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

Identification of two plastid transit peptides for construction of pollen-inactivation system in rice

Hybrid seed production technology (SPT) is achieved through the utilization of a recessive nuclear male-sterile mutant transformed with a transgenic cassette comprising three essential components: the wild-type gene to restore the fertility of the male-sterile mutant, an α-amylase gene to disrupt transgenic pollen grains, and red fluorescence protein gene DsRed to distinguish the transgenic seeds from the nontransgenic male sterile seeds. In rice, we establish the pollen disruption system by introducing an amyloplast targeting signal peptide (ASP) at the N-terminus of maize α-amylase protein ZM-AA1ΔSP (ZM-AA1 with the N-terminal signal peptide removed). The ASP facilitates the transport of ZM-AA1ΔSP protein into amyloplast where it degrades starch, resulting in disruption of the pollen fertility. To obtain such signal peptides for rice, we searched the rice proteins homologous to the defined wheat amyloplast proteins followed by protein-protein interaction network predictions and targeting signal peptides prediction. These analyses enabled the identification of four candidate ASPs in rice, which were designated as ASP1, ASP2, ASP3, and ASP4, respectively. ASP1 and ASP2, when linked with ZM-AA1ΔSP, exhibited the capability to disrupt transgenic pollen grains, whereas ASP3 and ASP4 did not produce this effect. Interestingly, the localization experiments showed that ASP3 and ASP4 were able to target the proteins into chloroplast. The ASP1 and ASP2 sequences provide valuable tools for genetic engineering of the rice male-sterile system, which will contribute to the hybrid rice breeding and production.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01471-y.

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.

Ectopic expression of pigeonpea Orf147 gene imparts partial sterility in Cicer arietinum

Orf147, a cytotoxic peptide, has been found to cause cytoplasmic male sterility (CMS) in Cajanus cajanifolius (pigeonpea). In our study, Orf147 was introduced into self-pollinating Cicer arietinum (chickpea) using Agrobacterium-mediated transformation for induction of CMS. The stable integration and expression of the transgene has been assessed through PCR and qRT-PCR analysis. In addition, phenotypic sterility analysis has been performed, considering developmental parameters like flower development, pod formation and flower drop. Transgene inheritance analysis demonstrates that out of the five PCR positive events in the T₀ generation, two events have segregated according to the Mendelian segregation ratio (3:1) in the T₂ generation. Further, pollen viability test using microscopic analysis confirms the induction of partial CMS in transgenic chickpea. The study holds significant value regarding the heterosis of self-pollinating legumes like chickpea. As a part of the prospect, exploring inducible promoters of species-specific or related legumes would be the next step to developing a two-line hybrid system.

Construction of a Novel Female Sterility System for Hybrid Rice

The main constraints of current hybrid rice technology using male sterility (MS) are the low yield and high labor costs of hybrid rice seed (HRS) production. Therefore, there is an urgent need for innovative new hybrid rice technology. Fortunately, we discovered a unique spontaneous sporophytic female-sterile rice mutant controlled by a single recessive locus in the nucleus. Because female-sterile mutant lines cannot produce any selfed-seeds but their pollen has totally normal functions, female sterility (FS) lines may be considered ideal pollen donors to replace the female-fertile pollen donor parent lines currently used in the HRS production. In this study, a genetically engineered FS-based system was constructed to propagate a pure transgene-free FS line using a bentazon herbicide screening. Additionally, the ability of the FS + MS (FM)-line system, with mixed plantings of FS and MS lines, to produce HRS was tested. The pilot field experiment results showed that HRS of the FM-line system was more efficient compared with the conventional FS to MS strip planting control mode. Thus, this study provides new insights into genetic engineering technology and a promising strategy for the utilization of FS in hybrid rice.

Rice β-glucosidase Os12BGlu38 is required for synthesis of intine cell wall and pollen fertility

Glycoside hydrolase family1 β-glucosidases play a variety of roles in plants, but their in planta functions are largely unknown in rice (Oryza sativa). In this study, the biological function of Os12BGlu38, a rice β-glucosidase, expressed in bicellular to mature pollen, was examined. Genotype analysis of progeny of the self-fertilized heterozygous Os12BGlu38 T-DNA mutant, os12bglu38-1, found no homozygotes and a 1:1 ratio of wild type to heterozygotes. Reciprocal cross analysis demonstrated that Os12BGlu38 deficiency cannot be inherited through the male gamete. In cytological analysis, the mature mutant pollen appeared shrunken and empty. Histochemical staining and TEM showed that mutant pollen lacked intine cell wall, which was rescued by introduction of wild-type Os12BGlu38 genomic DNA. Metabolite profiling analysis revealed that cutin monomers and waxes, the components of the pollen exine layer, were increased in anthers carrying pollen of os12bglu38-1 compared with wild type and complemented lines. Os12BGlu38 fused with green fluorescent protein was localized to the plasma membrane in rice and tobacco. Recombinant Os12BGlu38 exhibited β-glucosidase activity on the universal substrate p-nitrophenyl β-d-glucoside and some oligosaccharides and glycosides. These findings provide evidence that function of a plasma membrane-associated β-glucosidase is necessary for proper intine development.

