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• Environment-sensitive genic male sterility (EGMS) lines are used widely in two-line hybrid breeding in rice (Oryza sativa). At present, photoperiod-sensitive genic male sterility (PGMS) lines and thermo-sensitive genic male sterility (TGMS) lines are predominantly used in two-line hybrid rice, with humidity-sensitive genic male sterility (HGMS) lines rarely being reported. • Here, it is shown that HUMIDITY-SENSITIVE GENIC MALE STERILITY 1 (HMS1), encoding a β-ketoacyl-CoA synthase, plays key roles in the biosynthesis of very-long-chain fatty acids (VLCFAs) and HGMS in rice. • The hms1 mutant displayed decreased seed setting under low humidity, but normal seed setting under high humidity. HMS1 catalyzed the biosynthesis of the C26 and C28 VLCFAs, contributing to the formation of bacula and tryphine in the pollen wall, which protect the pollen from dehydration. Under low-humidity conditions, hms1 pollen showed poor adhesion and reduced germination on the stigmas, which could be rescued by increasing humidity. HMS1-INTERACTING PROTEIN (HMS1I) interacted with HMS1 to coregulate HGMS. Furthermore, both japonica and indica rice varieties with defective HMS1 exhibited HGMS, suggesting that hms1 potentially could be used in hybrid breeding. • The results herein reveal the novel mechanism of VLCFA-mediated pollen wall formation, which protects pollen from low-humidity stress in rice, and has a potential use in hybrid crop breeding.

Hybrid breeding can help us to meet the challenge of feeding a growing world population with limited agricultural land. The demand for soybean is expected to grow; however, the hybrid soybean is still in the process of commercialization even though considerable progress has been made in soybean genome and genetic studies in recent years. Here, we summarize recent advances in male sterility-based breeding programs and the current status of hybrid soybean breeding. A number of male-sterile lines with cytoplasmic male sterility (CMS), genic-controlled photoperiod/thermo-sensitive male sterility, and stable nuclear male sterility (GMS) have been identified in soybean. More than 40 hybrid soybean varieties have been bred using the CMS three-line hybrid system and the cultivation of hybrid soybean is still under way. The key to accelerating hybrid soybean breeding is to increase the out-crossing rate in an economical way. This review outlines current problems with the hybrid soybean breeding systems and explores the current efforts to make the hybrid soybean a commercial success.

[This corrects the article DOI: 10.3389/fpls.2021.635397.].

Phased small-interfering RNAs (phasiRNAs) are a special class of small RNAs, which are generated in 21- or 24-nt intervals from transcripts of precursor RNAs. Although phasiRNAs have been found in a range of organisms, their biological functions in plants have yet to be uncovered. Here we show that phasiRNAs generated by the photopheriod-sensetive genic male sterility 1 (Pms1) locus were associated with photoperiod-sensitive male sterility (PSMS) in rice, a germplasm that started the two-line hybrid rice breeding. The Pms1 locus encodes a long-noncoding RNA PMS1T that was preferentially expressed in young panicles. PMS1T was targeted by miR2118 to produce 21-nt phasiRNAs that preferentially accumulated in the PSMS line under long-day conditions. A single nucleotide polymorphism in PMS1T nearby the miR2118 recognition site was critical for fertility change, likely leading to differential accumulation of the phasiRNAs. This result suggested possible roles of phasiRNAs in reproductive development of rice, demonstrating the potential importance of this RNA class as regulators in biological processes.

Hybrid breeding has emerged as a pivotal strategy to enhance wheat crop yield, a critical step to meet the escalating food demand for the growing global population. Heterosis in wheat can boost crop yield; however, harnessing heterosis in bread wheat is complex and hindered by the species' inherent tendency for self-pollination, high genome ploidy, and limitation of male sterile lines. In contrast, the availability of genetic male sterility, and altering reproductive biology such as anther extrusion and floret opening, is challenging but could facilitate outcrossing. Despite the advancements in sterility systems and molecular tools, an efficient and environmentally stable wheat hybrid production system is still lacking. In this review, we examine the advantages and limitations of different male sterility sources utilized to date including, chemical hybridizing agents (CHAs), cytoplasmic male sterility (CMS), nuclear genic male sterility (NGMS), and environmental-sensitive male sterility (ESMS). Furthermore, we explore the potential of molecular tools such as marker-assisted selection (MAS), genome editing, and other genetic engineering approaches to accelerate hybrid wheat breeding efforts. Future research directions are proposed to develop robust, cost-effective systems by integrating conventional and molecular approaches with advanced screening methods including cytogenomics and next generation sequencing (NGS), which can reliably help to produce stable, high-yielding, and resilient hybrid wheat cultivars compared to current open-pollinated varieties. Collectively, these efforts are vital to achieve the food demands for escalating population under climate change scenario.

