A separation defect of tapetum cells and microspore mother cells results in male sterility in Brassica napus: the role of abscisic acid in early anther development

Transcriptomics reveals a core transcriptional network of K-type cytoplasmic male sterility microspore abortion in wheat (Triticum aestivum L.)

BACKGROUND

Cytoplasmic male sterility (CMS) plays a crucial role in hybrid production. K-type CMS, a cytoplasmic male sterile line of wheat with the cytoplasms of Aegilops kotschyi, is widely used due to its excellent characteristics of agronomic performance, easy maintenance and easy restoration. However, the mechanism of its pollen abortion is not yet clear.

RESULTS

In this study, wheat K-type CMS MS(KOTS)-90-110 (MS line) and it's fertile near-isogenic line MR (KOTS)-90-110 (MR line) were investigated. Cytological analysis indicated that the anthers of MS line microspore nucleus failed to divide normally into two sperm nucleus and lacked starch in mature pollen grains, and the key abortive period was the uninucleate stage to dinuclear stage. Then, we compared the transcriptome of MS line and MR line anthers at these two stages. 11,360 and 5182 differentially expressed genes (DEGs) were identified between the MS and MR lines in the early uninucleate and binucleate stages, respectively. Based on GO enrichment and KEGG pathways analysis, it was evident that significant transcriptomic differences were "plant hormone signal transduction", "MAPK signaling pathway" and "spliceosome". We identified 17 and 10 DEGs associated with the IAA and ABA signal transduction pathways, respectively. DEGs related to IAA signal transduction pathway were downregulated in the early uninucleate stage of MS line. The expression level of DEGs related to ABA pathway was significantly upregulated in MS line at the binucleate stage compared to MR line. The determination of plant hormone content and qRT-PCR further confirmed that hormone imbalance in MS lines. Meanwhile, 1 and 2 DEGs involved in ABA and Ethylene metabolism were also identified in the MAPK cascade pathway, respectively; the significant up regulation of spliceosome related genes in MS line may be another important factor leading to pollen abortion.

CONCLUSIONS

We proposed a transcriptome-mediated pollen abortion network for K-type CMS in wheat. The main idea is hormone imbalance may be the primary factor, MAPK cascade pathway and alternative splicing (AS) may also play important regulatory roles in this process. These findings provided intriguing insights for the molecular mechanism of microspore abortion in K-type CMS, and also give useful clues to identify the crucial genes of CMS in wheat.

Disruption of the Contents of Endogenous Hormones Cause Pollen Development Obstruction and Abortion in Male-Sterile Hybrid Lily Populations

Lilies are well-known flowers with large anthers and a high quantity of pollen that easily contaminates clothing and tepals. The anthers need to be artificially removed, leading to production problems. Cultivating male-sterile or pollen-free lilies could solve these problems. The key period of male sterility in a specific male-sterile hybrid lily population was determined through cytological observation. The contents of hormones, soluble sugar, soluble protein, and proline were determined by high-performance liquid chromatography, tandem mass spectrometry and colorimetry. Transcriptome sequencing was used to identify the genes with altered expression. The key period of male sterility was determined to be the microspore mother and tetrad stages. The hormone contents were abnormal in the sterile line compared with the fertile line. The indole-3-acetic acid (IAA) content was higher in the sterile line than in the fertile line at all stages, while the gibberellic acid 4 (GA4) content showed the opposite result. Abscisic acid (ABA) accumulated in the sterile line in both the microspore mother and tetrad stages, and the zeatin riboside (ZR) content in the sterile line increased at the microspore mother stage but decreased at the tetrad stage. The contents of soluble sugar, soluble protein and proline were higher in the fertile line than in the sterile line. Genes involved in auxin and ABA synthesis and signalling pathways were highly expressed in the male-sterile line. Our data suggested that abnormal contents of hormones in the microspore mother and tetrad stages resulted in pollen abortion in a male-sterile hybrid lily population, which indicated that the hormone balance in specific stages plays critical functions in pollen development in lilies.

Glyphosate-Induced Abscisic Acid Accumulation Causes Male Sterility in Sea Island Cotton

Sea Island cotton is the best quality tetraploid cultivated cotton in the world, in terms of fiber quality. Glyphosate is a widely used herbicide in cotton production, and the improper use of herbicides has led to pollen abortion in sea island cotton and, consequently, to a dramatic decrease in yield; however, the mechanism remains unclear. In this study, different concentrations (0, 3.75, 7.5, 15, and 30 g/L) of glyphosate were applied to CP4-EPSPS transgenic sea island cotton Xinchang 5 in 2021 and 2022 at Korla, with 15 g/L glyphosate chosen as the suitable concentration. By comparing the paraffin sections of 2-24 mm anthers in the 15 g/L glyphosate treatment group with those in the water control group, we showed that the key period of anther abortion after glyphosate treatment was the formation and development of tetrads, which corresponded to 8-9 mm buds. Transcriptome sequencing analysis of the treated and control anthers revealed a significant enrichment of differentially expressed genes in phytohormone-related pathways, in particular abscisic acid response and regulation pathways. Additionally, after treatment with 15 g/L of glyphosate, there was a significant increase in the amount of abscisic acid in the anthers in the 8-9 mm buds. Further analysis of the differential expression of abscisic acid response and regulatory genes, an abscisic acid response gene GbTCP14 (Gbar_A11G003090) was identified, which was significantly upregulated in buds with 15 g/L glyphosate treatment than the control, and it could be a key candidate gene for the subsequent research involving male sterility induced by glyphosate in sea island cotton.

