Effect of modified endogenous ethylene production on sex expression, bisexual flower development and fruit production in melon (Cucumis melo L.)

Developmental stages and episode-specific regulatory genes in andromonoecious melon flower development

BACKGROUND AND AIMS

Given the lack of specific studies on floral development in melon (Cucumis melo L.), we carried out an extensive study involving morphological and transcriptomic analyses to characterize floral development in this species.

METHODS

Using an andromonoecious line, we analysed the development of floral buds in male and hermaphrodite flowers with both light microscopy and scanning electron microscopy. Based on flower lengths, we established a correlation between the developmental stages and four main episodes of floral development and conducted an extensive RNA sequencing analysis of these episodes.

KEY RESULTS

We identified 12 stages of floral development, from the appearance of the floral meristems to anthesis. The main structural differences between male and hermaphrodite flowers appeared between stages 6 and 7; later stages of development leading to the formation of organs and structures in both types of flowers were also described. We analysed the gene expression patterns of the four episodes in flower development to find the genes that were specific to each given episode. Among others, we identified genes that defined the passage from one episode to the next according to the ABCDE model of floral development.

CONCLUSIONS

This work combines a detailed morphological analysis and a comprehensive transcriptomic study to enable characterization of the structural and molecular mechanisms that determine the floral development of an andromonoecious genotype in melon. Taken together, our results provide a first insight into gene regulation networks in melon floral development that are crucial for flowering and pollen formation, highlighting potential targets for genetic manipulation to improve crop yield of melon in the future.

Molecular Marker-Assisted Mapping, Candidate Gene Identification, and Breeding in Melon (Cucumis melo L.): A Review

Melon (Cucumis melo L.) is an important crop that is cultivated worldwide for its fleshy fruit. Understanding the genetic basis of a plant's qualitative and quantitative traits is essential for developing consumer-favored varieties. This review presents genetic and molecular advances related to qualitative and quantitative phenotypic traits and biochemical compounds in melons. This information guides trait incorporation and the production of novel varieties with desirable horticultural and economic characteristics and yield performance. This review summarizes the quantitative trait loci, candidate genes, and development of molecular markers related to plant architecture, branching patterns, floral attributes (sex expression and male sterility), fruit attributes (shape, rind and flesh color, yield, biochemical compounds, sugar content, and netting), and seed attributes (seed coat color and size). The findings discussed in this review will enhance demand-driven breeding to produce cultivars that benefit consumers and melon breeders.

Identification of novel sex determination loci in Japanese weedy melon

KEY MESSAGE

Japanese weedy melon exhibits unique sex expression with interactions between previously reported sex determination genes and two novel loci. Sex expression contributes to fruit quality and yield in the Cucurbitaceae. In melon, orchestrated regulation by sex determination genes explains the mechanism of sex expression, resulting in a great variety of sexual morphologies. In this study, we examined the Japanese weedy melon UT1, which does not follow the reported model of sex expression. We conducted QTL analysis using F₂ plants for flower sex on the main stem and the lateral branch and mapped "occurrence of pistil-bearing flower on the main stem" locus on Chr. 3 (Opbf3.1) and "type of pistil-bearing flower" (female or bisexual) loci on Chr. 2 (tpbf2.1) and Chr. 8 (tpbf8.1). The Opbf3.1 included the known sex determination gene CmACS11. Sequence comparison of CmACS11 between parental lines revealed three nonsynonymous SNPs. A CAPS marker developed from one of the SNPs was closely linked to the occurrence of pistil-bearing flowers on the main stem in two F₂ populations with different genetic backgrounds. The UT1 allele on Opbf3.1 was dominant in F₁ lines from crosses between UT1 and diverse cultivars and breeding lines. This study suggests that Opbf3.1 and tpbf8.1 may promote the development of pistil and stamen primordia by inhibiting CmWIP1 and CmACS-7 functions, respectively, making the UT1 plants hermaphrodite. The results of this study provide new insights into the molecular mechanisms of sex determination in melons and considerations for the application of femaleness in melon breeding.

