Hormonal interactions and gene regulation can link monoecy and environmental plasticity to the evolution of dioecy in plants

Hung out to dry: diminished flowers offer less to pollinators and us

This article is a Commentary on Barman et al. (2024), 244: 1013–1023.

Why not XY? Male monoecious sexual phenotypes challenge the female monoecious paradigm in Cannabis sativa L

Monoecy in Cannabis sativa L. has long been considered an industrially important trait due to the increased uniformity it offers and was thought to be exclusively associated with XX females. The isolation and characterisation of a monoecious individual with XY chromosomes sourced from non-proprietary germplasm is reported for the first time. The chromosomal make up of this trait was confirmed through inflorescence structure, growth habit, PCR analysis and sexual phenotypes of progeny from a series of targeted crosses. The identification of an XY monoecious phenotype widens our understanding of monoecy in Cannabis and has important implications for breeding, particularly for producing F1-hybrid seed.

Gender specific SNP markers in Coscinium fenestratum (Gaertn.) Colebr. for resource augmentation

BACKGROUND

Unregulated extraction of highly traded medicinal plant species results in drastic decline of the natural resources and alters viable sex ratio of populations. Conservation and long-term survival of such species, require gender specific restoration programs to ensure reproductive success. However, it is often difficult to differentiate sex of individuals before reaching reproductive maturity. C. fenestratum is one of the medicinally important and overexploited dioecious woody liana, with a reproductive maturity of 15 years. Currently, no information is available to identify sex of C. fenestratum in seedling stage while augmenting the resources. Thus, the current study envisages to utilize transcriptomics approach for gender differentiation which is imperative for undertaking viable resource augmentation programmes.

METHODS AND RESULTS

Gender specific SNPs with probable role in sexual reproduction/sex determination was located using comparative transcriptomics approach (sampling male and female individuals), alongside gene ontology and annotation. Nine sets of primers were synthesized from 7 transcripts (involved in sexual reproduction/other biological process) containing multiple SNP variants. Out of the nine primer pairs, only one SNP locus with no available information of its role in reproduction, showed consistent and accurate results (males-heterozygous and females-homozygous), in the analyzed 40 matured individuals of known sexes. Thus validated the efficiency of this SNP marker in differentiating male and female individuals.

CONCLUSIONS

The study could identify SNPs linked to the loci with apparent role in gender differentiation. This SNP marker can be used for early sexing of seedlings for in-situ conservation and resource augmentation of C. fenestratum in Kerala, India.

Seasonal variation of two floral patterns in Clematis 'Vyvyan Pennell' and its underlying mechanism

BACKGROUND

Floral patterns are crucial for insect pollination and plant reproduction. Generally, once these patterns are established, they exhibit minimal changes under natural circumstances. However, the Clematis cultivar' Vyvyan Pennell', the apetalous lineage in the Ranunculaceae family, produces two distinct types of flowers during different seasons. The regulatory mechanism responsible for this phenomenon remains largely unknown. In this study, we aim to shed light on this floral development with shifting seasonal patterns by conducting extensive morphological, transcriptomic, and hormone metabolic analyses. Our findings are anticipated to contribute valuable insights into the diversity of flowers in the Ranunculaceae family.

RESULTS

The morphological analysis revealed that the presence of extra petaloid structures in the spring double perianth was a result of the transformation of stamens covered with trichomes during the 5th developmental stage. A de novo reference transcriptome was constructed by comparing buds and organs within double and single perianth from both seasons. A total of 209,056 unigenes were assembled, and 5826 genes were successfully annotated in all six databases. Among the 69,888 differentially expressed genes from the comparative analysis, 48 genes of utmost significance were identified. These critical genes are associated with various aspects of floral development. Interestingly, the A-, B-, and C-class genes exhibited a wider range of expression and were distinct within two seasons. The determination of floral organ identity was attributed to the collaborative functioning of all the three classes genes, aligning with a modified "fading border model". The phytohormones GA3, salicylic acid, and trans-zeatin riboside may affect the formation of the spring double perianth, whereas GA7 and abscisic acid may affect single flowers in autumn.

CONCLUSIONS

We presumed that the varying temperatures between the two seasons served as the primary factor in the alteration of floral patterns, potentially affecting the levels of plant hormones and expressions of organ identity genes. However, a more thorough investigation is necessary to fully comprehend the entire regulatory network. Nonetheless, our study provides some valuable informations for understanding the underlying mechanism of floral pattern alterations in Clematis.

Nutrition regulates sex expression in a gender diphasy plant, Lilium concolor var. megalanthum

Introduction

The evolution and maintenance of plant polymorphism have always received much attention. Gender diphasy is a rare sexual system. Plant individuals with gender diphasy can adjust the resource allocation of different functional organs according to the changes of environmental conditions to regulate the sex expression of individuals, and the sex expression can be converted between years. However, our understanding of sex expression in plants is still very insufficient. In this study, we explored whether the perennial plant Lilium concolor var. megalanthum has a gender diphasy system and whether environmental resource availability affects its resource allocation and sex expression.

Method

By collecting the bulbs of two sexual phenotypes (male and hermaphrodite) in the field and simulating the application of different levels of nutrients under the same habitat conditions, the growth and reproduction indexes and sex expression of plants in two years (2021 and 2022) were measured to evaluate the resource allocation strategy and sex expression pattern of Lilium concolor var. megalanthum.

Results

We found that the sex expression of Lilium concolor var. megalanthum was variable in continuous years. Under limited resources, Lilium concolor var. megalanthum increases the biomass resources of the leaves and has a longer flowering period. Resource availability regulates the growth, reproduction and sex expression of Lilium concolor var. megalanthum. Bulb size is the main factor affecting its growth, reproduction and sex expression after accumulating sufficient resources.

Discussion

This study confirms that Lilium concolor var. megalanthum has gender diphasy system. There is a strong trade-off between growth and reproduction under limited resources. Nutrient levels can regulate the reproduction and sex expression process of Lilium concolor var. megalanthum. With the growth of Lilium concolor var. megalanthum in consecutive years, the size dependence of bulbs may be the decisive factor in its sex expression.

Identifying Genes Associated with Female Flower Development of Phellodendron amurense Rupr. Using a Transcriptomics Approach

Phellodendron amurense Rupr., a species of Rutaceae, is a nationally protected and valuable medicinal plant. It is generally considered to be dioecious. With the discovery of monoecious P. amurense, the phenomenon that its sex development is regulated by epigenetics has been revealed, but the way epigenetics affects the sex differentiation of P. amurense is still unclear. In this study, we investigated the effect of DNA methylation on the sexual development of P. amurense. The young inflorescences of male plants were treated with the demethylation agent 5-azaC, and the induced female flowers were obtained. The induced female flowers' morphological functions and transcriptome levels were close to those of normally developed plants. Genes associated with the development of female flowers were studied by comparing the differences in transcriptome levels between the male and female flowers. Referring to sex-related genes reported in other plants, 188 candidate genes related to the development of female flowers were obtained, including sex-regulating genes, genes related to the formation and development of sexual organs, genes related to biochemical pathways, and hormone-related genes. RPP0W, PAL3, MCM2, MCM6, SUP, PIN1, AINTEGUMENTA, AINTEGUMENTA-LIKE6, AGL11, SEUSS, SHI-RELATED SEQUENCE 5, and ESR2 were preliminarily considered the key genes for female flower development. This study has demonstrated that epigenetics was involved in the sex regulation of P. amurense, with DNA methylation as one of its regulatory modes. Moreover, some candidate genes related to the sexual differentiation of P. amurense were obtained with analysis. These results are of great significance for further exploring the mechanism of sex differentiation of P. amurense and studying of sex differentiation of plants.