Cold Stress Response Mechanisms in Anther Development

Unlike animals that can escape threats, plants must endure and adapt to biotic and abiotic stresses in their surroundings. One such condition, cold stress, impairs the normal growth and development of plants, in which most phases of reproductive development are particularly susceptible to external low temperature. Exposed to uncomfortably low temperature at the reproductive stage, meiosis, tapetal programmed cell death (PCD), pollen viability, and fertilization are disrupted, resulting in plant sterility. Of them, cold-induced tapetal dysfunction is the main cause of pollen sterility by blocking nutrition supplements for microspore development and altering their timely PCD. Further evidence has indicated that the homeostatic imbalances of hormones, including abscisic acid (ABA) and gibberellic acid (GA), and sugars have occurred in the cold-treated anthers. Among them, cold stress gives rise to the accumulation of ABA and the decrease of active GA in anthers to affect tapetal development and represses the transport of sugar to microspores. Therefore, plants have evolved lots of mechanisms to alleviate the damage of external cold stress to reproductive development by mainly regulating phytohormone levels and sugar metabolism. Herein, we discuss the physiological and metabolic effects of low temperature on male reproductive development and the underlying mechanisms from the perspective of molecular biology. A deep understanding of cold stress response mechanisms in anther development will provide noteworthy references for cold-tolerant crop breeding and crop production under cold stress.

Two wrongs make a right: heat stress reversion of a male-sterile Brassica napus line

Male-sterile lines play important roles in plant breeding to obtain hybrid vigour. The male sterility Lembke (MSL) system is a thermosensitive genic male sterility system of Brassica napus and is one of the main systems used in European rapeseed breeding. Interestingly, the MSL system shows high similarity to the 9012AB breeding system from China, including the ability to revert to fertile in high temperature conditions. Here we demonstrate that the MSL system is regulated by the same restorer of fertility gene BnaC9-Tic40 as the 9012AB system, which is related to the translocon at the inner envelope membrane of chloroplasts 40 (TIC40) from Arabidopsis. The male sterility gene of the MSL system was also identified to encode a chloroplast-localized protein which we call BnChimera; this gene shows high sequence similarity to the sterility gene previously described for the 9012AB system. For the first time, a direct protein interaction between BnaC9-Tic40 and the BnChimera could be demonstrated. In addition, we identify the corresponding amino acids that mediate this interaction and suggest how BnaC9-Tic40 acts as the restorer of fertility. Using an RNA-seq approach, the effects of heat treatment on the male fertility restoration of the C545 MSL system line were investigated. These data demonstrate that many pollen developmental pathways are affected by higher temperatures. It is hypothesized that heat stress reverses the male sterility via a combination of slower production of cell wall precursors in plastids and a slower flower development, which ultimately results in fertile pollen. The potential breeding applications of these results are discussed regarding the use of the MSL system in producing thermotolerant fertile plants.

Hormonal Signaling in the Progamic Phase of Fertilization in Plants

Pollen–pistil interaction is a basic process in the reproductive biology of flowering plants and has been the subject of intense fundamental research that has a pronounced practical value. The phytohormones ethylene (ET) and cytokinin (CK) together with other hormones such as auxin, gibberellin (GA), jasmonic acid (JA), abscisic acid (ABA), and brassinosteroids (BRs) influence different stages of plant development and growth. Here, we mainly focus on the information about the ET and CK signaling in the progamic phase of fertilization. This signaling occurs during male gametophyte development, including tapetum (TAP) cell death, and pollen tube growth, including synergid programmed cell death (PCD) and self-incompatibility (SI)-induced PCD. ET joins the coordination of successive events in the developing anther, including the TAP development and cell death, anther dehiscence, microspore development, pollen grain maturation, and dehydration. Both ET and CK take part in the regulation of E. ET signaling accompanies adhesion, hydration, and germination of pollen grains in the stigma and growth of pollen tubes in style tissues. Thus, ET production may be implicated in the pollination signaling between organs accumulated in the stigma and transmitted to the style and ovary to ensure successful pollination. Some data suggest that ET and CK signaling are involved in S-RNase-based SI.

Cell wall Glycine-rich Protein2 is involved in tapetal differentiation and pollen maturation

The tapetum plays important roles in anther development by providing materials for pollen-wall formation and nutrients for pollen development. Here, we report the characterization of a male-sterile mutant of glycine-rich protein 2 (OsGRP2), which exhibits irregular cell division and dysfunction of the tapetum. GRP is a cellwall structural protein present in the cell walls of diverse plant species, but its function is unclear in pollen development. We found that few GRP genes are expressed in rice and thus focused on one highly expressed gene, OsGRP2. The tapetal cell walls of an OsGRP2 mutant did not thicken at the pollen mothercell stage, as a result, pollen maturation and fertility rate decreased. High OsGRP2 expression was detected in male-floral organs, and OsGRP2 was distributed in the tapetum. OsGRP2 participated in establishment of the cellwall network during early tapetum development. In conclusion, our results indicate that OsGRP2 plays important roles in the differentiation and function of the tapetum.

Identification of fertility-related genes for maize CMS-S via Bulked Segregant RNA-Seq

Cytoplasmic male sterility (CMS) is extensively used in maize hybrid production, and identification of genes related to fertility restoration for CMS is important for hybrid breeding. The fertility restoration of S type CMS is governed by several loci with major and minor effects, while the mechanism of fertility restoration for CMS-S is still unknown. In this study, BSR-Seq was conducted with two backcrossing populations with the fertility restoration genes, Rf3 and Rf10, respectively. Genetic mapping via BSR-Seq verified the positions of the two loci. A total of 353 and 176 differentially expressed genes (DEGs) between the male fertility and male sterile pools were identified in the populations with Rf3 and Rf10, respectively. In total, 265 DEGs were co-expressed in the two populations, which were up-regulated in the fertile plants, and they might be related to male fertility involving in anther or pollen development. Moreover, 35 and seven DEGs were specifically up-regulated in the fertile plants of the population with Rf3 and Rf10, respectively. Function analysis of these DEGs revealed that jasmonic acid (JA) signal pathway might be involved in the Rf3 mediated fertility restoration for CMS-S, while the small ubiquitin-related modifier system could play a role in the fertility restoration of Rf10.