Distinct metabolic profiling is correlated with bisexual flowers formation resulting from exogenous ethephon induction in melon (Cucumis melo L.)

Melon (Cucumis melo L.) is an agronomically important vegetable. Most cultivars of melon are andromonoecious and bisexual flowers only emerged from the leaf axil of lateral branches. However, the regulatory mechanism contributing to the occurrence of bisexual flowers were still obscure. In this study, ethephon was applied in two common cultivars of melon. In control without ethephon treatment, no bisexual flower was made in the main stem. However, 6.56 ± 1.42 and 6.63 ± 0.55 bisexual flowers were respectively induced in main stem of 'Yangjiaocui-QX' and 'Lvbao' after ethephon treatment, and induced bisexual flowers distributed in 12-20 nodes of main stem. During the formation of bisexual flowers, 41 metabolites were significantly up-regulated and 98 metabolites were significantly down-regulated. According to the KEGG enrichment analysis of 139 different metabolites, a total of 30 pathways were mapped and KEGG terms of "Phenylalanine, tyrosine and tryptophan biosynthesis", "Phenylalanine metabolism" and "Flavone and flavonol biosynthesis" were significantly enriched. In three significantly enriched KEGG terms, shikimic acid, L-tryptophan, L-phenylalanine, and kaempferol were significantly up-regulated while L-tyrosine, 4-hydroxycinnami acid and luteolin were significantly down-regulated in ET compared to CK. Different metabolites were also classified depend on major class features and 14 classes were acquired. The results of metabonomics and endogenous hormone identification indicated that ethylene could enhance the concentration of salicylic acid, methyl jasmonate, ABA and IAA. This study provided an important theoretical foundation for inducing bisexual flowers in main stem and breeding new varieties of melon in future.

The Ethylene Biosynthesis Gene CpACO1A: A New Player in the Regulation of Sex Determination and Female Flower Development in Cucurbita pepo

A methanesulfonate-generated mutant has been identified in Cucurbita pepo that alters sex determination. The mutation converts female into hermaphrodite flowers and disrupts the growth rate and maturation of petals and carpels, delaying female flower opening, and promoting the growth rate of ovaries and the parthenocarpic development of the fruit. Whole-genome resequencing allowed identification of the causal mutation of the phenotypes as a missense mutation in the coding region of CpACO1A, which encodes for a type I ACO enzyme that shares a high identity with Cucumis sativus CsACO3 and Cucumis melo CmACO1. The so-called aco1a reduced ACO1 activity and ethylene production in the different organs where the gene is expressed, and reduced ethylene sensitivity in flowers. Other sex-determining genes, such as CpACO2B, CpACS11A, and CpACS27A, were differentially expressed in the mutant, indicating that ethylene provided by CpACO1A but also the transcriptional regulation of CpACO1A, CpACO2B, CpACS11A, and CpACS27A are responsible for determining the fate of the floral meristem toward a female flower, promoting the development of carpels and arresting the development of stamens. The positive regulation of ethylene on petal maturation and flower opening can be mediated by inducing the biosynthesis of JA, while its negative control on ovary growth and fruit set could be mediated by its repressive effect on IAA biosynthesis.

Systematic genome-wide analysis of the ethylene-responsive ACS gene family: Contributions to sex form differentiation and development in melon and watermelon

Ethylene is an important regulatory phytohormone for sex differentiation and flower development. As the rate-limiting enzyme encoding genes in ethylene biosynthesis, ACS gene family has been well studied in cucumber; however, little is known in other cucurbit crops, such as melon and watermelon, which show diverse sex types in the field. Here, we identified and characterized eight ACS genes each in the genomes of melon and watermelon. According to the conserved serine residues at C-terminal, all the ACS genes could be characterized into three groups, which were supported by the exon-intron organizations and conserved motif distributions. ACS genes displayed diverse tissue-specific expression patterns among four melon and three watermelon sex types. Furthermore, a comparative expression analysis in the shoot apex identified orthologous pairs with potential functions in sex determination, e.g., ACS1s and ACS6s. All ACS orthologs in melon and watermelon exhibited similar expression patterns in monoecious and gynoecious genotypes, except for ACS11s and ACS12s. As expected, the majority of ACS genes were responsive to exogenous ethephon; however, some orthologs exhibited opposite expression patterns, such as ACS1s, ACS9s, and ACS10s. Collectively, our findings provide valuable ACS candidates related to flower development in various sex types of melon and watermelon.