Regulatory mechanism of MeGI on sexuality in Diospyros oleifera

Dioecy system is an important strategy for maintaining genetic diversity. The transcription factor MeGI, contributes to dioecy by promoting gynoecium development in Diospyros lotus and D. kaki. However, the function of MeGI in D. oleifera has not been identified. In this study, we confirmed that MeGI, cloned from D. oleifera, repressed the androecium development in Arabidopsis thaliana. Subsequently, chromatin immunoprecipitation-sequencing (ChIP-seq), DNA affinity purification-sequencing (DAP-seq), and RNA-seq were used to uncover the gene expression response to MeGI. The results showed that the genes upregulated and downregulated in response to MeGI were mainly enriched in the circadian rhythm-related and flavonoid biosynthetic pathways, respectively. Additionally, the WRKY DNA-binding protein 28 (WRKY28) gene, which was detected by ChIP-seq, DAP-seq, and RNA-seq, was emphasized. WRKY28 has been reported to inhibit salicylic acid (SA) biosynthesis and was upregulated in MeGI-overexpressing A. thaliana flowers, suggesting that MeGI represses the SA level by increasing the expression level of WRKY28. This was confirmed that SA level was lower in D. oleifera female floral buds than male. Overall, our findings indicate that the MeGI mediates its sex control function in D. oleifera mainly by regulating genes in the circadian rhythm, SA biosynthetic, and flavonoid biosynthetic pathways.

Genomic profiling of dioecious Amaranthus species provides novel insights into species relatedness and sex genes

BACKGROUND

Amaranthus L. is a diverse genus consisting of domesticated, weedy, and non-invasive species distributed around the world. Nine species are dioecious, of which Amaranthus palmeri S. Watson and Amaranthus tuberculatus (Moq.) J.D. Sauer are troublesome weeds of agronomic crops in the USA and elsewhere. Shallow relationships among the dioecious Amaranthus species and the conservation of candidate genes within previously identified A. palmeri and A. tuberculatus male-specific regions of the Y (MSYs) in other dioecious species are poorly understood. In this study, seven genomes of dioecious amaranths were obtained by paired-end short-read sequencing and combined with short reads of seventeen species in the family Amaranthaceae from NCBI database. The species were phylogenomically analyzed to understand their relatedness. Genome characteristics for the dioecious species were evaluated and coverage analysis was used to investigate the conservation of sequences within the MSY regions.

RESULTS

We provide genome size, heterozygosity, and ploidy level inference for seven newly sequenced dioecious Amaranthus species and two additional dioecious species from the NCBI database. We report a pattern of transposable element proliferation in the species, in which seven species had more Ty3 elements than copia elements while A. palmeri and A. watsonii had more copia elements than Ty3 elements, similar to the TE pattern in some monoecious amaranths. Using a Mash-based phylogenomic analysis, we accurately recovered taxonomic relationships among the dioecious Amaranthus species that were previously identified based on comparative morphology. Coverage analysis revealed eleven candidate gene models within the A. palmeri MSY region with male-enriched coverages, as well as regions on scaffold 19 with female-enriched coverage, based on A. watsonii read alignments. A previously reported FLOWERING LOCUS T (FT) within A. tuberculatus MSY contig was also found to exhibit male-enriched coverages for three species closely related to A. tuberculatus but not for A. watsonii reads. Additional characterization of the A. palmeri MSY region revealed that 78% of the region is made of repetitive elements, typical of a sex determination region with reduced recombination.

CONCLUSIONS

The results of this study further increase our understanding of the relationships among the dioecious species of the Amaranthus genus as well as revealed genes with potential roles in sex function in the species.

The S-Gene YUC6 Pleiotropically Determines Male Mating Type and Pollen Size in Heterostylous Turnera (Passifloraceae): A Novel Neofunctionalization of the YUCCA Gene Family

In heterostylous, self-incompatible Turnera species, a member of the YUCCA gene family, YUC6, resides at the S-locus and has been hypothesized to determine the male mating type. YUCCA gene family members synthesize the auxin, indole-3-acetic acid, via a two-step process involving the TAA gene family. Consequently, it has been speculated that differences in auxin concentration in developing anthers are the biochemical basis underlying the male mating type. Here, we provide empirical evidence that supports this hypothesis. Using a transgenic knockdown approach, we show that YUC6 acts pleiotropically to control both the male physiological mating type and pollen size, but not the filament length dimorphism associated with heterostyly in Turnera. Using qPCR to assess YUC6 expression in different transgenic lines, we demonstrate that the level of YUC6 knockdown correlates with the degree of change observed in the male mating type. Further assessment of YUC6 expression through anther development, in the knockdown lines, suggests that the male mating type is irreversibly determined during a specific developmental window prior to microsporogenesis, which is consistent with the genetically sporophytic nature of this self-incompatibility system. These results represent the first gene controlling male mating type to be characterized in any species with heterostyly.

Transcriptome Analysis Reveals the Regulatory Networks of Cytokinin in Promoting Floral Feminization in Castanea henryi

Castanea henryi is a monoecious plant with a low female-to-male ratio, which limits its yield. The phytohormone cytokinin (CK) plays a crucial role in flower development, especially gynoecium development. Here, the feminizing effect of CK on the development of C. henryi was confirmed by the exogenous spraying of N-(2-chloro-4-pyridyl)-N'-phenylurea (CPPU). Spraying CPPU at 125 mg·L^-1 thrice changed the male catkin into a pure female catkin, whereas at 5 mg·L^-1 and 25 mg·L^-1, only a part of the male catkin was transformed into a female catkin. A comparative transcriptome analysis of male catkins subjected to CPPU was performed to study the mechanism of the role of CKs in sex differentiation. Using Pearson's correlation analysis between hormone content and hormone synthesis gene expression, four key genes, LOG1, LOG3, LOG7 and KO, were identified in the CK and GA synthesis pathways. Moreover, a hub gene in the crosstalk between JA and the other hormone signaling pathways, MYC2, was identified, and 15 flowering-related genes were significantly differentially expressed after CPPU treatment. These results suggest that CK interacts with other phytohormones to determine the sex of C. henryi, and CK may directly target floral organ recognition genes to control flower sex.

The distribution of sexual function in the flowering plant: from monoecy to dioecy

In flowering plants, male and female functions are usually closely associated in the same flowers, as predicted by resource allocation theory. However, the benefits of outbreeding can lead to unisexual flowers and the physiological control of their distribution across the plant (monoecy). Monoecy is thought to be a major route to dioecy (separation of sexual function of different individuals). The developmental and functional problems associated with unisexual flowers may thus be solved at the level of the evolution of monoecy. Consequently, the evolution of dioecy from monoecy requires mutations in only a single gene. Here various scenarios (conceptual models) are presented for the evolution of monoecy and dioecy, including scenarios consistent with known cases of single-gene control of dioecy, such as in Populus, and the artificial breeding of dioecy from monoecy experimentally achieved in Zea and Cucumis. Attention is also drawn here to the phenomenon of pleogamy, the minor or occasional occurrence of additional sex morphs within a species, which may provide important information about the genetic and developmental control of various sexual systems. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.