Two young genes reshape a novel interaction network in Brassica napus

New genes often drive the evolution of gene interaction networks. In Brassica napus, the widely used genic male sterile breeding system 7365ABC is controlled by two young genes, Bnams4^b and BnaMs3. However, the interaction mechanism of these two young genes remains unclear. Here, we confirmed that Bnams4^b interacts with the nuclear localised E3 ligase BRUTUS (BTS). Ectopic expression of AtBRUTUS (AtBTS) and comparison between Bnams4^b -transgenic Arabidopsis and bts mutants suggested that Bnams4^b may drive translocation of BTS to cause various toxic defects. BnaMs3 gained an exclusive interaction with the plastid outer-membrane translocon Toc33 compared with Bnams3 and AtTic40, and specifically compensated for the toxic effects of Bnams4^b . Heat shock treatment also rescued the sterile phenotype, and high temperature suppressed the interaction between Bnams4^b and BTS in yeast. Furthermore, the ubiquitin system and TOC (translocon at the outer envelope membrane of chloroplasts) component accumulation were affected in Bnams4^b -transgenic Arabidopsis plants. Taken together, these results indicate that new chimeric Bnams4^b carries BTS from nucleus to chloroplast, which may disrupt the normal ubiquitin-proteasome system to cause toxic effects, and these defects can be compensated by BnaMs3-Toc33 interaction or environmental heat shock. It reveals a scenario in which two population-specific coevolved young genes reshape a novel interaction network in plants.

Genome-wide identification, phylogeny and expression analysis of the PME and PMEI gene families in maize

Pectins, the major components of cell walls in plants, are synthesized and secreted to cell walls as highly methyl-esterified polymers and then demethyl-esterified by pectin methylesterases (PMEs). The PMEs are spatially regulated by pectin methylesterase inhibitors (PMEIs). In this study, 43 and 49 putative PME and PMEI genes were identified in maize, respectively. Gene structure and motif analysis revealed that members in the same paralogous pairs or in the same subgroup generally had common motif compositions and gene structure patterns, which indicates functional similarity between the closely related ZmPME/PMEI genes. Gene ontology annotation analysis showed that most of the ZmPME/PMEI genes are involved in cell wall modification and pectin catabolic process with molecular functions of pectinesterase or pectinesterase inhibitor activities. There are 35 ZmPME/PMEI genes expressed higher in anthers than in other tissues from the NimbleGen maize microarray data, and the semiq-RT-PCR assay revealed most of these ZmPME/PMEIs specially expressed in anthers and pollens, indicating they possibly had role in anther and pollen development. In addition, these ZmPME/PMEI genes were highly expressed in the fertile anthers, while lowly or no expressed in sterile anthers. This further indicated these genes might be involved in the development of anther and pollen.

Construction of restorer lines and molecular mapping for restorer gene of hau cytoplasmic male sterility in Brassica napus

Successfully constructing restorer lines for the hau CMS line and molecular mapping of Rfh to a 94 kb candidate region on chromosome A03 in Brassica napus. Cytoplasmic male sterility is a general phenomenon in almost 200 species, and the interaction between chimeric genes in mitochondria and restorer genes in nucleus may be responsible for restoration of male fertility. Orf288 has been identified as a CMS-associated gene in the hau CMS line of Brassica napus and Brassica juncea; however, the restorer lines/genes have not been found yet. We therefore have successfully constructed two restorer lines in B. napus by extensive testcrossing and have mapped a major restorer gene Rfh to a physical distance of 94 kb on chromosome A03 by whole-genome resequencing and molecular markers. We found that the restorer line is indeed restored to male fertility at histological level. Comparative genomics and collinearity analysis between close relatives revealed that rearrangements and recombination may have happened and thus caused the production of Rfh or components of the restoration of fertility complex. Meanwhile, nuclear backgrounds with multiple loci and temperature were related to the variation and instability of restoration of fertility in three different populations. Our study provides new sights into the coevolution between restorer genes and CMS-associated genes as well as the cultivation of superior hybrids via molecular breeding.

Abscisic acid prevents pollen abortion under high-temperature stress by mediating sugar metabolism in rice spikelets

Heat stress at the pollen mother cell (PMC) meiotic stage leads to pollen sterility in rice, in which the reactive oxygen species (ROS) and sugar homeostasis are always adversely affected. This damage is reversed by abscisic acid (ABA), but the mechanisms underlying the interactions among the ABA, sugar metabolism, ROS and heat shock proteins in rice spikelets under heat stress are unclear. Two rice genotypes, Zhefu802 (a recurrent parent) and fgl (its near-isogenic line) were subjected to heat stress of 40°C after pre-foliage sprayed with ABA and its biosynthetic inhibitor fluridone at the meiotic stage of PMC. The results revealed that exogenous application of ABA reduced pollen sterility caused by heat stress. This was achieved through various means, including: increased levels of soluble sugars, starch and non-structural carbohydrates, markedly higher relative expression levels of heat shock proteins (HSP24.1 and HSP71.1) and genes related to sugar metabolism and transport, such as sucrose transporters (SUT) genes, sucrose synthase (SUS) genes and invertase (INV) genes as well as increased antioxidant activities and increased content of adenosine triphosphate and endogenous ABA in spikelets. In short, exogenous application of ABA prior to heat stress enhanced sucrose transport and accelerated sucrose metabolism to maintain the carbon balance and energy homeostasis, thus ABA contributed to heat tolerance in rice.

Metabolic Alterations in Male-Sterile Potato as Compared to Male-Fertile

The common potato, Solanum tuberosum L., is the fourth most important agricultural crop worldwide. Until recently, vegetative propagation by tubers has been the main method of potato cultivation. A shift of interest to sexual potato reproduction by true botanical seeds is due to the appearance of a new hybrid seed breeding strategy whose successful application for many crop species has been supported by male sterility. This investigation was focused on the study of differences in the metabolite profiles of anthers at the mature pollen stage from male-fertile and male-sterile genotypes of S. tuberosum. Application of gas chromatography coupled with a mass spectrometry method allowed detection of metabolic profiles for 192 compounds. Further data analysis with several libraries fully identified 75 metabolites; a similar amount was defined up to the classes. Metabolic profiles in the anthers of fertile genotypes were significantly distinguished from male-sterile ones by the accumulation of carbohydrates, while the anthers of sterile genotypes contained a higher amount of amino acids. In comparison with male-fertile plants, male-sterile genotypes had undeveloped pollen grain characters; i.e., smaller grain size, a thicker exine, “permanent tetrads” that failed to disintegrate into microspores, and the absence of pollen apertures that might be due to a disorder in the metabolism of carbohydrates and fatty acids.