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.

To be a male or a female flower, a question of ethylene in cucurbits

Within the Cucurbitaceae family, most of its species develop unisexual female and male flowers, either on the same plant (monoecy) or on different plants (dioecy). As in other plant families, these two sex morphotypes have evolved from hermaphrodite species; however, many evolutionary events have occurred in cucurbits allowing easy conversion from dioecy to monoecy and vice versa. The variability in sex morphotypes is higher in the domesticated species of the family, which together with recent advances in genomics, make cucurbits an ideal model to study the genetic and molecular mechanisms that control sex determination in plants. Conventional studies demonstrated that ethylene was the master regulator of sex determination in cucurbits, although some cultivated species may respond differently to ethylene action. In this article, we survey the new advances in hormonal and genetic control of sex determination in cucurbit species, control which establishes the ethylene biosynthesis and signaling genes as being those that determine the floral meristem towards a male, female or hermaphrodite flower. The interactions between these genes are integrated into a model that explains the occurrence and distribution of unisexal and hermaphrodite flowers within the different sex morphotypes. We underline the significance of this scientific progress with regard to breeding programs for agronomically-important sex-associated traits.

Transcriptional and Hormonal Responses in Ethephon-Induced Promotion of Femaleness in Pumpkin

The number and proportion of female flowers per plant can directly influence the yield and economic benefits of cucurbit crops. Ethephon is often used to induce female flowers in cucurbits. However, the mechanism through which it affects floral sex differentiation in pumpkin is unknown. We found that the application of ethephon on shoot apical meristem of pumpkin at seedling stage significantly increased the number of female flowers and expedited the appearance of the first female flower. These effects were further investigated by transcriptome and hormone analyses of plants sprayed with ethephon. A total of 647 differentially expressed genes (DEGs) were identified, among which 522 were upregulated and 125 were downregulated. Gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) analysis indicated that these genes were mainly enriched in plant hormone signal transduction and 1-aminocyclopropane-1-carboxylate oxidase (ACO). The results suggests that ethylene is a trigger for multiple hormone signaling, with approximately 4.2% of the identified DEGs involved in ethylene synthesis and multiple hormone signaling. Moreover, ethephon significantly reduced the levels of jasmonic acid (JA), jasmonoyl-L-isoleucine (JA-ILE), and para-topolin riboside (pTR) but increased the levels of 3-indoleacetamide (IAM). Although the level of 1-aminocyclopropanecarboxylic acid was not changed, the expression of ACO genes, which code for the enzyme catalyzing the key rate-limiting step in ethylene production, was significantly upregulated after ethephon treatment. The results indicate that the ethephon affects the transcription of ethylene synthesis and signaling genes, and other hormone signaling genes, especially auxin responsive genes, and modulates the levels of auxin, jasmonic acid, and cytokinin (CK), which may together contribute to femaleness.

Transcriptome profiling reveals the occurrence mechanism of bisexual flowers in melon (Cucumis melo L.)

Most cultivated melons are andromonoecies in which male flowers arose both in main stem and lateral branches but bisexual flowers only emerged from the leaf axils of lateral branches. However, bisexual flowers emerged in leaf axils of main stem after ethephon treatment. Therefore, the mechanism regulating the occurrence of bisexual flowers were investigated by performing transcriptome analysis in two comparison sets: shoot apex of main stem (MA) versus that of lateral branches (LA), and shoot apex of main stem after ethephon treatment (Eth) versus control (Cont). KEGG results showed that genes involved in "plant hormone signal transduction", "MAPK signaling pathway" and "carbon metabolism" were significantly upregulated both in LA and Eth. Further, details of DEGs involved in ethylene signaling pathway were surveyed and six genes were co-upregulated in two comparison sets. Among these, CmERF1, downstream in ethylene signaling pathway, showed the most significantly difference and expressed higher in bisexual buds than that in male buds. Furthermore, fifteen DEGs were found to contain GCC box or CRT/DRE cis-element for CmERF1 in their putative promoter region, and these DEGs involved in several plant hormones signaling pathway, camalexin synthesis, carbon metabolism and plant pathogen interaction.

