Sugar signalling is involved in the sex expression response of monoecious cucumber to low temperature

Utilization of marker-assisted backcrossing to generate new gynoecious cucumber lines with genetic heritage similar to the indigenous Vietnamese cucumber

The marker-assisted backcrossing (MAB) can help to transfer an interested allele at a target locus from a donor to a recipient line. Gynoecious is a pivotal trait of cucumber since commercial F1 hybrid seeds produced with gynoecious line as one of the parents are high-yield and affordable. This study aims to transfer the F locus encoded for gynoecious trait to Vietnamese domesticated cucumbers by marker-assisted backcrossing. Two monoecious cucumber lines, A1 (Ha Giang, Vietnam) A2 (Yen Bai, Vietnam), and two gynoecious cucumber lines, B1 (Plantgene, India) and B2 (Hue, Vietnam) were utilized as the starting materials. BCAT marker (located on the F locus) and 52 SSRs (spread across seven chromosomes and tightly linked with some crucial horticultural traits) were used as the foreground and background markers, respectively. With this, phenotype selection for fruit and leaf sizes was also applied. First, using phenotypic screening and foreground marker, A1 (Ha Giang, Vietnam) and B1 (Plantgene, India) were selected as donor and recurrent parents for backcrossing. Then, after two backcrosses followed by two self-pollinations, four gynoecious C cucumber lines were created. These C lines have leaf sizes slightly bigger than the recurrent parent. Importantly, their fruit length is the same or longer than A1 (Ha Giang, Vietnam). These new gynoecious lines could be used as material lines for producing commercial F1 hybrid seeds.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01481-w.

Revisit and explore the ethylene-independent mechanism of sex expression in cucumber (Cucumis sativus)

KEY MESSAGE

This review provides a thorough and comprehensive perspective on the topic of cucumber sexual expression. Specifically, insights into sex expression mediated by pathways other than ethylene are highlighted. Cucumber (Cucumis sativus L.) is a common and important commercial crop that is cultivated and consumed worldwide. Additionally, this species is commonly used as a model for investigating plant sex expression. Cucumbers exhibit a variety of floral arrangements, comprising male, female, and hermaphroditic (bisexual) flowers. Generally, cucumber plants that produce female flowers are typically preferred due to their significant impact on the overall output. Various environmental conditions, such as temperature, light quality, and photoperiod, have been also shown to influence the sex expression in this species. Multiple lines of evidence indicate that ethylene and its biosynthesis genes are crucial in regulating cucumber sex expression. Gibberellins, another well-known phytohormone, can similarly influence cucumber sex expression via an ethylene-independent route. Further studies employing the next-generation sequencing technology also visualized a deeper slice of the molecular mechanism such as the role of the cell cycle program in the cucumber sex expression. This review aims to provide an overview of the sex expression of cucumber including its underlying molecular mechanism and regulatory aspects based on recent investigations.

Comparative RNA-Seq Analysis between Monoecious and Androecious Plants Reveals Regulatory Mechanisms Controlling Female Flowering in Cucurbita pepo

In the monoecious Cucurbita pepo, the transition to female flowering is the time at which the plant starts the production of female flowers after an initial male phase of development. Ethylene plays an essential role in this process since some ethylene deficient and ethylene-insensitive mutants are androecious and only produce male flowers. To gain insight into the molecular mechanisms regulating the specification and early development of female flowers, we have compared the transcriptomic changes occurring in the shoot apices of WT and androecious ethylene-insensitive etr1b mutant plants upon female flowering transition. There were 1160 female flowering-specific DEGs identified in WT plants upon female flowering, and 284 of them were found to be modulated by the ethylene-insensitive etr1b mutation. The function of these DEGs indicated that female flower specification depends on the adoption of a transcriptional program that includes previously identified sex-determining genes in the ethylene pathway, but also genes controlling the biosynthesis and signaling pathways of other phytohormones, and those encoding for many different transcription factors. The transcriptomic changes suggested that gibberellins play a negative role in female flowering, while ethylene, auxins, ABA and cytokinins are positive regulators. Transcription factors from 34 families, including NAC, ERF, bHLH, bZIP, MYB and C2H2/CH3, were found to be regulating female flowering in an ethylene-dependent or -independent manner. Our data open a new perspective of the molecular mechanisms that control the specification and development of female flowers in C. pepo.