Cold Influences Male Reproductive Development in Plants: A Hazard to Fertility, but a Window for Evolution

Being sessile organisms, plants suffer from various abiotic stresses including low temperature. In particular, male reproductive development of plants is extremely sensitive to cold which may dramatically reduce viable pollen shed and plant fertility. Cold stress disrupts stamen development and prominently interferes with the tapetum, with the stress-responsive hormones ABA and gibberellic acid being greatly involved. In particular, low temperature stress delays and/or inhibits programmed cell death of the tapetal cells which consequently damages pollen development and causes male sterility. On the other hand, studies in Arabidopsis and crops have revealed that ectopically decreased temperature has an impact on recombination and cytokinesis during meiotic cell division, implying a putative role for temperature in manipulating plant genomic diversity and architecture during the evolution of plants. Here, we review the current understanding of the physiological impact of cold stress on the main male reproductive development processes including tapetum development, male meiosis and gametogenesis. Moreover, we provide insights into the genetic factors and signaling pathways that are involved, with putative mechanisms being discussed.

Genome-Wide DNA Methylation Comparison between Brassica napus Genic Male Sterile Line and Restorer Line

DNA methylation is an essential epigenetic modification that dynamically regulates gene expression during plant development. However, few studies have determined the DNA methylation profiles of male-sterile rapeseed. Here, we conducted a global comparison of DNA methylation patterns between the rapeseed genic male sterile line 7365A and its near-isogenic fertile line 7365B by whole-genome bisulfite sequencing (WGBS). Profiling of the genome-wide DNA methylation showed that the methylation level in floral buds was lower than that in leaves and roots. Besides, a total of 410 differentially methylated region-associated genes (DMGs) were identified in 7365A relative to 7365B. Traditional bisulfite sequencing polymerase chain reaction (PCR) was performed to validate the WGBS data. Eleven DMGs were found to be involved in anther and pollen development, which were analyzed by quantitative PCR. In particular, Bnams4 was hypo-methylated in 7365A, and its expression was up-regulated, which might affect other DMGs and thus control the male sterility. This study provided genome-wide DNA methylation profiles of floral buds and important clues for revealing the molecular mechanism of genic male sterility in rapeseed.

Comparative RNA-Seq analysis on the regulation of cucumber sex differentiation under different ratios of blue and red light

Cucumber (Cucumis sativus L.) is a typical monoecism vegetable with individual male and female flowers, which has been used as a plant model for sex determination. It is well known that light is one of the most important environmental stimuli, which control the timing of the transition from vegetative growth to reproductive development. However, whether light controls sex determination remains elusive. To unravel this problem, we performed high-throughput RNA-Seq analyses, which compared the transcriptomes of shoot apices between R2B1(Red light:Blue light = 2:1)-treated and R4B1(Red light:Blue light = 4:1)-treated cucumber seedlings. Results showed that the higher proportion of blue light in the R2B1 treatment significantly induced the formation of female flowers and accelerated female flowering time in this whole study. The genes related to flowering time, such as flowering locus T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1), were up-regulated after R2B1 treatment. Furthermore, the transcriptome analysis showed that up-regulation and down-regulation of specific DEGs (the differentially expressed genes) were primarily the result of plant hormone signal transduction after treatments. The specific DEGs related with auxin formed the highest percentage of DEGs in the plant hormone signal transduction. In addition, the expression levels of transcription factors also changed after R2B1 treatment. Thus, sex differentiation affected by light quality might be induced by plant hormone signal transduction and transcription factors. These results provide a theoretical basis for further investigation of the regulatory mechanism of female flower formation under different light qualities in cucumber seedlings.

ABA and IAA control microsporogenesis in Petunia hybrida L

The formation of fertile male gametophyte is known to require timely degeneration of polyfunctional tapetum tissue. The last process caused by the programmed cell death (PCD) is a part of the anther program maturation which leads to sequential anther tissue destruction coordinated with pollen differentiation. In the present work, distribution of abscisic acid (ABA) and indole-3-acetic acid (IAA) in developing anthers of male-fertile and male-sterile lines of petunia (Petunia hybrida L.) was analyzed by using the immunohistochemical method. It was established that the development of fertile male gametophyte was accompanied by monotonous elevation of ABA and IAA levels in reproductive cells and, in contrast, their monotonous lowering in tapetum cells and the middle layers. Abortion of microsporocytes in the meiosis prophase in the sterile line caused by premature tapetum degeneration along with complete maintenance of the middle layers was accompanied by dramatic, twofold elevation in the levels of both the phytohormones in reproductive cells. The data obtained allowed us to conclude that at the meiosis stage ABA and IAA are involved in the PCD of microsporocytes.

Mutations in the CDS and promoter of BjuA07.CLV1 cause a multilocular trait in Brassica juncea

Multilocular trait has recently attracted considerable attention for its potential to increase yield. Our previous studies indicated that two genes (Bjln1 and Bjln2) are responsible for multilocular siliques in Brassica juncea and the Bjln1 gene has been delimited to a 208-kb region. In present study, the Bjln1 gene was successfully isolated using the map-based cloning method. Complementation test indicated that the BjuA07.CLV1 (equivalent to BjLn1) could rescue the multilocular phenotype and generate bilocular siliques. Two amino acids changes at positions 28 and 63 in BjuA07.clv1 as well as a 702-bp deletion in its promoter have been proved to affect the carpel numbers. Microscopic analyses suggested that BjuA07.CLV1 is involved in the maintenance of shoot and floral meristem size. The expression level of BjuA07.clv1 was significantly reduced in the SAM. Furthermore, WUS, CLV2, CLV3, RPK2 and POL, key genes in the CLV/WUS signal pathway, showed lower expression level in the multilocular plants. These data suggest that the mutations in the CDS and promoter of BjuA07.clv1 reduced its function and expression level, which disturbed CLV/WUS signal pathway, thereby leading to the enlargement of the shoot and floral meristem and resulting in the multilocular siliques.

