodes for a cysteine protease inhibitor, from a wild peanut, Arachis diogoi and developed a plant gene based restoration system for cysteine protease induced male sterile transgenic tobacco plants. We confirmed the interaction between the cysteine protease and a cystatin of the wild peanut, A. diogoi through in silico modeling and yeast two-hybrid assay. Pollen from primary transgenic tobacco plants expressing cystatin gene under the tapetum specific promoter- TA29 restored fertility on cysteine protease induced male sterile tobacco plants developed earlier. This has confirmed the in vivo interaction of cysteine protease and cystatin in the tapetal cells, and the inactivation of cysteine protease and modulation of its negative effects on pollen fertility. Both the cysteine protease and cystatin genes are of plant origin in contrast to the analogous barnase-barstar system that deploys genes of prokaryotic origin. Because of the deployment of genes of plant origin, this system might not face biosafety problems in developing hybrids in food crops.