Transcriptomic analysis reveals ethylene signal transduction genes involved in pistil development of pumpkin

Development of female flowers is an important process that directly affects the yield of Cucubits. Little information is available on the sex determination and development of female flowers in pumpkin, a typical monoecious plant. In the present study, we used aborted and normal pistils of pumpkin for RNA-Seq analysis and determined the differentially expressed genes (DEGs) to gain insights into the molecular mechanism underlying pistil development in pumpkin. A total of 3,817 DEGs were identified, among which 1,341 were upregulated and 2,476 were downregulated. The results of transcriptome analysis were confirmed by real-time quantitative RT-PCR. KEGG enrichment analysis showed that the DEGs were significantly enriched in plant hormone signal transduction and phenylpropanoid biosynthesis pathway. Eighty-four DEGs were enriched in the plant hormone signal transduction pathway, which accounted for 12.54% of the significant DEGs, and most of them were annotated as predicted ethylene responsive or insensitive transcription factor genes. Furthermore, the expression levels of four ethylene signal transduction genes in different flower structures (female calyx, pistil, male calyx, stamen, leaf, and ovary) were investigated. The ethyleneresponsive DNA binding factor, ERDBF3, and ethylene responsive transcription factor, ERTF10, showed the highest expression in pistils and the lowest expression in stamens, and their expression levels were 78- and 162-times more than that in stamens, respectively. These results suggest that plant hormone signal transduction genes, especially ethylene signal transduction genes, play an important role in the development of pistils in pumpkin. Our study provides a theoretical basis for further understanding of the mechanism of regulation of ethylene signal transduction genes in pistil development and sex determination in pumpkin.

Comparative Transcriptome Analysis of Male and Female Conelets and Development of Microsatellite Markers in Pinus bungeana, an Endemic Conifer in China

The sex determination in gymnosperms is still poorly characterized due to the lack of genomic/transcriptome resources and useful molecular genetic markers. To enhance our understanding of the molecular mechanisms of the determination of sexual recognition of reproductive structures in conifers, the transcriptome of male and female conelets were characterized in a Chinese endemic conifer species, Pinus bungeana Zucc. ex Endl. The 39.62 Gb high-throughput sequencing reads were obtained from two kinds of sexual conelets. After de novo assembly of the obtained reads, 85,305 unigenes were identified, 53,944 (63.23%) of which were annotated with public databases. A total of 12,073 differentially expressed genes were detected between the two types of sexes in P. bungeana, and 5766 (47.76%) of them were up-regulated in females. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enriched analysis suggested that some of the genes were significantly associated with the sex determination process of P. bungeana, such as those involved in tryptophan metabolism, zeatin biosynthesis, and cysteine and methionine metabolism, and the phenylpropanoid biosynthesis pathways. Meanwhile, some important plant hormone pathways (e.g., the gibberellin (GA) pathway, carotenoid biosynthesis, and brassinosteroid biosynthesis (BR) pathway) that affected sexual determination were also induced in P. bungeana. In addition, 8791 expressed sequence tag-simple sequence repeats (EST-SSRs) from 7859 unigenes were detected in P. bungeana. The most abundant repeat types were dinucleotides (1926), followed by trinucleotides (1711). The dominant classes of the sequence repeat were A/T (4942) in mononucleotides and AT/AT (1283) in dinucleotides. Among these EST-SSRs, 84 pairs of primers were randomly selected for the characterization of potential molecular genetic markers. Finally, 19 polymorphic EST-SSR primers were characterized. We found low to moderate levels of genetic diversity (N_A = 1.754; H_O = 0.206; H_E = 0.205) across natural populations of P. bungeana. The cluster analysis revealed two distinct genetic groups for the six populations that were sampled in this endemic species, which might be caused by the fragmentation of habitats and long-term geographic isolation among different populations. Taken together, this work provides important insights into the molecular mechanisms of sexual identity in the reproductive organs of P. bungeana. The molecular genetic resources that were identified in this study will also facilitate further studies in functional genomics and population genetics in the Pinus species.