BnaA.bZIP1 Negatively Regulates a Novel Small Peptide Gene, BnaC.SP6, Involved in Pollen Activity

Small peptides secreted to the extracellular matrix control many aspects of the plant's physiological activities which were identified in Arabidopsis thaliana, called ATSPs. Here, we isolated and characterized the small peptide gene Bna.SP6 from Brassica napus. The BnaC.SP6 promoter was cloned and identified. Promoter deletion analysis suggested that the -447 to -375 and -210 to -135 regions are crucial for the silique septum and pollen expression of BnaC.SP6, respectively. Furthermore, the minimal promoter region of p158 (-210 to -52) was sufficient for driving gene expression specifically in pollen and highly conserved in Brassica species. In addition, BnaA.bZIP1 was predominantly expressed in anthers where BnaC.SP6 was also expressed, and was localized to the nuclei. BnaA.bZIP1 possessed transcriptional activation activity in yeast and protoplast system. It could specifically bind to the C-box in p158 in vitro, and negatively regulate p158 activity in vivo. BnaA.bZIP1 functions as a transcriptional repressor of BnaC.SP6 in pollen activity. These results provide novel insight into the transcriptional regulation of BnaC.SP6 in pollen activity and the pollen/anther-specific promoter regions of BnaC.SP6 may have their potential agricultural application for new male sterility line generation.

Comprehensive Transcriptome Analysis of Phytohormone Biosynthesis and Signaling Genes in the Flowers of Chinese Chinquapin (Castanea henryi)

Chinese chinquapin (Castanea henryi) nut provides a rich source of starch and nutrients as food and feed, but its yield is restricted by a low ratio of female to male flowers. Little is known about the developmental programs underlying sex differentiation of the flowers. To investigate the involvement of phytohormones during sex differentiation, we described the morphology of male and female floral organs and the cytology of flower sex differentiation, analyzed endogenous levels of indole-3-acetic acid (IAA), gibberellins (GAs), cytokinins (CKs), and abscisic acid (ABA) in the flowers, investigated the effects of exogenous hormones on flower development, and evaluated the expression profiles of genes related to biosyntheses and signaling pathways of these four hormones using RNA-Seq combined with qPCR. Morphological results showed that the flowers consisted of unisexual and bisexual catkins, and could be divided into four developmental stages. HPLC results showed that CK accumulated much more in the female flowers than that in the male flowers, GA and ABA showed the opposite results, while IAA did not show a tendency. The effects of exogenous hormones on sex differentiation were consistent with those of endogenous hormones. RNA-Seq combined with qPCR analyses suggest that several genes may play key roles in hormone biosynthesis and sex differentiation. This study presents the first comprehensive report of phytohormone biosynthesis and signaling during sex differentiation of C. henryi, which should provide a foundation for further mechanistic studies of sex differentiation in Castanea Miller species and other nonmodel plants.

Comparative transcript profiling of fertile and sterile flower buds from multiple-allele-inherited male sterility in Chinese cabbage (Brassica campestris L. ssp. pekinensis)

We studied the underlying causes of multiple-allele-inherited male sterility in Chinese cabbage (Brassica campestris L. ssp. pekinensis) by identifying differentially expressed genes (DEGs) related to pollen sterility between fertile and sterile flower buds. In this work, we verified the stages of sterility microscopically and then performed transcriptome analysis of mRNA isolated from fertile and sterile buds using Illumina HiSeq 2000 platform sequencing. Approximately 80% of ~229 million high-quality paired-end reads were uniquely mapped to the reference genome. In sterile buds, 699 genes were significantly up-regulated and 4096 genes were down-regulated. Among the DEGs, 28 pollen cell wall-related genes, 54 transcription factor genes, 45 phytohormone-related genes, 20 anther and pollen-related genes, 212 specifically expressed transcripts, and 417 DEGs located on linkage group A07 were identified. Six transcription factor genes BrAMS, BrMS1, BrbHLH089, BrbHLH091, BrAtMYB103, and BrANAC025 were identified as putative sterility-related genes. The weak auxin signal that is regulated by BrABP1 may be one of the key factors causing pollen sterility observed here. Moreover, several significantly enriched GO terms such as "cell wall organization or biogenesis" (GO:0071554), "intrinsic to membrane" (GO:0031224), "integral to membrane" (GO:0016021), "hydrolase activity, acting on ester bonds" (GO:0016788), and one significantly enriched pathway "starch and sucrose metabolism" (ath00500) were identified in this work. qRT-PCR, PCR, and in situ hybridization experiments validated our RNA-seq transcriptome analysis as accurate and reliable. This study will lay the foundation for elucidating the molecular mechanism(s) that underly sterility and provide valuable information for studying multiple-allele-inherited male sterility in the Chinese cabbage line 'AB01'.

Transcriptional regulation of male-sterility in 7B-1 male-sterile tomato mutant

The 7B-1 tomato (Solanum lycopersicum L. cv Rutgers) is a male-sterile mutant with enhanced tolerance to abiotic stress, which makes it a potential candidate for hybrid seed breeding and stress engineering. To underline the molecular mechanism regulating the male-sterility in 7B-1, transcriptomic profiles of the 7B-1 male-sterile and wild type (WT) anthers were studied using mRNA sequencing (RNA-Seq). In total, 768 differentially expressed genes (DEGs) were identified, including 132 up-regulated and 636 down-regulated transcripts. Gene ontology (GO) enrichment analysis of DEGs suggested a general impact of the 7B-1 mutation on metabolic processes, such as proteolysis and carbohydrate catabolic process. Sixteen candidates with key roles in regulation of anther development were subjected to further analysis using qRT-PCR and in situ hybridization. Cytological studies showed several defects associated with anther development in the 7B-1 mutant, including unsynchronized anther maturation, dysfunctional meiosis, arrested microspores, defect in callose degradation and abnormal tapetum development. TUNEL assay showed a defect in programmed cell death (PCD) of tapetal cells in 7B-1 anthers. The present study provides insights into the transcriptome of the 7B-1 mutant. We identified several genes with altered expression level in 7B-1 (including beta-1,3 glucanase, GA2oxs, cystatin, cysteine protease, pectinesterase, TA29, and actin) that could potentially regulate anther developmental processes, such as meiosis, tapetum development, and cell-wall formation/degradation.