Labile sex expression in angiosperm species with sex chromosomes

Here, we review the literature on sexual lability in dioecious angiosperm species with well-studied sex chromosomes. We distinguish three types of departures from strict dioecy, concerning either a minority of flowers in some individuals (leakiness) or the entire individual, which can constantly be bisexual or change sex. We found that for only four of the 22 species studied, reports of lability are lacking. The occurrence of lability is only weakly related to sex chromosome characteristics (number of sex-linked genes, age of the non-recombining region). These results contradict the naive idea that lability is an indication of the absence or the recent evolution of sex chromosomes, and thereby contribute to a growing consensus that sex chromosomes do not necessarily fix sex determination once and for all. We discuss some implications of these findings for the evolution of sex chromosomes, and suggest that more species with well-characterized lability should be studied with genomic data and tools. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.

A derived ZW chromosome system in Amborella trichopoda, representing the sister lineage to all other extant flowering plants

The genetic basis and evolution of sex determination in dioecious plants is emerging as an active area of research with exciting advances in genome sequencing and analysis technologies. As the sole species within the sister lineage to all other extant flowering plants, Amborella trichopoda is an important model for understanding the evolution and development of flowers. Plants typically produce only male or female flowers, but sex determination mechanisms are unknown for the species. Sequence data derived from plants of natural origin and an F1 mapping population were used to identify sex-linked genes and the nonrecombining region. Amborella trichopoda has a ZW sex determination system. Analysis of genes in a 4 Mb nonrecombining sex-determination region reveals recent divergence of Z and W gametologs, and few Z- and W-specific genes. The sex chromosomes of A. trichopoda evolved less than 16.5 Myr ago, long after the divergence of the extant angiosperms.

The Patterns of Male and Female Flowers in Flowering Stage May Not Be Optimal Resource Allocation for Fruit and Seed Growth

Changes in the proportions of male and female flowers in monoecious plants in response to external environmental conditions are directly related to the reproductive fitness of plants. The monoecious cucumber (Cucumber sativus) plant was used in this study to assess the responses of sex differentiation and the breeding process to nutrient supply and the degree of artificial pollination using pollen solutions of different concentrations. We found that the nutrient supply significantly improved the number of female flowers, while pollination treatments did not obviously increase the number of male flowers. Continuous pollination changed the number of female flowers especially in the later stage of the pollination experiment. Therefore, pollination changed the ratio of male and female flowers in the flowering stage of cucumber. Pollination treatment affected the fruit growth, seed set, and fruit yield. The number of fruit, fruit set percentage, and total seeds per plant did not increase with the pollination level, but individual fruit weight and seed number in one fruit did increase. The differentiation of male and female flowers in the flowering stage of cucumber is a response to nutrient and pollination resources, but this response is not the optimal resource allocation for subsequent fruit development and seed maturity, which suggests that the response of plants to external environment resources is short-term and direct.

VviPLATZ1 is a major factor that controls female flower morphology determination in grapevine

Plant genetic sex determinants that mediate the transition to dioecy are predicted to be diverse, as this type of mating system independently evolved multiple times in angiosperms. Wild Vitis species are dioecious with individuals producing morphologically distinct female or male flowers; whereas, modern domesticated Vitis vinifera cultivars form hermaphrodite flowers capable of self-pollination. Here, we identify the VviPLATZ1 transcription factor as a key candidate female flower morphology factor that localizes to the Vitis SEX-DETERMINING REGION. The expression pattern of this gene correlates with the formation reflex stamens, a prominent morphological phenotype of female flowers. After generating CRISPR/Cas9 gene-edited alleles in a hermaphrodite genotype, phenotype analysis shows that individual homozygous lines produce flowers with reflex stamens. Taken together, our results demonstrate that loss of VviPLATZ1 function is a major factor that controls female flower morphology in Vitis.

Unlike wild Vitis species, which produce either female or male flowers, modern grapevine cultivars form hermaphrodite flowers for self-pollination. Here, the authors report that the VviPLATZ1 (plant AT-rich sequence-and zinc-binding protein1) transcription factor functions in controlling female flower morphology determination.

Transcriptome-based analysis of the hormone regulation mechanism of gender differentiation in Juglans mandshurica Maxim

Juglans mandshurica Maxim is a hermaphroditic plant belonging to the genus Juglans in the family Juglandaceae. The pollination period of female flowers is different from the loose powder period of male flowers on the same tree. In several trees, female flowers bloom first, whereas in others, male flowers bloom first. In this study, male and female flower buds of J. mandshurica at the physiological differentiation stage were used. Illumina-based transcriptome sequencing was performed, and the quality of the sequencing results was evaluated and analyzed. A total of 138,138 unigenes with an average length of 788 bp were obtained. There were 8,116 differentially expressed genes (DEGs); 2,840 genes were upregulated, and 5,276 genes were downregulated. The DEGs were classified by Gene Ontology and analyzed by Kyoto Encyclopedia of Genes and Genomes. The signal transduction factors involved in phytohormone synthesis were selected. The results displayed that ARF and SAUR were expressed differently in the auxin signaling pathway. Additionally, DELLA protein (a negative regulator of gibberellin), the cytokinin synthesis pathway, and A-ARR were downregulated. On April 2nd, the contents of IAA, GA, CTK, ETH and SA in male and female flower buds of two types of J. mandshurica were opposite, and there were obvious genes regulating gender differentiation. Overall, we found that the sex differentiation of J. mandshurica was related to various hormone signal transduction pathways, and hormone signal transduction plays a leading role in regulation.

Characterization and expression pattern of the trehalose-6-phosphate synthase and trehalose-6-phosphate phosphatase gene families in Populus

Trehalose plays an important role in plant metabolism, growth development, and stress tolerance. Trehalose-6-phosphate synthase gene (TPS) and trehalose-6-phosphate phosphatase gene (TPP) are vital for the synthesis of trehalose. Populus is a prominent perennial woody plant, in which systematic genome-wide analysis of the TPS and TPP family is limited. In this study, 13 PtTPS and 10 PtTPP genes were identified in the Populus genome. Phylogenetic analysis indicated PtTPS and PtTPP genes were both divided into two subfamilies, and gene members of each subfamily have highly conserved intron structures. Analysis of cis-acting elements showed that PtTPS and PtTPP genes were involved in plant hormones and environmental stress responses. Expression profiles also found PtTPSs and PtTPPs expressed differently in response to salt stress, cold, mechanical damage, salicylic acid, and methyl jasmonate treatment. Furthermore, reverse transcription quantitative real-time PCR results found PtTPSs and PtTPPs displayed a specific expression pattern in the seven developmental stages of Populus male and female floral buds. This work will not only lead a foundation on reveal the functions of PtTPS and PtTPP gene families in trehalose regulation of poplar but also provide references to related trehalose research in other perennial plants.