Transcriptomic Analysis Reveals Candidate Genes for Female Sterility in Pomegranate Flowers

Pomegranate has two types of flowers on the same plant: functional male flowers (FMF) and bisexual flowers (BF). BF are female-fertile flowers that can set fruits. FMF are female-sterile flowers that fail to set fruit and that eventually drop. The putative cause of pomegranate FMF female sterility is abnormal ovule development. However, the key stage at which the FMF pomegranate ovules become abnormal and the mechanism of regulation of pomegranate female sterility remain unknown. Here, we studied ovule development in FMF and BF, using scanning electron microscopy to explore the key stage at which ovule development was terminated and then analyzed genes differentially expressed (differentially expressed genes - DEGs) between FMF and BF to investigate the mechanism responsible for pomegranate female sterility. Ovule development in FMF ceased following the formation of the inner integument primordium. The key stage for the termination of FMF ovule development was when the bud vertical diameter was 5.0-13.0 mm. Candidate genes influencing ovule development may be crucial factors in pomegranate female sterility. INNER OUTER (INO/YABBY4) (Gglean016270) and AINTEGUMENTA (ANT) homolog genes (Gglean003340 and Gglean011480), which regulate the development of the integument, showed down-regulation in FMF at the key stage of ovule development cessation (ATNSII). Their upstream regulator genes, such as AGAMOUS-like (AG-like) (Gglean028014, Gglean026618, and Gglean028632) and SPOROCYTELESS (SPL) homolog genes (Gglean005812), also showed differential expression pattern between BF and FMF at this key stage. The differential expression of the ethylene response signal genes, ETR (ethylene-resistant) (Gglean022853) and ERF1/2 (ethylene-responsive factor) (Gglean022880), between FMF and BF indicated that ethylene signaling may also be an important factor in the development of pomegranate female sterility. The increase in BF observed after spraying with ethephon supported this interpretation. Results from qRT-PCR confirmed the findings of the transcriptomic analysis.

A major quantitative trait locus conferring subgynoecy in cucumber

A major QTL conditioning high degree of femaleness in cucumber was identified by marker analysis and next generation sequencing. Cucumber (Cucumis sativus L.) is a model species for sex determination studies, and its yield is associated with the degree of femaleness. Subgynoecy represents a sex form with a high degree of femaleness for which the genetic basis remains elusive. In this study, genetic analysis in the F2 and BC1 populations developed from a cross between subgynoecious S-2-98 and monoecious M95 suggested a quantitative nature of subgynoecy. Application of simple sequence repeat markers between subgynoecious and monoecious bulks constructed from BC1 plants identified three QTLs: sg3.1, sg6.1, and sg6.2. The major QTL sg3.1 contributed to 54.6% of the phenotypic variation, and its presence was confirmed by genome-wide comparison of SNP profiles between parental lines and a subgynoecious bulk constructed from BC6 plants. Using PCR-based markers developed from the SNP profile, sg3.1 was further delimited to a genomic region of 799 kb. The genetic basis of subgynoecy revealed here shall shed light on the development of elite cultivars with high yield potential.

Effect of CRC::etr1-1 transgene expression on ethylene production, sex expression, fruit set and fruit ripening in transgenic melon (Cucumis melo L.)