Identification and Analysis of MS5d: A Gene That Affects Double-Strand Break (DSB) Repair during Meiosis I in Brassica napus Microsporocytes

Here, we report the identification of the Brassica-specific gene MS5d, which is responsible for male sterility in Brassica napus. The MS5d gene is highly expressed in the microsporocyte and encodes a protein that localizes to the nucleus. Light microscopy analyses have demonstrated that the MS5d gene affects microsporocyte meiosis in the thermosensitive genic male sterility line TE5A. Sequence comparisons and genetic complementation revealed a C-to-T transition in MS5d, encoding a Leu-to-Phe (L281F) substitution and causing abnormal male meiosis in TE5A. These findings suggest arrested meiotic chromosome dynamics at pachytene. Furthermore, immunofluorescence analyses showed that double-strand break (DSB) formation and axial elements were normal but that DSB repair and spindle behavior were aberrant in TE5A meiocytes. Collectively, our results indicate that MS5d likely encodes a protein required for chromosomal DSB repair at early stages of meiosis in B. napus.

Helitron-like transposons contributed to the mating system transition from out-crossing to self-fertilizing in polyploid Brassica napus L

The mating system transition in polyploid Brassica napus (AACC) from out-crossing to selfing is a typical trait to differentiate it from their diploid progenitors. Elucidating the mechanism of mating system transition has profound consequences for understanding the speciation and evolution in B. napus. Functional complementation experiment has shown that the insertion of 3.6 kb into the promoter of self-incompatibility male determining gene, BnSP11-1 leads to its loss of function in B. napus. The inserted fragment was found to be a non-autonomous Helitron transposon. Further analysis showed that the inserted 3.6 kb non-autonomous Helitron transposon was widely distributed in B. napus accessions which contain the S haplotype BnS-1. Through promoter deletion analysis, an enhancer and a putative cis-regulatory element (TTCTA) that were required for spatio-temporal specific expression of BnSP11-1 were identified, and both might be disrupted by the insertion of Helitron transposon. We suggested that the insertion of Helitron transposons in the promoter of BnSP11-1 gene had altered the mating system and might facilitated the speciation of B. napus. Our findings have profound consequences for understanding the self-compatibility in B. napus as well as for the trait variations during evolutionary process of plant polyploidization.

Ectopic Expression of BnaC.CP20.1 Results in Premature Tapetal Programmed Cell Death in Arabidopsis

Tapetal programmed cell death (PCD) is essential in pollen grain development, and cysteine proteases are ubiquitous enzymes participating in plant PCD. Although the major papain-like cysteine proteases (PLCPs) have been investigated, the exact functions of many PLCPs are still poorly understood in PCD. Here, we identified a PLCP gene, BnaC.CP20.1, which was closely related to XP_013596648.1 from Brassica oleracea. Quantitative real-time PCR analysis revealed that BnaC.CP20.1 expression was down-regulated in male-sterile lines in oilseed rape, suggesting a connection between this gene and male sterility. BnaC.CP20.1 is especially active in the tapetum and microspores in Brassica napus from the uninucleate stage until formation of mature pollen grains during anther development. On expression of BnaC.CP20.1 prior to the tetrad stage, BnA9::BnaC.CP20.1 transgenic lines in Arabidopsis thaliana showed a male-sterile phenotype with shortened siliques containing fewer or no seeds by self-crossing. Scanning electron microscopy indicated that the reticulate exine was defective in aborted microspores. Callose degradation was delayed and microspores were not released from the tetrad in a timely fashion. Additionally, the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay indicated that BnaC.CP20.1 ectopic expression led to premature tapetal PCD. Transmission electron microscopy analyses further demonstrated that the pollen abortion was due to the absence of tectum connections to the bacula in the transgenic anthers. These findings suggest that timely expression of BnaC.CP20.1 is necessary for tapetal degeneration and pollen wall formation.

Heterodimer Formation of BnPKSA or BnPKSB with BnACOS5 Constitutes a Multienzyme Complex in Tapetal Cells and is Involved in Male Reproductive Development in Brassica napus

Multienzyme associations localized to specific subcellular sites are involved in several critical functions in cellular metabolism, such as plant survival and reproduction. To date, few multienzyme complexes involved in male fertility have been examined in Brassica napus Here, we reported that in B. napus, the members of a multienzyme complex work in an interaction pattern different from that in Arabidopsis thaliana for sporopollenin biosynthesis. 7365A, a male-sterile mutant with a relatively smooth anther cuticle, was found to have a dramatic reduction in both cutin monomers and wax composition. Proteomic comparison between the mutant 7365A and wild-type 7365B showed down-regulation of three sporopollenin biosynthetic enzymes, namely BnPKSA, BnPKSB and BnTKPR; these enzymes were tightly co-expressed with BnACOS5. BnPKSA and BnPKSB showed similar expression patterns but distinct accumulation levels, suggesting that they had partially distinct functions during sporopollenin biosynthesis. In vitro and in vivo analyses demonstrated that BnPKSB directly interacted with BnPKSA and BnACOS5, but no such interactions were found in the present investigation for BnTKPR1. Interestingly, the interaction between PKSA and PKSB has not been discovered in Arabidopsis, which may indicate a new interaction representing an additional efficient regulation method in B. napus Taken together, we propose that BnPKSA and BnPKSB may comprise a heterodimer combined with BnACOS5, constituting a sporopollenin metabolon in tapetal cells that is related to male reproductive development in B. napus.