Physiological Changes and Differential Gene Expression of Tea Plants (Camellia sinensis (L.) Kuntze var. niaowangensis Q.H. Chen) Under Cold Stress

Low temperature is an important factor that affects the growth and reproduction of tea plants [Camellia sinensis (L.) Kuntze]. In this study, Yunwu Tribute Tea cutting seedlings [Camellia sinensis (L.) Kuntze var. niaowangensis Q.H. Chen] were subjected to different low-temperature treatments in Guizhou Province, China, and the changes in physiological indicators of the leaves were measured to investigate the physiological response and cold tolerance of this variety. Under cold stress, the peak of antioxidant enzyme activity appeared on the third day of treatment at 1°C, indicating that Yunwu Tribute Tea could improve the resistance to cold stress through an increase in enzyme activity within a low-temperature range. However, after 3 days treatment at 1°C, the tolerance of plant had been exceeded; the ability to resist cold stress disappeared, and enzyme activity decreased. When the temperature or duration of stress exceeded the maximum tolerance of the plant, the synthesis of soluble substances decreased in concert with their protective effects. Under cold conditions, Yunwu Tribute Tea could increase the production of abscisic acid growth inhibitors and reduce those of indoleacetic acid, gibberellin, and other growth promoting substances to manage cold stress by regulating the balance of growth regulators in the plant. Five differential genes were screened as candidate genes from the Yunwu Tribute Tea cold stress transcriptome (DW, 1°C) for fluorescence quantitative analysis. The results showed that the changes in levels of expression of these genes under continuous cold stress significantly positively correlated with the corresponding physiological indicators. Nevertheless, the levels of expression of the Yunwu Tribute Tea polyphenol oxidase (PPO) gene and the gibberellin 3β-dioxygenase gene (G3O2) were reversely inhibited under cold stress. The result was consistent with the corresponding physiological indicators, and it provides a basis for the study of cold resistance mechanisms in tea plants.

Epigenetics drive the evolution of sex chromosomes in animals and plants

We review how epigenetics affect sex chromosome evolution in animals and plants. In a few species, sex is determined epigenetically through the action of Y-encoded small RNAs. Epigenetics is also responsible for changing the sex of individuals through time, even in species that carry sex chromosomes, and could favour species adaptation through breeding system plasticity. The Y chromosome accumulates repeats that become epigenetically silenced which leads to an epigenetic conflict with the expression of Y genes and could accelerate Y degeneration. Y heterochromatin can be lost through ageing, which activates transposable elements and lowers male longevity. Y chromosome degeneration has led to the evolution of meiotic sex chromosome inactivation in eutherians (placentals) and marsupials, and dosage compensation mechanisms in animals and plants. X-inactivation convergently evolved in eutherians and marsupials via two independently evolved non-coding RNAs. In Drosophila, male X upregulation by the male specific lethal (MSL) complex can spread to neo-X chromosomes through the transposition of transposable elements that carry an MSL-binding motif. We discuss similarities and possible differences between plants and animals and suggest future directions for this dynamic field of research. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

Plant sex chromosomes defy evolutionary models of expanding recombination suppression and genetic degeneration

Hundreds of land plant lineages have independently evolved separate sexes in either gametophytes (dioicy) or sporophytes (dioecy), but 43% of all dioecious angiosperms are found in just 34 entirely dioecious clades, suggesting that their mode of sex determination evolved a long time ago. Here, we review recent insights on the molecular mechanisms that underlie the evolutionary change from individuals that each produce male and female gametes to individuals specializing in the production of just one type of gamete. The canonical model of sex chromosome evolution in plants predicts that two sex-determining genes will become linked in a sex-determining region (SDR), followed by expanding recombination suppression, chromosome differentiation and, ultimately, degeneration. Experimental work, however, is showing that single genes function as master regulators in model systems, such as the liverwort Marchantia and the angiosperms Diospyros and Populus. In Populus, this type of regulatory function has been demonstrated by genome editing. In other systems, including Actinidia, Asparagus and Vitis, two coinherited factors appear to independently regulate female and male function, yet sex chromosome differentiation has remained low. We discuss the best-understood systems and evolutionary pathways to dioecy, and present a meta-analysis of the sizes and ages of SDRs. We propose that limited sexual conflict explains why most SDRs are small and sex chromosomes remain homomorphic. It appears that models of increasing recombination suppression with age do not apply because selection favours mechanisms in which sex determination depends on minimal differences, keeping it surgically precise.

Phytohormone and integrated mRNA and miRNA transcriptome analyses and differentiation of male between hermaphroditic floral buds of andromonoecious Diospyros kaki Thunb

BACKGROUND

Persimmon (Diospyros kaki Thunb.) has various labile sex types, and studying its sex differentiation can improve breeding efficiency. However, studies on sexual regulation patterns in persimmon have focused mainly on monoecy and dioecy, whereas little research has been published on andromonoecy. In order to reveal the sex differentiation regulation mechanism of andromonoecious persimmon, we performed histological and cytological observations, evaluated OGI and MeGI expression and conducted phytohormones assays and mRNA and small RNA transcriptome analyses of the male and hermaphroditic floral buds of the andromonoecious persimmon 'Longyanyeshi 1'.

RESULTS

Stages 2 and 4 were identified as the critical morphological periods for sex differentiation of 'Longyanyeshi 1' by histological and cytological observation. At both stages, OGI was differentially expressed in male and hermaphroditic buds, but MeGI was not. This was different from their expressions in dioecious and monoecious persimmons. Meantime, the results of phytohormones assays showed that high IAA, ABA, GA3, and JA levels at stage 2 may have promoted male floral bud differentiation. However, high JA levels at stage 4 and high ZT levels at stages 2 and 4 may have promoted hermaphroditic floral bud differentiation. In these phytohormone biosynthesis and signaling pathways, 52 and 54 differential expression genes (including Aux/IAA, ARFs, DELLA, AHP, A-ARR, B-ARR, CYP735A, CRE1, PP2C, JAZ, MYC2, COI1, CTR1, SIMKK, ACO, and MPK6) were identified, respectively. During the development of male floral buds, five metacaspases genes may have been involved in pistil abortion. In addition, MYB, FAR1, bHLH, WRKY, and MADS transcription factors might play important roles in persimmon floral bud sex differentiation. Noteworthy, miR169v_1, miR169e_3, miR319_1, and miR319 were predicted to contribute to phytohormone biosynthesis and signaling pathways and floral organogenesis and may also regulate floral bud sex differentiation.

CONCLUSION

The present study revealed the differences in morphology and phytohormones content between male and hermaphroditic floral buds of 'Longyanyeshi 1' during the process of sex differentiation, and identified a subset of candidate genes and miRNAs putatively associated with its sex differentiation. These findings can provide a foundation for molecular regulatory mechanism researching on andromonoecious persimmon.

The rapid dissolution of dioecy by experimental evolution

Evolutionary transitions from hermaphroditism to dioecy have been common in flowering plants,^1,2 but recent analysis also points to frequent reversions from dioecy to hermaphroditism.^2-4 Here, we use experimental evolution to expose a mechanism for such reversions, validating an explanation for the scattered phylogenetic distribution of dioecy. We removed males from dioecious populations of the wind-pollinated plant Mercurialis annua and allowed natural selection to act on the remaining females that occasionally produced male flowers; such "leaky" sex expression is common in both males and females of dioecious plants.⁵ Over the course of four generations, females evolved a 23-fold increase in average male flower production. This phenotypic masculinization of females coincided with the evolution of partial self-fertilization, high average seed set in the continued absence of males, and a capacity to sire progeny when males were re-introduced into their populations. Our study thus validates a mechanism for the rapid dissolution of dioecy and the evolution of functional hermaphroditism under conditions that may frequently occur during periods of low population density, repeated colonization, or range expansion.^6,7 Our results illustrate the power of natural selection, acting in replicated experimental populations, to bring about transitions in the mating behavior of plants.