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.

Effects of Different Chemicals on Sexual Regulation in Persimmon (Diospyros kaki Thunb.) Flowers

Research on crop sexuality is important for establishing systems for germplasm innovation and cultivating improved varieties. In this study, androecious persimmon trees were treated with various concentrations of ethrel (100, 500, and 1,000 mg/L) and zeatin (1, 5, and 10 mg/L) to investigate the morphological, physiological, and molecular characteristics of persimmon. Ethrel at 1,000 mg/L and zeatin at 10 mg/L both significantly reduced the stamen length and pollen grain diameter in androecious trees. Ethrel treatment also led to reduced stamen development with degenerated cellular contents; zeatin treatment promoted the development of arrested pistils via maintaining relatively normal mitochondrial morphology. Both treatments altered carbohydrate, amino acid, and endogenous phytohormone contents, as well as genes associated with hormone production and floral organ development. Thereafter, we explored the combined effects of four chemicals, including ethrel and zeatin, as well as zebularine and 5-azacytidine, both of which are DNA methylation inhibitors, on androecious persimmon flower development. Morphological comparisons showed that stamen length, pollen viability, and pollen grain diameter were significantly inhibited after combined treatment. Large numbers of genes involving in carbohydrate metabolic, mitogen-activated protein kinase (MAPK) signaling, and ribosome pathways, and metabolites including uridine monophosphate (UMP) and cyclamic acid were identified in response to the treatment, indicating complex regulatory mechanisms. An association analysis of transcriptomic and metabolomic data indicated that ribosomal genes have distinct effects on UMP and cyclamic acid metabolites, explaining how male floral buds of androecious persimmon trees respond to these exogenous chemicals. These findings extend the knowledge concerning sexual differentiation in persimmon; they also provide a theoretical basis for molecular breeding, high-yield cultivation, and quality improvement in persimmon.

Functional Characterization of a Cucumber (Cucumis sativus L.) Vacuolar Invertase, CsVI1, Involved in Hexose Accumulation and Response to Low Temperature Stress

Cucumber (Cucumis sativus L.), an important vegetable plant species, is susceptible to low temperature stress especially during the seedling stage. Vacuolar invertase (VI) plays important roles in plant responses to abiotic stress. However, the molecular and biochemical mechanisms of VI function in cucumber, have not yet been completely understood and VI responses to low temperature stress and it functions in cold tolerance in cucumber seedlings are also in need of exploration. The present study found that hexose accumulation in the roots of cucumber seedlings under low temperature stress is closely related to the observed enhancement of invertase activity. Our genome-wide search for the vacuolar invertase (VI) genes in cucumber identified the candidate VI-encoding gene CsVI1. Expression profiling of CsVI1 showed that it was mainly expressed in the young roots of cucumber seedlings. In addition, transcriptional analysis indicated that CsVI1 expression could respond to low temperature stress. Recombinant CsVI1 proteins purified from Pichia pastoris and Nicotiana benthamiana leaves could hydrolyze sucrose into hexoses. Further, overexpression of CsVI1 in cucumber plants could increase their hexose contents and improve their low temperature tolerance. Lastly, a putative cucumber invertase inhibitor was found could form a complex with CsVI1. In summary, these results confirmed that CsVI1 functions as an acid invertase involved in hexose accumulation and responds to low temperature stress in cucumber seedlings.

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.

Comparative transcriptome analysis of male and female flowers in Spinacia oleracea L

Background

Dioecious spinach (Spinacia oleracea L.), a commercial and nutritional vegetable crop, serves as a model for studying the mechanisms of sex determination and differentiation in plants. However, this mechanism is still unclear. Herein, based on PacBio Iso-seq and Illumina RNA-seq data, comparative transcriptome analysis of male and female flowers were performed to explore the sex differentiation mechanism in spinach.