OsERF2 controls rice root growth and hormone responses through tuning expression of key genes involved in hormone signaling and sucrose metabolism

Root determines plant distribution, development progresses, stress response, as well as crop qualities and yields, which is under the tight control of genetic programs and environmental stimuli. Ethylene responsive factor proteins (ERFs) play important roles in plant growth and development. Here, the regulatory function of OsERF2 involved in root growth was investigated using the gain-function mutant of OsERF2 (nsf2857) and the artificial microRNA-mediated silenced lines of OsERF2 (Ami-OsERF2). nsf2857 showed short primary roots compared with the wild type (WT), while the primary roots of Ami-OsERF2 lines were longer than those of WT. Consistent with this phenotype, several auxin/cytokinin responsive genes involved in root growth were downregulated in nsf2857, but upregulated in Ami-OsERF2. Then, we found that nsf2857 seedlings exhibited decreased ABA accumulation and sensitivity to ABA and reduced ethylene-mediated root inhibition, while those were the opposite in Ami-ERF2 plants. Moreover, several key genes involved in ABA synthesis were downregulated in nsf2857, but unregulated in Ami-ERF2 lines. In addition, OsERF2 affected the accumulation of sucrose and UDPG by mediating expression of key genes involved in sucrose metabolism. These results indicate that OsERF2 is required for the control of root architecture and ABA- and ethylene-response by tuning expression of series genes involved in sugar metabolism and hormone signaling pathways.

Map-based cloning reveals the complex organization of the BnRf locus and leads to the identification of BnRf(b), a male sterility gene, in Brassica napus

Sequencing of BAC clones reveals the complex organization of the BnRf locus and allowed us to clone BnRf (b) , which encodes a nucleus-localized chimeric protein BnaA7.mtHSP70-1-like. The male sterility in an extensively used genic male sterility (GMS) line (9012A) in Brassica napus was regarded to be conferred by BnMs3/Bnms3 and the multiallelic BnRf locus including three alleles. We previously mapped BnRf to a 13.8 kb DNA fragment on the B. napus chromosome A7. In the present study, we isolated bacterial artificial chromosome clones individually covering the restorer allele BnRf (a) and the male-sterile allele BnRf (b) , and revealed that the candidate regions of BnRf (a) and BnRf (b) show complex structural variations relative to the maintainer allele BnRf (c). By analyzing the recombination events and the newly developed markers, we delimited BnRf (a) to a 35.9 kb DNA fragment that contained seven predicted open-reading frames (ORFs). However, genetic transformation of the ORF G14 from both the male-sterile and restorer lines into wild-type Arabidopsis plants led to a stable male-sterile phenotype matching a 9012A-derived GMS line (RG206A); moreover, the male sterility caused by G14 could be fully recovered by the restorer gene BnMs3. These facts indicate that BnRf (b) corresponds to G14 while BnRf (a) likely associates with another flanking ORF. G14 encodes a nucleus-localized chimeric protein designated as BnaA7.mtHSP70-1-like. Ectopic expression of G14 in Arabidopsis negatively regulates some vital genes responsible for tapetum degeneration, and delayed programmed cell death of tapetum and led to the developmental arrest of tetrads. Our work not only presents new insights on the hereditary model of sterility control but also lays a solid foundation for dissecting the molecular basis underlying male sterility and restoration in 9012A.

Identification of the tapetum/microspore-specific promoter of the pathogenesis-related 10 gene and its regulation in the anther of Lilium longiflorum

A tapetum/microspore-specific pathogenesis-related (PR) 10 gene was previously identified in lily (Lilium longiflorum Thunb.) anthers. In situ hybridization and RNA blot analysis indicated that the lily PR10 genes are expressed specifically and differentially in the tapetum of the anther wall and in microspores during anther development. The accumulation of PR10 transcripts was exogenously induced by gibberellic acid (GA) and was suppressed by ethylene. Studies using inhibitors of GA and ethylene revealed that the lily PR10 is modulated by an antagonistic interaction between GA and ethylene. The treatment of norbornadien, an ethylene inhibitor, caused the tapetum to become densely cytoplasmic and highly polarized, whereas uniconazole, an inhibitor of GA biosynthesis, arrested tapetal development to a status close to that of control. The expression of the lily PR10g promoter in transgenic Arabidopsis was determined using the β-glucuronidase (GUS) reporter gene indicated that the decisive fragment required for anther specificity is located -1183 bp to -880 bp upstream of the transcription start site. The PR10gPro::barnase transgenic lines exhibited complete male sterility because of the disruption of the tapetum and the deformation of microspore/pollen. The anther specificity of lily PR10 highlights the importance of the tapetum/microspore-specific PR10g promoter for future biotechnological and agricultural applications.

Comprehensive analysis of genic male sterility-related genes in Brassica rapa using a newly developed Br300K oligomeric chip

To identify genes associated with genic male sterility (GMS) that could be useful for hybrid breeding in Chinese cabbage (Brassicarapa ssp. pekinensis), floral bud transcriptome analysis was carried out using a B. rapa microarray with 300,000 probes (Br300K). Among 47,548 clones deposited on a Br300K microarray with seven probes of 60 nt length within the 3' 150 bp region, a total of 10,622 genes were differentially expressed between fertile and sterile floral buds; 4,774 and 5,848 genes were up-regulated over 2-fold in fertile and sterile buds, respectively. However, the expression of 1,413 and 199 genes showed fertile and sterile bud-specific features, respectively. Genes expressed specifically in fertile buds, possibly GMS-related genes, included homologs of several Arabidopsis male sterility-related genes, genes associated with the cell wall and synthesis of its surface proteins, pollen wall and coat components, signaling components, and nutrient supplies. However, most early genes for pollen development, genes for primexine and callose formation, and genes for pollen maturation and anther dehiscence showed no difference in expression between fertile and sterile buds. Some of the known genes associated with Arabidopsis pollen development showed similar expression patterns to those seen in this study, while others did not. BrbHLH89 and BrMYP99 are putative GMS genes. Additionally, 17 novel genes identified only in B. rapa were specifically and highly expressed only in fertile buds, implying the possible involvement in male fertility. All data suggest that Chinese cabbage GMS might be controlled by genes acting in post-meiotic tapetal development that are different from those known to be associated with Arabidopsis male sterility.