Background Tetrad sterility in potato is caused by a specific cytoplasm, called TSC sto , derived from the Mexican wild tetraploid species Solanum stoloniferum . Different S. stoloniferum accessions crossed as females with S. tuberosum resulted in 12 fertile hybrids and 27 sterile hybrids exhibiting tetrad sterility. Results Whole-mitochondrial-genome sequencing was performed for two fertile hybrids and three hybrids exhibiting tetrad sterility. Two to seven contigs, with the total assembly lengths ranging from 462,716 to 535,375 bp, were assembled for each hybrid. Unlike for the reference mitochondrial genome (cv. Désirée), two different recombinant-type contigs (RC-I and RC-II) were identified. RC-I featured by the rpl5-ψrps14 gene joined to the nad6 gene, generating a novel intergenic region. Using a PCR marker (P-3), we found that this intergenic region occurred exclusively in interspecific hybrids exhibiting tetrad sterility and in their parental S. stoloniferum accessions. A part of this intergenic sequence was expressed in the pollen. From a large survey in which P-3 was applied to 129 accessions of 27 mostly Mexican wild species, RC-I was found in diploid S. verrucosum and polyploid species. From eight accessions of S. verrucosum used as females, 92 interspecific hybrids were generated, in which only those carrying RC-I exhibited tetrad sterility. Conclusions RC-I was clearly associated with tetrad sterility, and the RC-I-specific intergenic region likely contains a causal factor of tetrad sterility.

Summary 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.

Summary Genic‐cytoplasmic male sterility (CMS) due to interactions between nuclear and cytoplasmic genomes is a well‐known phenomenon in some Solanum spp. hybrids, but genes involved are not known. In this study, the chondriomes of two isonuclear male‐fertile and sterile somatic hybrids (SH9A and SH9B, respectively) between the common potato (S. tuberosum Group Tuberosum, tbr) and the wild species S. commersonii were sequenced and compared to those of parental species to identify mitochondrial genes involved in the expression of male sterility. A putative novel gene (orf125) was found only in tbr and in male‐sterile hybrids. Physical or functional deletion of orf125 by mtDNA editing in SH9B and its allotopic expression in SH9A clearly demonstrated that orf125 affects male fertility. Besides knockout mutants induced by mitoTALEN and DddA‐derived cytosine base editing, specific orf125 missense mutations generated by the latter approach also induced reversion to male fertility in edited SH9B plants, prompting further studies on ORF125 structure–function relationship. The organization of the mitochondrial genome region implicated in CMS was found to be conserved across all common potato accessions, while an identical copy of tbr orf125 was detected in accessions belonging to the S. berthaultii species complex (ber). Such findings corroborate the hypothesis that ber accessions with T/β cytoplasm outcrossed as female with Andean potato, giving rise to the differentiation of the Chilean potato, and highlight the origin of mitochondrial factors contributing to genic‐cytoplasmic male sterility in some tuber‐bearing Solanum hybrids. Our results contribute to the development of innovative breeding approaches in potato.

Male sterility forms the foundation of hybrid seed production technology in field crops. A variety of genetically controlled male sterility/fertility systems starting with cytoplasmic male sterility (CMS), genic male sterility (GMS) including conditional male sterility and transgenic-based male sterility have been developed and deployed for heterosis breeding over the past century. Here we review environment-sensitive genic male sterility (EGMS) and biotechnology-based male sterility systems and describe the underlying molecular mechanisms. Advances in crop genomics and discovery of a large number of nuclear genes governing anther/pollen development, which are shared across species, are helping design diverse types of male sterile lines suitable for different crop species and situations. In particular, gene editing offers quick and easy route to develop novel male sterility systems for hybrid seed production. We discuss the advantages and challenges of biotechnology-based male sterility systems and present alternative strategies to address concerns of transgenics. Finally, we propose development of functional male sterility systems based on pollen competition as the future area that holds great promise for heterosis breeding.

Summary 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.