Exploiting Genic Male Sterility in Rice: From Molecular Dissection to Breeding Applications

Rice (Oryza sativa L.) occupies a very salient and indispensable status among cereal crops, as its vast production is used to feed nearly half of the world's population. Male sterile plants are the fundamental breeding materials needed for specific propagation in order to meet the elevated current food demands. The development of the rice varieties with desired traits has become the ultimate need of the time. Genic male sterility is a predominant system that is vastly deployed and exploited for crop improvement. Hence, the identification of new genetic elements and the cognizance of the underlying regulatory networks affecting male sterility in rice are crucial to harness heterosis and ensure global food security. Over the years, a variety of genomics studies have uncovered numerous mechanisms regulating male sterility in rice, which provided a deeper and wider understanding on the complex molecular basis of anther and pollen development. The recent advances in genomics and the emergence of multiple biotechnological methods have revolutionized the field of rice breeding. In this review, we have briefly documented the recent evolution, exploration, and exploitation of genic male sterility to the improvement of rice crop production. Furthermore, this review describes future perspectives with focus on state-of-the-art developments in the engineering of male sterility to overcome issues associated with male sterility-mediated rice breeding to address the current challenges. Finally, we provide our perspectives on diversified studies regarding the identification and characterization of genic male sterility genes, the development of new biotechnology-based male sterility systems, and their integrated applications for hybrid rice breeding.

Choice of the Promoter for Tissue and Developmental Stage-Specific Gene Expression

Transgenic technologies belong to important tools of reverse genetics and biotechnology in plants. Targeted genetic modifications can reveal functions of genes of interest, change metabolic and regulatory pathways, or result in accumulation of valuable proteins or metabolites. However, to be efficient in targeted genetic modification, the chimeric gene construct should be designed properly. In particular, the promoters used to control transgene expression need to be carefully chosen. Most promoters in widely used vectors belong to strong and constitutively expressed variants. However, in many cases transgene expression has to be restricted to certain tissue, stage of development, or response to some internal or external stimuli. In turn, a large variety of tissue-specific promoters have been studied and information on their characteristics may be recovered from the literature. An appropriate promoter may be selected and used in genetic construct to optimize the transgene transcription pattern. We have previously designed the TGP database (TransGene Promoters, http://wwwmgs.bionet.nsc.ru/mgs/dbases/tgp/home.html ) collecting information from the publications in this field. Here we review the wide range of noncanonical tissue-specific and developmentally regulated promoters that might be used for transgene expression control.

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).

Dissipation Dynamics and Residue of Four Herbicides in Paddy Fields Using HPLC-MS/MS and GC-MS

The dissipation dynamics and residue of pyrazosulfuron-ethyl, bensulfuron-methyl, acetochlor, and butachlor in paddy fields at Good Agricultural Practices (GAP) condition were carefully investigated in this study. The four herbicides’ residues were determined based on a quick, easy, cheap, rugged, safe (QuEChERS) method coupled with HPLC-MS/MS and GC-MS. The limit of detection (LOD) for pyrazosulfuron-ethyl, bensulfuron-methyl, acetochlor, and butachlor in all matrices ranged from 0.04–1.0 ng. The limit of quantification (LOQ) of the four herbicides ranged from 0.01–0.1 mg/kg. Moreover, the average recoveries of the four herbicides ranged from 78.9–108% with relative standard deviations (RSDs) less than 15% at three different fortified levels for different matrices. The dissipation results indicated that the average half-lives (t_1/2) of the four herbicides in soil were in the range of 3.5–17.8 days, and more than 95% of the four herbicides dissipated within 5 days in water. Furthermore, the final residues of the four herbicides were all below the LOQ at harvest time. Such results highlight the dissipation dynamics and residue of the four herbicides in a rice cropping system and contribute to risk assessment as well as scientific guidance on the proper and safe application of herbicides in paddy fields.