Exploiting comparative mapping among Brassica species to accelerate the physical delimitation of a genic male-sterile locus (BnRf) in Brassica napus

The recessive genic male sterility (RGMS) line 9012AB has been used as an important pollination control system for rapeseed hybrid production in China. Here, we report our study on physical mapping of one male-sterile locus (BnRf) in 9012AB by exploiting the comparative genomics among Brassica species. The genetic maps around BnRf from previous reports were integrated and enriched with markers from the Brassica A7 chromosome. Subsequent collinearity analysis of these markers contributed to the identification of a novel ancestral karyotype block F that possibly encompasses BnRf. Fourteen insertion/deletion markers were further developed from this conserved block and genotyped in three large backcross populations, leading to the construction of high-resolution local genetic maps where the BnRf locus was restricted to a less than 0.1-cM region. Moreover, it was observed that the target region in Brassica napus shares a high collinearity relationship with a region from the Brassica rapa A7 chromosome. A BnRf-cosegregated marker (AT3G23870) was then used to screen a B. napus bacterial artificial chromosome (BAC) library. From the resulting 16 positive BAC clones, one (JBnB089D05) was identified to most possibly contain the BnRf (c) allele. With the assistance of the genome sequence from the Brassica rapa homolog, the 13.8-kb DNA fragment covering both closest flanking markers from the BAC clone was isolated. Gene annotation based on the comparison of microcollinear regions among Brassica napus, B. rapa and Arabidopsis showed that five potential open reading frames reside in this fragment. These results provide a foundation for the characterization of the BnRf locus and allow a better understanding of the chromosome evolution around BnRf.

BnMs3 is required for tapetal differentiation and degradation, microspore separation, and pollen-wall biosynthesis in Brassica napus

7365AB, a recessive genetic male sterility system, is controlled by BnMs3 in Brassica napus, which encodes a Tic40 protein required for tapetum development. However, the role of BnMs3 in rapeseed anther development is still largely unclear. In this research, cytological analysis revealed that anther development of a Bnms3 mutant has defects in the transition of the tapetum to the secretory type, callose degradation, and pollen-wall formation. A total of 76 down-regulated unigenes in the Bnms3 mutant, several of which are associated with tapetum development, callose degeneration, and pollen development, were isolated by suppression subtractive hybridization combined with a macroarray analysis. Reverse genetics was applied by means of Arabidopsis insertional mutant lines to characterize the function of these unigenes and revealed that MSR02 is only required for transport of sporopollenin precursors through the plasma membrane of the tapetum. The real-time PCR data have further verified that BnMs3 plays a primary role in tapetal differentiation by affecting the expression of a few key transcription factors, participates in tapetal degradation by modulating the expression of cysteine protease genes, and influences microspore separation by manipulating the expression of BnA6 and BnMSR66 related to callose degradation and of BnQRT1 and BnQRT3 required for the primary cell-wall degradation of the pollen mother cell. Moreover, BnMs3 takes part in pollen-wall formation by affecting the expression of a series of genes involved in biosynthesis and transport of sporopollenin precursors. All of the above results suggest that BnMs3 participates in tapetum development, microspore release, and pollen-wall formation in B. napus.

BnaC.Tic40, a plastid inner membrane translocon originating from Brassica oleracea, is essential for tapetal function and microspore development in Brassica napus

Here, we describe the characteristics of a Brassica napus male sterile mutant 7365A with loss of the BnMs3 gene, which exhibits abnormal enlargement of the tapetal cells during meiosis. Later in development, the absence of the BnMs3 gene in the mutant results in a loss of the secretory function of the tapetum, as suggested by abortive callose dissolution and retarded tapetal degradation. The BnaC.Tic40 gene (equivalent to BnMs3) was isolated by a map-based cloning approach and was confirmed by genetic complementation. Sequence analyses suggested that BnaC.Tic40 originated from BolC.Tic40 on the Brassica oleracea linkage group C9, whereas its allele Bnms3 was derived from BraA.Tic40 on the Brassica rapa linkage group A10. The BnaC.Tic40 gene is highly expressed in the tapetum and encodes a putative plastid inner envelope membrane translocon, Tic40, which is localized into the chloroplast. Transmission electron microscopy (TEM) and lipid staining analyses suggested that BnaC.Tic40 is a key factor in controlling lipid accumulation in the tapetal plastids. These data indicate that BnaC.Tic40 participates in specific protein translocation across the inner envelope membrane in the tapetal plastid, which is required for tapetal development and function.

Characterization of a caleosin expressed during olive (Olea europaea L.) pollen ontogeny

BACKGROUND

The olive tree is an oil-storing species, with pollen being the second most active site in storage lipid biosynthesis. Caleosins are proteins involved in storage lipid mobilization during seed germination. Despite the existence of different lipidic structures in the anther, there are no data regarding the presence of caleosins in this organ to date. The purpose of the present work was to characterize a caleosin expressed in the olive anther over different key stages of pollen ontogeny, as a first approach to unravel its biological function in reproduction.

RESULTS

A 30 kDa caleosin was identified in the anther tissues by Western blot analysis. Using fluorescence and transmission electron microscopic immunolocalization methods, the protein was first localized in the tapetal cells at the free microspore stage. Caleosins were released to the anther locule and further deposited onto the sculptures of the pollen exine. As anthers developed, tapetal cells showed the presence of structures constituted by caleosin-containing lipid droplets closely packed and enclosed by ER-derived cisternae and vesicles. After tapetal cells lost their integrity, the caleosin-containing remnants of the tapetum filled the cavities of the mature pollen exine, forming the pollen coat. In developing microspores, this caleosin was initially detected on the exine sculptures. During pollen maturation, caleosin levels progressively increased in the vegetative cell, concurrently with the number of oil bodies. The olive pollen caleosin was able to bind calcium in vitro. Moreover, PEGylation experiments supported the structural conformation model suggested for caleosins from seed oil bodies.

CONCLUSIONS

In the olive anther, a caleosin is expressed in both the tapetal and germ line cells, with its synthesis independently regulated. The pollen oil body-associated caleosin is synthesized by the vegetative cell, whereas the protein located on the pollen exine and its coating has a sporophytic origin. The biological significance of the caleosin in the reproductive process in species possessing lipid-storing pollen might depend on its subcellular emplacement. The pollen inner caleosin may be involved in OB biogenesis during pollen maturation. The protein located on the outside might rather play a function in pollen-stigma interaction during pollen hydration and germination.