Results

Compared with published genome of spinach, 10,800 transcripts were newly annotated; alternative splicing, alternative polyadenylation and lncRNA were analyzed for the first time, increasing the diversity of spinach transcriptome. A total of 2965 differentially expressed genes were identified between female and male flowers at three early development stages. The differential expression of RNA splicing-related genes, polyadenylation-related genes and lncRNAs suggested the involvement of alternative splicing, alternative polyadenylation and lncRNA in sex differentiation. Moreover, 1946 male-biased genes and 961 female-biased genes were found and several candidate genes related to gender development were identified, providing new clues to reveal the mechanism of sex differentiation. In addition, weighted gene co-expression network analysis showed that auxin and gibberellin were the common crucial factors in regulating female or male flower development; however, the closely co-expressed genes of these two factors were different between male and female flower, which may result in spinach sex differentiation.

Conclusions

In this study, 10,800 transcripts were newly annotated, and the alternative splicing, alternative polyadenylation and long-noncoding RNA were comprehensively analyzed for the first time in spinach, providing valuable information for functional genome study. Moreover, candidate genes related to gender development were identified, shedding new insight on studying the mechanism of sex determination and differentiation in plant.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07277-4.

Comparative transcriptome analysis reveals new molecular pathways for cucumber genes related to sex determination

Transcriptome data and qPCR analysis revealed new insight into genes regulatory mechanism related to cucumber sex determination. Cucumber (Cucumis sativus L.) is an economically important crop cultivated worldwide. Enhancing the genomic resources for cucumber may enable the regulation of traits relevant to crop productivity and quality. Sequencing technologies and bioinformatics tools provide opportunities for the development of such resources. The aims of this study were to identify and characterize the genes involved in sex determination and flower morphogenesis in cucumber isogenic lines that differed regarding flower sex type. We obtained transcripts for 933 genes related to shoot apex development, among which 310 were differentially expressed genes (DEGs) among the male, female, and hermaphroditic lines. We performed gene ontology and molecular network analyses and explored the DEGs related to already known processes like: hormone synthesis and signaling, lipid and sugar metabolism; and also newly discovered processes related to cell wall, membrane, and cytoskeleton modifications; ion homeostasis which appears to be important for ethylene perception and signaling, and genes expression mediated by transcription factors related to floral organ identities. We proposed a new model of regulatory mechanism network of sex development in cucumber. Our results may be useful for clarifying the molecular genetics and the functional mechanisms underlying the sex determination processes.

Gene Interactions Regulating Sex Determination in Cucurbits

The family Cucurbitaceae includes many economically important crops, such as cucumber (Cucumis sativus), melon (Cucumis melo), watermelon (Citrullus lanatus), and zucchini (Cucurbita pepo), which share homologous gene pathways that control similar phenotypes. Sex determination is a research hotspot associated with yield and quality, and the genes involved are highly orthologous and conserved in cucurbits. In the field, six normal sex types have been categorized according to the distribution of female, male, or bisexual flowers in a given plant. To date, five orthologous genes involved in sex determination have been cloned, and their various combinations and expression patterns can explain all the identified sex types. In addition to genetic mechanisms, ethylene controls sex expression in this family. Two ethylene signaling components have been identified recently, which will help us to explore the ethylene signaling-mediated interactions among sex-related genes. This review discusses recent advances relating to the mechanism of sex determination in cucurbits and the prospects for research in this area.

MicroRNAs and their targets in cucumber shoot apices in response to temperature and photoperiod

BACKGROUND

The cucumber is one of the most important vegetables worldwide and is used as a research model for study of phloem transport, sex determination and temperature-photoperiod physiology. The shoot apex is the most important plant tissue in which the cell fate and organ meristems have been determined. In this study, a series of whole-genome small RNA, degradome and transcriptome analyses were performed on cucumber shoot apical tissues treated with high vs. low temperature and long vs. short photoperiod.