Ethylene is a key factor regulating sex expression in cucurbits. Commercial melons (Cucumis melo L.) are typically andromonoecious, producing male and bisexual flowers. Our prior greenhouse studies of transgenic melon plants expressing the dominant negative ethylene perception mutant gene, etr1-1, under control of the carpel- and nectary-primordia targeted CRAB'S CLAW (CRC) promoter showed increased number and earlier appearance of carpel-bearing flowers. To further investigate this phenomenon which could be potentially useful for earlier fruit production, we observed CRC::etr1-1 plants in the field for sex expression, fruit set, fruit development, and ripening. CRC::etr1-1 melon plants showed increased number of carpel-bearing open flowers on the main stem and earlier onset by 7-10 nodes. Additional phenotypes observed in the greenhouse and field were conversion of approximately 50% of bisexual buds to female, and elongated ovaries and fruits. Earlier and greater fruit set occurred on the transgenic plants. However, CRC::etr1-1 plants had greater abscission of young fruit, and smaller fruit, so that final yield (kg/plot) was equivalent to wild type. Earlier fruit set in line M5 was accompanied by earlier appearance of ripe fruit. Fruit from line M15 frequently did not exhibit external ripening processes of rind color change and abscission, but when cut open, the majority showed a ripe or overripe interior accompanied by elevated internal ethylene. The non-ripening external phenotype in M15 fruit corresponded with elevated etr1-1 transgene expression in the exocarp. These results provide insight into the role of ethylene perception in carpel-bearing flower production, fruit set, and ripening.

Floral primordia-targeted ACS (1-aminocyclopropane-1-carboxylate synthase) expression in transgenic Cucumis melo implicates fine tuning of ethylene production mediating unisexual flower development

Floral primordia-targeted expression of the ethylene biosynthetic gene, ACS , in melon suggests that differential timing and ethylene response thresholds combine to promote carpels, inhibit stamens, and prevent asexual bud formation. Typical angiosperm flowers produce both male and female reproductive organs. However, numerous species have evolved unisexuality. Melons (Cucumis melo L.) can produce varying combinations of male, female or bisexual flowers. Regardless of final sex, floral development begins with sequential initiation of all four floral whorls; unisexuality results from carpel or stamen primordia arrest regulated by the G and A loci, respectively. Ethylene, which promotes femaleness, is a key factor regulating sex expression. We sought to further understand the location, timing, level, and relationship to sex gene expression required for ethylene to promote carpel development or inhibit stamen development. Andromonoecious melons (GGaa) were transformed with the ethylene biosynthetic enzyme gene, ACS (1-aminocyclopropane-1-carboxylate synthase), targeted for expression in stamen and petal, or carpel and nectary, primordia using Arabidopsis APETALA3 (AP3) or CRABSCLAW (CRC) promoters, respectively. CRC::ACS plants did not exhibit altered sex phenotype. AP3::ACS melons showed increased femaleness manifested by gain of a bisexual-only phase not seen in wild type, decreased male buds and flowers, and loss of the initial male-only phase. In extreme cases, plants became phenotypically hermaphrodite, rather than andromonoecious. A reduced portion of buds progressed beyond initial whorl formation. Both the ACS transgene and exogenous ethylene reduced the expression of the native carpel-suppressing gene, G, while elevating expression of the stamen-suppressing gene, A. These results show ethylene-mediated regulation of key sex expression genes and suggest a mechanism by which temporally regulated ethylene production and differential ethylene response thresholds can promote carpels, inhibit stamens, and prevent the formation of asexual buds.

Role of ethylene receptors during senescence and ripening in horticultural crops

The past two decades have been rewarding in terms of deciphering the ethylene signal transduction and functional validation of the ethylene receptor and downstream genes involved in the cascade. Our knowledge of ethylene receptors and its signal transduction pathway provides us a robust platform where we can think of manipulating and regulating ethylene sensitivity by the use of genetic engineering and making transgenic. This review focuses on ethylene perception, receptor mediated regulation of ethylene biosynthesis, role of ethylene receptors in flower senescence, fruit ripening and other effects induced by ethylene. The expression behavior of the receptor and downstream molecules in climacteric and non climacteric crops is also elaborated upon. Possible strategies and recent advances in altering the ethylene sensitivity of plants using ethylene receptor genes in an attempt to modulate the regulation and sensitivity to ethylene have also been discussed. Not only will these transgenic plants be a boon to post-harvest physiology and crop improvement but, it will also help us in discovering the mechanism of regulation of ethylene sensitivity.