The Diversity and Dynamics of Sex Determination in Dioecious Plants

The diversity of inflorescences among flowering plants is captivating. Such charm is not only due to the variety of sizes, shapes, colors, and flowers displayed, but also to the range of reproductive systems. For instance, hermaphrodites occur abundantly throughout the plant kingdom with both stamens and carpels within the same flower. Nevertheless, 10% of flowering plants have separate unisexual flowers, either in different locations of the same individual (monoecy) or on different individuals (dioecy). Despite their rarity, dioecious plants provide an excellent opportunity to investigate the mechanisms involved in sex expression and the evolution of sex-determining regions (SDRs) and sex chromosomes. The SDRs and the evolution of dioecy have been studied in many species ranging from Ginkgo to important fruit crops. Some of these studies, for example in asparagus or kiwifruit, identified two sex-determining genes within the non-recombining SDR and may thus be consistent with the classical model for the evolution of dioecy from hermaphroditism via gynodioecy, that predicts two successive mutations, the first one affecting male and the second one female function, becoming linked in a region of suppressed recombination. On the other hand, aided by genome sequencing and gene editing, single factor sex determination has emerged in other species, such as persimmon or poplar. Despite the diversity of sex-determining mechanisms, a tentative comparative analysis of the known sex-determining genes and candidates in different species suggests that similar genes and pathways may be employed repeatedly for the evolution of dioecy. The cytokinin signaling pathway appears important for sex determination in several species regardless of the underlying genetic system. Additionally, tapetum-related genes often seem to act as male-promoting factors when sex is determined via two genes. We present a unified model that synthesizes the genetic networks of sex determination in monoecious and dioecious plants and will support the generation of hypothesis regarding candidate sex determinants in future studies.

Enhanced leaky sex expression in response to pollen limitation in the dioecious plant Mercurialis annua

In dioecious plants, males and females frequently show ‘leaky’ sex expression, with individuals occasionally producing flowers of the opposite sex. This leaky sex expression may have enabled the colonization of oceanic islands by dioecious plant species, and it is likely to represent the sort of variation upon which selection acts to bring about evolutionary transitions from dioecy to hermaphroditism. Although leakiness is commonly reported for dioecious species, it is not known whether it has plastic component. The question is interesting because males or females with an ability to enhance their leakiness plastically in the absence of mates would have an advantage of being able to produce progeny by self‐fertilization. Here, we demonstrate that leaky sex expression in the wind‐pollinated dioecious herb Mercurialis annua is plastically responsive to its mating context. We compared experimental populations of females growing either with or without males. Females growing in the absence of males were leakier in their sex expression than controls growing with males, producing more than twice as many male flowers. Our results thus provide a striking instance of plasticity in the reproductive behaviour of plants that is likely adaptive. We consider how females might sense their mating environment as a function of pollen availability, and we discuss possible constraints on the evolution of plasticity in sex expression when the environmental signals that individuals receive are unreliable.

Comparative transcriptomic analysis of the tea plant (Camellia sinensis) reveals key genes involved in pistil deletion

Background

The growth process of the tea plant (Camellia sinensis) includes vegetative growth and reproductive growth. The reproductive growth period is relatively long (approximately 1.5 years), during which a large number of nutrients are consumed, resulting in reduced tea yield and quality, accelerated aging, and shortened economic life of the tea plant. The formation of unisexual and sterile flowers can weaken the reproductive growth process of the tea plant. To further clarify the molecular mechanisms of pistil deletion in the tea plant, we investigated the transcriptome profiles in the pistil-deficient tea plant (CRQS), wild tea plant (WT), and cultivated tea plant (CT) by using RNA-Seq.

Results

A total of 3683 differentially expressed genes were observed between CRQS and WT flower buds, with 2064 upregulated and 1619 downregulated in the CRQS flower buds. These genes were mainly involved in the regulation of molecular function and biological processes. Ethylene synthesis–related ACC synthase genes were significantly upregulated and ACC oxidase genes were significantly downregulated. Further analysis revealed that one of the WIP transcription factors involved in ethylene synthesis was significantly upregulated. Moreover, AP1 and STK, genes related to flower development, were significantly upregulated and downregulated, respectively.

Conclusions

The transcriptome analysis indicated that the formation of flower buds with pistil deletion is a complex biological process. Our study identified ethylene synthesis, transcription factor WIP, and A and D-class genes, which warrant further investigation to understand the cause of pistil deletion in flower bud formation.

The evolving role of abscisic acid in cell function and plant development over geological time

Abscisic acid (ABA) is found in a wide diversity of organisms, yet we know most about the hormonal action of this compound in the ecologically dominant and economically important angiosperms. In angiosperms, ABA regulates a suite of critical responses from desiccation tolerance through to seed dormancy and stomatal closure. Work exploring the function of key genes in the ABA signalling pathway of angiosperms has revealed that this signal transduction pathway is ancient, yet considerable change in the physiological roles of this hormone have occurred over geological time. With recent advances in our capacity to characterise gene function in non-angiosperms we are on the cusp of revealing the origins of this critical hormonal signalling pathway in plants, and understanding how a simple hormone may have shaped land plant diversity, ecology and adaptation over the past 500 million years.

A single gene underlies the dynamic evolution of poplar sex determination

Although hundreds of plant lineages have independently evolved dioecy (that is, separation of the sexes), the underlying genetic basis remains largely elusive¹. Here we show that diverse poplar species carry partial duplicates of the ARABIDOPSIS RESPONSE REGULATOR 17 (ARR17) orthologue in the male-specific region of the Y chromosome. These duplicates give rise to small RNAs apparently causing male-specific DNA methylation and silencing of the ARR17 gene. CRISPR-Cas9-induced mutations demonstrate that ARR17 functions as a sex switch, triggering female development when on and male development when off. Despite repeated turnover events, including a transition from the XY system to a ZW system, the sex-specific regulation of ARR17 is conserved across the poplar genus and probably beyond. Our data reveal how a single-gene-based mechanism of dioecy can enable highly dynamic sex-linked regions and contribute to maintaining recombination and integrity of sex chromosomes.

Sex-Related Differences in Growth, Herbivory, and Defense of Two Salix Species

Sex-related differences in sex ratio, growth, and herbivory are widely documented in many dioecious plants. The common pattern is for males to grow faster than females and to be less well-defended against herbivores, but Salix is an exception. To study sex-related differences in the patterns of resource allocation for growth and defense in willows, we conducted a large-scale field experiment to investigate the flowering sex ratio, mortality, growth traits, insect herbivory and content of defensive substances in three Salix populations comprising two species. Results demonstrate that the two Salix suchowensis Cheng populations have a female bias in the sex ratio, whereas no bias is found in the S. triandra L. population. Male individuals in the S. suchowensis populations have significantly higher mortality rates than females. However, the mortality rate of S. triandra population has no gender difference. This finding may be one of the explanations for the difference in sex ratio between the two species. The females are larger in height, ground diameter, and biomass, and have a higher nutritional quality (N concentration) than males in both species. Nevertheless, slow-growing males have a higher concentration of the defense chemical (total phenol) and lower degrees of insect herbivory than females. Additionally, biomass is positively correlated with herbivory and negatively correlated with defense in the two willow species. It is concluded that the degrees of herbivory would have a great influence on resource allocation for growth and defense. Meanwhile, it also provides important implications for understanding the evolution of dioecy.