RESULTS

A total of 164 known miRNAs derived from 68 families and 203 novel miRNAs from 182 families were identified. Their 4611 targets were predicted using psRobot and TargetFinder, amongst which 349 were validated by degradome sequencing. Fourteen targets of six miRNAs were differentially expressed between the treatments. A total of eight known and 16 novel miRNAs were affected by temperature and photoperiod. Functional annotations revealed that "Plant hormone signal transduction" pathway was significantly over-represented in the miRNA targets. The miR156/157/SBP-Boxes and novel-mir153/ethylene-responsive transcription factor/senescence-related protein/aminotransferase/acyl-CoA thioesterase are the two most credible miRNA/targets combinations modulating the plant's responsive processes to the temperature-photoperiod changes. Moreover, the newly evolved, cucumber-specific novel miRNA (novel-mir153) was found to target 2087 mRNAs by prediction and has 232 targets proven by degradome analysis, accounting for 45.26-58.88% of the total miRNA targets in this plant. This is the largest sum of genes targeted by a single miRNA to the best of our knowledge.

CONCLUSIONS

These results contribute to a better understanding of the miRNAs mediating plant adaptation to combinations of temperature and photoperiod and sheds light on the recent evolution of new miRNAs in cucumber.

Temperature and photoperiod changes affect cucumber sex expression by different epigenetic regulations

BACKGROUND

Cucumbers (Cucumis sativus) are known for their plasticity in sex expression. DNA methylation status determines gene activity but is susceptible to environmental condition changes. Thus, DNA methylation-based epigenetic regulation may at least partially account for the instability of cucumber sex expression. Do temperature and photoperiod that are the two most important environmental factors have equal effect on cucumber sex expression by similar epigenetic regulation mechanism? To answer this question, we did a two-factor experiment of temperature and photoperiod and generated methylome and transcriptome data from cucumber shoot apices.

RESULTS

The seasonal change in the femaleness of a cucumber core germplasm collection was investigated over five consecutive years. As a result, 71.3% of the 359 cucumber accessions significantly decreased their femaleness in early autumn when compared with spring. High temperature and long-day photoperiod treatments, which mimic early autumn conditions, are both unfavorable for female flower formation, and temperature is the predominant factor. High temperatures and long-day treatments both predominantly resulted in hypermethylation compared to demethylation, and temperature effect was decisive. The targeted cytosines shared in high-temperature and long-day photoperiod treatment showed the same change in DNA methylation level. Moreover, differentially expressed TEs (DETs) and the predicted epiregulation sites were clustered across chromosomes, and importantly, these sites were reproducible among different treatments. Essentially, the photoperiod treatment preferentially and significantly influenced flower development processes, while temperature treatment produced stronger responses from phytohormone-pathway-related genes. Cucumber AGAMOUS was likely epicontrolled exclusively by photoperiod while CAULIFLOWER A and CsACO₃ were likely epicontrolled by both photoperiod and temperature.

CONCLUSIONS

Seasonal change of sex expression is a germplasm-wide phenomenon in cucumbers. High temperature and long-day photoperiod might have the same effect on the methylome via the same mechanism of gene-TE interaction but resulted in different epicontrol sites that account for different mechanisms between temperature- and photoperiod-dependent sex expression changes.

Transcriptomic insights into the blue light-induced female floral sex expression in cucumber (Cucumis sativus L.)

In cucurbitaceous crops, sex differentiation of flower buds is a crucial developmental process that directly affects fruit yield. Here we showed that the induction of female flower was the highest in the blue light-treated monoecious cucumber plants compared with that in other light qualities (white, green and red). High-throughput RNA-Seq analysis of the shoot apexes identified a total of 74 differently-expressed genes (DEGs), in which 52 up-regulated and 22 down-regulated under the blue light compared with that in white light. The DEGs were mainly involved in metabolic pathways, biosynthesis of secondary metabolites, plant hormone signal transduction, starch and sucrose metabolism and phenylpropanoid biosynthesis. While the  ethylene and gibberellins synthesis and signaling related genes were down-regulated, the abscisic acid and auxin signal transduction pathways were up-regulated by the blue light treatment. Furthermore, the blue light treatment up-regulated the transcription of genes relating to photosynthesis, starch and sucrose metabolism. Meanwhile, the blue light suppressed the GA₃ concentration but promoted the concentrations of auxin and photosynthetic pigments. Taken together, the results suggest that the blue light-induced female floral sex expression is closely associated with the blue light-induced changes in abscisic acid, auxin, gibberellins, photosynthesis, starch and sucrose metabolism pathways, which is potentially different from the traditional ethylene-dependent pathway.