Histological study of organogenesis in Cucumis melo L. after genetic transformation: why is it difficult to obtain transgenic plants?

Melon (Cucumis melo L.) is widely considered as a recalcitrant species for genetic transformation. In this study, we developed different regeneration and transformation protocols and we examined the regeneration process at different steps by histological studies. The highest regeneration rate (1.13 ± 0.02 plants per explant) was obtained using cotyledon explants of the 'Védrantais' genotype on Murashige and Skoog (MS) medium supplemented with 0.2 mg/l 6-benzylaminopurine (BAP) and 0.2 mg/l dimethylallylaminopurine (2-iP). Agrobacterium tumefaciens-mediated transformations with the uidA reporter gene were realized on cotyledon explants cultivated in these conditions: 70-90% of explants expressed a transient GUS activity during the early stages of regeneration, however, only few transgenic plants were obtained (1.8-4.5% of stable transformation with the GV2260pBI101 strain). These results revealed a low capacity of melon GUS-positive cells to regenerate transgenic plants. To evaluate the influence of the Agrobacterium infection on plant regeneration, histological analyses were conducted on explants 2, 7, 15, and 28 days after co-culture with the GV2260pBI101 strain. Genetic transformation occurred in epidermal and sub-epidermal cells and reached the meristematic structures expressing a high level of GUS activity during 14 days of culture; but after this period, most of the meristematic structures showed premature cell vacuolization and disorganization. This disruption of the GUS-positive meristematic areas could be responsible of the difficulties encountered to regenerate melon plants after genetic transformation.

Cloning and characterisation of two CTR1-like genes in Cucurbita pepo: regulation of their expression during male and female flower development

Ethylene is an essential regulator of flower development in Cucurbita pepo, controlling the sexual expression, and the differentiation and maturation of floral organs. To study the action mechanism of ethylene during the male and female flower development, we have identified two CTR1 homologues from C. pepo, CpCTR1 and CpCTR2, and analysed their expressions during female and male flower development and in response to external treatments with ethylene. CpCTR1 and CpCTR2 share a high homology with plant CTR1-like kinases, but differ from other related kinases such as the Arabidopsis EDR1 and the tomato LeCTR2. The C-terminal ends of both CpCTR1 and CpCTR2 have all the conserved motifs of Ser/Thr kinase domains, including the ATP-binding signature and the protein kinase active site consensus sequence, which suggests that CpCTR1 and CpCTR2 could have the same function as CTR1 in ethylene signalling. The transcripts of both genes were detected in different organs of the plant, including roots, leaves and shoots, but were mostly accumulated in mature flowers. During the development of male and female flowers, CpCTR1 and CpCTR2 expressions were concomitant with ethylene production, which indicates that both genes could be upregulated by ethylene, at least in flowers. Moreover, external treatments with ethylene, although did not alter the expression of these two genes in seedlings and leaves, were able to upregulate their expression in flowers. In the earlier stages of flower development, when ethylene production is very low, the expression of CpCTR1 and CpCTR2 is higher in male floral organs, which agrees with the role of these genes as negative regulators of ethylene signalling, and explain the lower ethylene sensitivity of male flowers in comparison with female flowers. The function of the upregulation of these two genes in later stages of female flower development, when the production of ethylene is also increased, is discussed.

Sex chromosomes in land plants

Sex chromosomes in land plants can evolve as a consequence of close linkage between the two sex determination genes with complementary dominance required to establish stable dioecious populations, and they are found in at least 48 species across 20 families. The sex chromosomes in hepatics, mosses, and gymnosperms are morphologically heteromorphic. In angiosperms, heteromorphic sex chromosomes are found in at least 19 species from 4 families, while homomorphic sex chromosomes occur in 20 species from 13 families. The prevalence of the XY system found in 44 out of 48 species may reflect the predominance of the evolutionary pathway from gynodioecy towards dioecy. All dioecious species have the potential to evolve sex chromosomes, and reversions back from dioecy to various forms of monoecy, gynodioecy, or androdioecy have also occurred. Such reversals may occur especially during the early stages of sex chromosome evolution before the lethality of the YY (or WW) genotype is established.