Genome-Wide Analysis Reveals Stress and Hormone Responsive Patterns of JAZ Family Genes in Camellia Sinensis

JAZ (Jasmonate ZIM-domain) proteins play pervasive roles in plant development and defense reaction. However, limited information is known about the JAZ family in Camellia sinensis. In this study, 12 non-redundant JAZ genes were identified from the tea plant genome database. Phylogenetic analysis showed that the 12 JAZ proteins belong to three groups. The cis-elements in promoters of CsJAZ genes and CsJAZ proteins interaction networks were also analyzed. Quantitative RT–PCR analysis showed that 7 CsJAZ genes were preferentially expressed in roots. Furthermore, the CsJAZ expressions were differentially induced by cold, heat, polyethylene glycol (PEG), methyl jasmonate (MeJA), and gibberellin (GA) stimuli. The Pearson correlations analysis based on expression levels showed that the CsJAZ gene pairs were differentially expressed under different stresses, indicating that CsJAZs might exhibit synergistic effects in response to various stresses. Subcellular localization assay demonstrated that CsJAZ3, CsJAZ10, and CsJAZ11 fused proteins were localized in the cell nucleus. Additionally, the overexpression of CsJAZ3, CsJAZ10, and CsJAZ11 in E. coli enhanced the growth of recombinant cells under abiotic stresses. In summary, this study will facilitate the understanding of the CsJAZ family in Camellia sinensis and provide new insights into the molecular mechanism of tea plant response to abiotic stresses and hormonal stimuli.

Developmental basis for flower sex determination and effects of cytokinin on sex determination in Plukenetia volubilis (Euphorbiaceae)

KEY MESSAGE

Cytokinin might be an important factor to regulate floral sex at the very early stage of flower development in sacha inchi. Sacha inchi (Plukenetia volubilis, Euphorbiaceae) is characterized by having female and male flowers in a thyrse with particular differences. The mechanisms involved in the development of unisexual flowers are very poorly understood. In this study, the inflorescence and flower development of P. volubilis were investigated using light microscopy and scanning electron microscopy. We also investigated the effects of cytokinin on flower sex determination by exogenous application of 6-benzyladenine (BA) in P. volubilis. The floral development of P. volubilis was divided into eight stages, and the first morphological divergence between the male and female flowers was found to occur at stage 3. Both female and male flowers can be structurally distinguished by differences in the shape and size of the flower apex after sepal primordia initiation. There are no traces of gynoecia in male flowers or of androecia in female flowers. Exogenous application of BA effectively induced gynoecium primordia initiation and female flower development, especially at the early flower developmental stages. We propose that flower sex is determined earlier and probably occurs before flower initiation, either prior to or at inflorescence development due to the difference in the position of the female and male primordia in the inflorescence and in the time of the female and male primordia being initiated. The influence of cytokinin on female primordia during flower development in P. volubilis strongly suggests a feminization role for cytokinin in sex determination. These results indicate that cytokinin could modify the fate of the apical meristem of male flower and promote the formation of carpel primordia in P. volubilis.

De novo genome assembly and Hi-C analysis reveal an association between chromatin architecture alterations and sex differentiation in the woody plant Jatropha curcas

BACKGROUND

Chromatin architecture is an essential factor regulating gene transcription in different cell types and developmental phases. However, studies on chromatin architecture in perennial woody plants and on the function of chromatin organization in sex determination have not been reported.

RESULTS

Here, we produced a chromosome-scale de novo genome assembly of the woody plant Jatropha curcas with a total length of 379.5 Mb and a scaffold N50 of 30.7 Mb using Pacific Biosciences long reads combined with genome-wide chromosome conformation capture (Hi-C) technology. Based on this high-quality reference genome, we detected chromatin architecture differences between monoecious and gynoecious inflorescence buds of Jatropha. Differentially expressed genes were significantly enriched in the changed A/B compartments and topologically associated domain regions and occurred preferentially in differential contact regions between monoecious and gynoecious inflorescence buds. Twelve differentially expressed genes related to flower development or hormone synthesis displayed significantly different genomic interaction patterns in monoecious and gynoecious inflorescence buds. These results demonstrate that chromatin organization participates in the regulation of gene transcription during the process of sex differentiation in Jatropha.

CONCLUSIONS

We have revealed the features of chromatin architecture in perennial woody plants and investigated the possible function of chromatin organization in Jatropha sex differentiation. These findings will facilitate understanding of the regulatory mechanisms of sex determination in higher plants.

A unique chromosome translocation disrupting ClWIP1 leads to gynoecy in watermelon

To understand sex determination in watermelon (Citrullus lanatus), a spontaneous gynoecious watermelon mutant, XHBGM, was selected from the monoecious wild type XHB. Using map-based cloning, resequencing and fluorescence in situ hybridization analysis, a unique chromosome translocation between chromosome 2 and chromosome 3 was found in XHBGM. Based on the breakpoint location in chromosome 2, a putative C2H2 zinc finger transcription factor gene, ClWIP1 (gene ID Cla008537), an orthologue of the melon gynoecy gene CmWIP1, was disrupted. Using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated system 9 to edit ClWIP1, we obtained gynoecious watermelon lines. Functional studies showed that ClWIP1 is expressed specifically in carpel primordia and is related to the abortion of carpel primordia in early floral development. To identify the cellular and metabolic processes associated with ClWIP1, we compared the shoot apex transcriptomes of two gynoecious mutants and their corresponding wild types. Transcriptome analysis showed that differentially expressed genes related to the ethylene and cytokinin pathways were upregulated in the gynoecious mutants. This study explores the molecular mechanism of sex determination in watermelon and provides a theoretical and technical basis for breeding elite gynoecious watermelon lines.

Epigenetic aspects of floral homeotic genes in relation to sexual dimorphism in the dioecious plant Mercurialis annua

In plants, dioecy characterizes species that carry male and female flowers on separate plants and it occurs in about 6% of angiosperms; however, the molecular mechanisms that underlie dioecy are essentially unknown. The ability for sex-reversal by hormone application raises the hypothesis that the genes required for the expression of both sexes are potentially functional but are regulated by epigenetic means. In this study, proteomic analysis of nuclear proteins isolated from flower buds of females, males, and feminized males of the dioecious plant Mercurialis annua revealed differential expression of nuclear proteins that are implicated in chromatin structure and function, including floral homeotic proteins. Focusing on floral genes, we found that class B genes were mainly expressed in male flowers, while class D genes, as well as SUPERMAN-like genes, were mainly expressed in female flowers. Cytokinin-induced feminization of male plants was associated with down-regulation of male-specific genes concomitantly with up-regulation of female-specific genes. No correlation was found between the expression of class B and D genes and the changes in DNA methylation or chromatin conformation of these genes. Thus, we could not confirm DNA methylation or chromatin conformation of floral genes to be the major determinant regulating sexual dimorphisms. Instead, determination of sex in M. annua might be controlled upstream of floral genes by one or more sex-specific factors that affect hormonal homeostasis. A comprehensive model is proposed for sex-determination in M. annua.

Proteomic analysis of gametophytic sex expression in the fern Ceratopteris thalictroides

Ceratopteris thalictroides, a model fern, has two kinds of gametophytes with different sex expression: male and hermaphrodite. Hermaphroditic gametophytes have one or several archegonia beneath the growing point and a few antheridia at the base or margin. Male gametophytes show a spoon-like shape with much longer than the width and produce many antheridia at the margin and surface. The results of chlorophyll fluorescence detection showed that the photochemical efficiency of hermaphrodites was higher than that of males. By using two-dimensional electrophoresis and mass spectrometry, the differentially abundant proteins in hermaphroditic and male gametophytes were identified. A total of 1136 ± 55 protein spots were detected in Coomassie-stained gels of proteins from hermaphroditic gametophytes, and 1130 ± 65 spots were detected in gels of proteins from male gametophytes. After annotation, 33 spots representing differentially abundant proteins were identified. Among these, proteins involved in photosynthesis and chaperone proteins were over-represented in hermaphrodites, whereas several proteins involved in metabolism were increased in male gametophytes in order to maintain their development under relatively nutritionally deficient conditions. Furthermore, the differentially abundant cytoskeletal proteins detected in this study, such as centrin and actin, may be involved in the formation of sexual organs and are directly related to sex expression. These differentially abundant proteins are important for maintaining the development of gametophytes of different sexes in C. thalictroides.