The novel ethylene-responsive factor CsERF025 affects the development of fruit bending in cucumber

Overexpression of CsERF025 induces fruit bending by promoting the production of ethylene. Cucumber fruit bending critically affects cucumber quality, but the mechanism that causes fruit bending remains unclear. To better understand this mechanism, we performed transcriptome analyses on tissues from the convex (C1) and concave (C2) sides of bending and straight (S) fruit at 2 days post anthesis (DPA). We identified a total of 281 differentially expressed genes (DEGs) from both the convex and concave sides of bent fruit that showed significantly different expression profiles relative to straight fruits. Of these 281 DEGs, 196 were up-regulated (C1/S_C2/S) and 85 were down-regulated (C1/S_C2/S). Among the 196 up-regulated DEGs, the transcriptional levels of genes related to ethylene biosynthesis and signaling pathways were significantly higher in bending fruit compared with straight fruit. CsERF025 showed the largest difference in expression between bending and straight fruit. CsERF025 is an AP2/ERF gene encoding a protein that localizes to the nucleus. Overexpression of this gene increased the bending rate of cucumber fruits and increased the angle of bending. CsERF025 increased both the expression of ethylene biosynthesis-related genes and the production of ethylene. The application of exogenous 1-aminocyclopropane-l-carboxylic acid (ACC) to straight fruits from control plants promoted fruit bending. Thus, CsERF025 enhances the production of ethylene and thereby promotes fruit bending in cucumber.

Expression and functional analysis of the transcription factor-encoding Gene CsERF004 in cucumber during Pseudoperonospora cubensis and Corynespora cassiicola infection

BACKGROUND

Cucumber downy mildew, caused by P. cubensis, is an important leaf disease that can severely affect cucumber production. In recent years, cucumber target spot, caused by C. cassiicola, has been reported in both Asia and Europe and is now considered as a major disease disrupting cucumber production. Single-disease-resistant cucumber varieties have been unable to satisfy production needs. To explore the molecular mechanisms of cucumber resistance to these two diseases, cucumber cultivars D9320 (resistant to downy mildew and target spot) and D0401 (susceptible to downy mildew and target spot) were used as experimental materials in this study. We used transcriptome sequencing technology to identify genes related to disease resistance and verified using transgenic technology.

RESULTS

We screened out the cucumber resistance-related gene CsERF004 using transcriptome sequencing technology. Induction by pathogens, salicylic acid (SA), and ethylene (ET) resulted in the up-regulation of CsERF004. Three treatments, namely, inoculation with C. cassiicola alone, inoculation with P. cubensis alone, and simultaneous inoculation with both pathogens, all resulted in the significant and sustained up-regulation of CsERF004 in the resistant cultivar D9320, during the early stage of infection. In the susceptible cultivar D0401, CsERF004 expression was also significantly up-regulated at the later stage of infection but to a lesser extent and for a shorter duration than in the resistant cultivar D9320. The CsERF004 gene encodes a protein localizes to the nucleus. The over-expression of CsERF004 in the susceptible cultivar D0401 resulted in the significant up-regulation of the CsPR1 and CsPR4 genes and increased the levels of SA and ET, which enhanced the resistance of cucumber to downy mildew and target spot.

CONCLUSIONS

Analyses of the CsERF004 expression pattern in disease-resistant and susceptible cucumber cultivars and transgenic validation indicate that CsERF004 confers resistance to P. cubensis and C. cassiicola. The findings of this study can help to better understanding of mechanisms of response to pathogens and in establishment the genetic basis for the development of cucumber broad-spectrum resistant cultivars.