Stamen development in Arabidopsis is arrested by organ-specific overexpression of a cucumber ethylene synthesis gene CsACO2

Cucumber (Cucumis sativus L.) has served as a model to understand hormone regulation in unisexual flower development since the 1950s and the role of ethylene in promoting female flower development has been well documented. Recent studies cloned the F-locus in gynoecious lines as an additional copy of the ACC synthase (ACS) gene, which further confirmed the role of ethylene in the promotion of female cucumber flowers. However, no direct evidence was generated to demonstrate that increases in endogenous ethylene production could induce female flowers by arresting stamen development. To clarify the relationship between ethylene production and stamen development, we overexpressed the ethylene synthesis cucumber gene CsACO2 to generate transgenic Arabidopsis, driven by the organ-specific promoter P ( AP3 ). We found that organ-specific overexpression of CsACO2 significantly affected stamen but not carpel development, similar to that in the female flowers of cucumber. Our results suggested that increases in ethylene production directly disturb stamen development. Additionally, our study revealed that among all floral organs, stamens respond most sensitively to exogenous ethylene.

Melon fruits: genetic diversity, physiology, and biotechnology features

Among Cucurbitaceae, Cucumis melo is one of the most important cultivated cucurbits. They are grown primarily for their fruit, which generally have a sweet aromatic flavor, with great diversity and size (50 g to 15 kg), flesh color (orange, green, white, and pink), rind color (green, yellow, white, orange, red, and gray), form (round, flat, and elongated), and dimension (4 to 200 cm). C. melo can be broken down into seven distinct types based on the previously discussed variations in the species. The melon fruits can be either climacteric or nonclimacteric, and as such, fruit can adhere to the stem or have an abscission layer where they will fall from the plant naturally at maturity. Traditional plant breeding of melons has been done for 100 years wherein plants were primarily developed as open-pollinated cultivars. More recently, in the past 30 years, melon improvement has been done by more traditional hybridization techniques. An improvement in germplasm is relatively slow and is limited by a restricted gene pool. Strong sexual incompatibility at the interspecific and intergeneric levels has restricted rapid development of new cultivars with high levels of disease resistance, insect resistance, flavor, and sweetness. In order to increase the rate and diversity of new traits in melon it would be advantageous to introduce new genes needed to enhance both melon productivity and melon fruit quality. This requires plant tissue and plant transformation techniques to introduce new or foreign genes into C. melo germplasm. In order to achieve a successful commercial application from biotechnology, a competent plant regeneration system of in vitro cultures for melon is required. More than 40 in vitro melon regeneration programs have been reported; however, regeneration of the various melon types has been highly variable and in some cases impossible. The reasons for this are still unknown, but this plays a heavy negative role on trying to use plant transformation technology to improve melon germplasm. In vitro manipulation of melon is difficult; genotypic responses to the culture method (i.e., organogenesis, somatic embryogenesis, etc.) as well as conditions for environmental and hormonal requirements for plant growth and regeneration continue to be poorly understood for developing simple in vitro procedures to culture and transform all C. melo genotypes. In many cases, this has to be done on an individual line basis. The present paper describes the various research findings related to successful approaches to plant regeneration and transgenic transformation of C. melo. It also describes potential improvement of melon to improve fruit quality characteristics and postharvest handling. Despite more than 140 transgenic melon field trials in the United States in 1996, there are still no commercial transgenic melon cultivars on the market. This may be a combination of technical or performance factors, intellectual property rights concerns, and, most likely, a lack of public acceptance. Regardless, the future for improvement of melon germplasm is bright when considering the knowledge base for both techniques and gene pools potentially useable for melon improvement.