The sex expression and sex determining mechanism in Pistacia species

Generally, Pistacia species are dioecious, but monoecious strains in several populations have been found, providing excellent models for studying sex differentiation and sex determination mechanisms. Although the mechanisms of sex determination and sex evolution have been extensively studied, related research on heterozygous woody plants is limited. Here, we discuss the expressions of various sex types, which showed broad diversity and complex instability. We have also reviewed the sex determination systems in the plant kingdom and the morphological, cytological, physiological, and molecular aspects of the sex-linked markers in Pistacia trees. Moreover, hypotheses to explain the origin of monoecy are discussed, which is more likely to be the interaction between sex-related genes and environment factors in female plants. Besides, further prospects for the utilization of monoecious resources and the research directions of sex determination mechanism are proposed. This study provides important information on sex expression and provides more insights into sex differentiation and determination.

Identification of quantitative trait loci governing subgynoecy in cucumber

QTL-seq analysis identified three major QTLs conferring subgynoecy in cucumbers. Furthermore, sequence and expression analyses predicted candidate genes controlling subgynoecy. The cucumber (Cucumis sativus L.) is a typical monoecious having individual male and female flowers, and sex differentiation is an important developmental process that directly affects its fruit yield. Subgynoecy represents a sex form with a high degree of femaleness and would have alternative use as gynoecy under limited resource conditions. Recently, many studies have been reported that QTL-seq, which integrates the advantages of bulked segregant analysis and high-throughput whole-genome resequencing, can be a rapid and cost-effective way of mapping QTLs. Segregation analysis in the F₂ and BC₁ populations derived from a cross between subgynoecious LOSUAS and monoecious BMB suggested the quantitative nature of subgynoecy in cucumbers. Both genome-wide SNP profiling of subgynoecious and monoecious bulks constructed from F₂ and BC₁ plants consistently identified three significant genomic regions, one on chromosome 3 (sg3.1) and another two on short and long arms of chromosome 1 (sg1.1 and sg1.2). Classical QTL analysis using the F₂ confirmed sg3.1 (R² = 42%), sg1.1 (R² = 29%) and sg1.2 (R² = 18%) as major QTLs. These results revealed the unique genetic inheritance of subgynoecious line LOSUAS through two distinct major QTLs, sg3.1 and sg1.1, which mainly increase degree of femaleness, while another QTL, sg1.2, contributes to decrease it. This study demonstrated that QTL-seq allows rapid and powerful detection of QTLs using preliminary generation mapping populations such as F₂ or BC₁ population and further that the identified QTLs could be useful for molecular breeding of cucumber lines with high yield potential.

A functional decomposition of sex inconstancy in the dioecious, colonizing plant Mercurialis annua

PREMISE

Plants with separate sexes often show "inconstant" or "leaky" sex expression, with females or males producing a few flowers of the opposite sex. The frequency and degree of such inconstancy may reflect residual hermaphroditic sex allocation after an evolutionary transition from combined to separate sexes. Sex inconstancy also represents a possible first step in the breakdown of dioecy back to hermaphroditism. In the Mercurialis annua (Euphorbiaceae) species complex, monoecy and androdioecy have evolved from dioecy in polyploid populations. Here, we characterize patterns of sex inconstancy in dioecious M. annua and discuss how sex inconstancy may have contributed to the breakdown of separate sexes in the genus.

METHODS

We measured sex inconstancy in three common gardens of M. annua over 2 years using a modification of Lloyd's phenotypic gender in terms of frequency and degree, with the degree calibrating inconstancy against the sex allocation of constant males and constant females, yielding a measure of gender that does not depend on the distribution of gender in the population.

RESULTS

Unusually for dioecious plants, the frequency of sex inconstancy in M. annua was greater in females, but its degree was greater for males in the 2 years of study. We suggest that this pattern is consistent with the maintenance of inconstancy in dioecious M. annua by selection for reproductive assurance under mate limitation.

CONCLUSIONS

Our study illustrates the utility of decomposing measures of sex inconstancy into its frequency and its degree and throws new light on the origin of variation in sexual systems in Mercurialis.

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.

Comparative transcriptome analysis reveals the regulatory networks of cytokinin in promoting the floral feminization in the oil plant Sapium sebiferum

BACKGROUND

Sapium sebiferum, whose seeds contain high level of fatty acids, has been considered as one of the most important oil plants. However, the high male to female flower ratio limited the seed yield improvement and its industrial potentials. Thus, the study of the sex determination in S. sebiferum is of significant importance in increasing the seed yield.

RESULTS

In this study, we demonstrated that in S. sebiferum, cytokinin (CK) had strong feminization effects on the floral development. Exogenous application with 6-benzylaminopurine (6-BA) or thidiazuron (TDZ) significantly induced the development of female flowers and increased the fruit number. Interestingly, the feminization effects of cytokinin were also detected on the androecious genotype of S. sebiferum which only produce male flowers. To further investigate the mechanism underlying the role of cytokinin in the flower development and sex differentiation, we performed the comparative transcriptome analysis of the floral buds of the androecious plants subjected to 6-BA. The results showed that there were separately 129, 352 and 642 genes differentially expressed at 6 h, 12 h and 24 h after 6-BA treatment. Functional analysis of the differentially expressed genes (DEGs) showed that many genes are related to the hormonal biosynthesis and signaling, nutrients translocation and cell cycle. Moreover, there were twenty one flowering-related genes identified to be differentially regulated by 6-BA treatment. Specifically, the gynoecium development-related genes SPATULA (SPT), KANADI 2 (KAN2), JAGGED (JAG) and Cytochrome P450 78A9 (CYP79A9) were significantly up-regulated, whereas the expression of PISTILLATA (PI), TATA Box Associated Factor II 59 (TAFII59) and MYB Domain Protein 108 (MYB108) that were important for male organ development was down-regulated in response to 6-BA treatment, demonstrating that cytokinin could directly target the floral organ identity genes to regulate the flower sex.

CONCLUSIONS

Our work demonstrated that cytokinin is a potential regulator in female flower development in S. sebiferum. The transcriptome analysis of the floral sex transition from androecious to monoecious in response to cytokinin treatment on the androecious S. sebiferum provided valuable information related to the mechanism of sex determination in the perennial woody plants.

Gender-specific expression of GIBBERELLIC ACID INSENSITIVE is critical for unisexual organ initiation in dioecious Spinacia oleracea

While unisexual flowers have evolved repeatedly throughout angiosperm families, the actual identification of sex-determining genes has been elusive, and their regulation within populations remains largely undefined. Here, we tested the mechanism of the feminization pathway in cultivated spinach (Spinacia oleracea), and investigated how this pathway may regulate alternative sexual development. We tested the effect of gibberellic acid (GA) on sex determination through exogenous applications of GA and inhibitors of GA synthesis and proteasome activity. GA concentrations in multiple tissues were estimated by enzyme-linked immunosorbent assay analysis. Gene function was investigated and pathway analysis was performed through virus-induced gene silencing. Relative gene expression levels were estimated by quantitative reverse transcription-polymerase chain reaction. Inhibition of GA production and proteasome activity feminized male flowers. However, there was no difference in GA content in tissues between males and females. We characterized a single DELLA family transcription factor gene (GIBBERELLIC ACID INSENSITIVE (SpGAI)) and observed inflorescence expression in females two-fold higher than in males. Reduction of SpGAI expression in females to male levels phenocopied exogenous GA application with respect to flower development. These results implicate SpGAI as the feminizing factor in spinach, and suggest that the feminizing pathway is epistatic to the masculinizing pathway. We present a unified model for alternative sexual development and discuss the implications for established theory.

De novo transcriptome assembly and comparative analysis between male and benzyladenine-induced female inflorescence buds of Plukenetia volubilis

Plukenetia volubilis is a promising oilseed crop due to its seeds being rich in unsaturated fatty acids, especially alpha-linolenic acid. P. volubilis is monoecious, with separate male and female flowers on the same inflorescence. We previously reported that male flowers were converted to female flowers by exogenous cytokinin (6-benzyladenine, 6-BA) treatment in P. volubilis. To identify candidate genes associated with floral sex differentiation of P. volubilis, we performed de novo transcriptome assembly and comparative analysis on control male inflorescence buds (MIB) and female inflorescence buds (FIB) induced by 6-BA using Illumina sequencing technology. A total of 57,664 unigenes with an average length of 979 bp were assembled from 104.1 million clean reads, and 45,235 (78.45%) unigenes were successfully annotated in the public databases. Notably, Gene Ontology analyses revealed that 4193 and 3880 unigenes were enriched in the categories of reproduction and reproductive processes, respectively. Differential expression analysis identified 1385 differentially expressed unigenes between MIB and FIB, of which six unigenes related to cytokinin and auxin signaling pathways and 16 important transcription factor (TF) genes including MADS-box family members were identified. In particular, several unigenes encoding important TFs, such as homologs of CRABS CLAW, RADIALIS-like 1, RADIALIS-like 2, HECATE 2, WUSCHEL-related homeobox 9, and SUPERMAN, were expressed at higher levels in FIB than in MIB. The expression patterns of the 36 selected unigenes revealed by transcriptome analysis were successfully validated by quantitative real-time PCR. This study not only provides comprehensive gene expression profiles of P. volubilis inflorescence buds, but also lays the foundation for research on the molecular mechanism of floral sex determination in P. volubilis and other monoecious plants.

On the rarity of dioecy in flowering plants

Dioecy, the coexistence of separate male and female individuals in a population, is a rare but phylogenetically widespread sexual system in flowering plants. While research has concentrated on why and how dioecy evolves from hermaphroditism, the question of why dioecy is rare, despite repeated transitions to it, has received much less attention. Previous phylogenetic and theoretical studies have suggested that dioecy might be an evolutionary dead end. However, recent research indicates that the phylogenetic support for this hypothesis is attributable to a methodological bias and that there is no evidence for reduced diversification in dioecious angiosperms. The relative rarity of dioecy thus remains a puzzle. Here, we review evidence for the hypothesis that dioecy might be rare not because it is an evolutionary dead end, but rather because it easily reverts to hermaphroditism. We review what is known about transitions between hermaphroditism and dioecy, and conclude that there is an important need to consider more widely the possibility of transitions away from dioecy, both from an empirical and a theoretical point of view, and by combining tools from molecular evolution and insights from ecology.

Deep analysis of wild Vitis flower transcriptome reveals unexplored genome regions associated with sex specification

RNA-seq of Vitis during early stages of bud development, in male, female and hermaphrodite flowers, identified new loci outside of annotated gene models, suggesting their involvement in sex establishment. The molecular mechanisms responsible for flower sex specification remain unclear for most plant species. In the case of V. vinifera ssp. vinifera, it is not fully understood what determines hermaphroditism in the domesticated subspecies and male or female flowers in wild dioecious relatives (Vitis vinifera ssp. sylvestris). Here, we describe a de novo assembly of the transcriptome of three flower developmental stages from the three Vitis vinifera flower types. The validation of de novo assembly showed a correlation of 0.825. The main goals of this work were the identification of V. v. sylvestris exclusive transcripts and the characterization of differential gene expression during flower development. RNA from several flower developmental stages was used previously to generate Illumina sequence reads. Through a sequential de novo assembly strategy one comprehensive transcriptome comprising 95,516 non-redundant transcripts was assembled. From this dataset 81,064 transcripts were annotated to V. v. vinifera reference transcriptome and 11,084 were annotated against V. v. vinifera reference genome. Moreover, we found 3368 transcripts that could not be mapped to Vitis reference genome. From all the non-redundant transcripts that were assembled, bioinformatics analysis identified 133 specific of V. v. sylvestris and 516 transcripts differentially expressed among the three flower types. The detection of transcription from areas of the genome not currently annotated suggests active transcription of previously unannotated genomic loci during early stages of bud development.

Light availability affects sex lability in a gynodioecious plant

PREMISE OF THE STUDY

Sex lability (i.e., gender diphasy) in plants is classically linked to the larger resource needs associated with the female sexual function (i.e., seed production) compared to the male function (i.e., pollen production). Sex lability in response to the environment is extensively documented in dioecious species, but has been largely overlooked in gynodioecious plants.

METHODS

Here, we tested whether environmental conditions induce sex lability in the gynodioecious Geranium sylvaticum. We conducted a transplantation experiment in the field where plants with different sex expression were reciprocally transplanted between high light and low light habitats. We measured plants' reproductive output and sex expression over four years.

KEY RESULTS

Our results show that sex expression was labile over the study period. The light level at the destination habitat had a significant effect on sexual expression and reproductive output, because plants decreased their reproductive output when transplanted to the low light habitat. Transplantation origin did not affect any parameter measured.

CONCLUSIONS

This study shows that sex expression in Geranium sylvaticum is labile and related to light availability. Sexually labile plants did not produce more seeds or pollen, and thus, there was no apparent fitness gain in sexually labile individuals. Sex lability in gynodioecious plants may be more common than previously believed because detection of sex lability necessitates data on the same individuals over time, which is rare in sexually dimorphic herbaceous plants.

Abscisic acid controlled sex before transpiration in vascular plants

Sexual reproduction in animals and plants shares common elements, including sperm and egg production, but unlike animals, little is known about the regulatory pathways that determine the sex of plants. Here we use mutants and gene silencing in a fern species to identify a core regulatory mechanism in plant sexual differentiation. A key player in fern sex differentiation is the phytohormone abscisic acid (ABA), which regulates the sex ratio of male to hermaphrodite tissues during the reproductive cycle. Our analysis shows that in the fern Ceratopteris richardii, a gene homologous to core ABA transduction genes in flowering plants [SNF1-related kinase2s (SnRK2s)] is primarily responsible for the hormonal control of sex determination. Furthermore, we provide evidence that this ABA-SnRK2 signaling pathway has transitioned from determining the sex of ferns to controlling seed dormancy in the earliest seed plants before being co-opted to control transpiration and CO₂ exchange in derived seed plants. By tracing the evolutionary history of this ABA signaling pathway from plant reproduction through to its role in the global regulation of plant-atmosphere gas exchange during the last 450 million years, we highlight the extraordinary effect of the ABA-SnRK2 signaling pathway in plant evolution and vegetation function.