The association of changes in DNA methylation with temperature-dependent sex determination in cucumber

Cucumber (Cucumis sativus L.) is characterized by its diverse and flexible sexual types. Here, we evaluated the effect of low temperature (LT) exposure on cucumber femaleness under short-day conditions. Shoot apices were subjected to whole-genome bisulfate sequencing (WGBS), mRNA-seq, and sRNA-seq. The results showed that temperature had a substantial and global impact on transposable element (TE)-related small RNA-directed DNA methylation (RdDM) mechanisms, resulting in large amounts of CHH-type cytosine demethylation. In the cucumber genome, TEs are common in regions near genes that are also subject to DNA demethylation. TE-gene interactions showed very strong reactions to LT treatment, as nearly 80% of the differentially methylated regions (DMRs) were distributed in genic regions. Demethylation near genes led to the co-ordinated expression of genes and TEs. More importantly, genome-wide de novo methylation changes also resulted in small amounts of CG- and CHG-type DMRs. Methylation changes in CG-DMRs located <600 bp from the transcription start and end sites (TSSs/TESs) negatively correlated with transcription changes in differentially expressed genes (DEGs), probably indicating epiregulation. Ethylene is called the 'sex hormone' of cucumbers. We observed the up-regulation of ethylene biosynthesis-related CsACO3 and the down-regulation of an Arabidopsis RAP2.4-like ethylene-responsive (AP2/ERF) transcription factor, demonstrating the inferred epiregulation. Our study characterized the response of the apex methylome to LT and predicted the possible epiregulation of temperature-dependent sex determination (TSD) in cucumber.

Trehalose-6-phosphate and SNF1-related protein kinase 1 are involved in the first-fruit inhibition of cucumber

In cucumber (Cucumis sativus L.), the preexisting fruits inhibit the growth of subsequent fruits. To study the mechanism underlying this phenomenon, we examined the sink activity, the level of free sugars, and the activity of SNF1-related protein kinase 1 (SnRK1) in the peduncles of two types of fruits. In the two-fruit cucumber plants, the growth rate and sink activity [evaluated by alkaline alpha-galactosidase (CsAGA) activity in the peduncle] of the first fruit were greater than those of the second fruit. The (14)C-labeling experiment revealed that assimilates produced by the leaves closer to the second fruit tended to move to the first fruit. Sucrose and trehalose-6-phosphate (T6P) levels in the peduncle of the first fruit were higher than those in the peduncle of the second fruit. The SnRK1 activity was lower in the peduncle of the first fruit than in that of the second fruit at 0-8 days after anthesis. The growth rate and sink activity of the second fruit were enhanced after the removal of the first fruit or after treatment with 6-benzyl aminopurine, as determined by comparison with an increase in the sucrose and T6P levels and a decrease in the SnRK1 activity in its peduncle. The SnRK1 activity was inhibited by T6P in an in vitro kinase assay, and the mRNA level of CsAGA1 in cucumber calli was up-regulated by exogenous trehalose treatment, confirming that the SnRK1 activity and CsAGA1 expression can be regulated by T6P levels. Our results suggest that the T6P- and SnRK1-mediated signaling functions are involved in the regulation of first-fruit inhibition in cucumber plants.

Involvement of ethylene in sex expression and female flower development in watermelon (Citrullus lanatus)

Although it is known that ethylene has a masculinizing effect on watermelon, the specific role of this hormone in sex expression and flower development has not been analyzed in depth. By using different approaches the present work demonstrates that ethylene regulates differentially two sex-related developmental processes: sexual expression, i.e. the earliness and the number of female flowers per plant, and the development of individual floral buds. Ethylene production in the shoot apex as well as in male, female and bisexual flowers demonstrated that the female flower requires much more ethylene than the male one to develop, and that bisexual flowers result from a decrease in ethylene production in the female floral bud. The occurrence of bisexual flowers was found to be associated with elevated temperatures in the greenhouse, concomitantly with a reduction of ethylene production in the shoot apex. External treatments with ethephon and AVG, and the use of Cucurbita rootstocks with different ethylene production and sensitivity, confirmed that, as occurs in other cucurbit species, ethylene is required to arrest the development of stamens in the female flower. Nevertheless, in watermelon ethylene inhibits the transition from male to female flowering and reduces the number of pistillate flowers per plant, which runs contrary to findings in other cucurbit species. The use of Cucurbita rootstocks with elevated ethylene production delayed the production of female flowers but reduced the number of bisexual flowers, which is associated with a reduced fruit set and altered fruit shape.

Molecular and functional characterization of CpACS27A gene reveals its involvement in monoecy instability and other associated traits in squash (Cucurbita pepo L.)

A number of Cucurbita pepo genotypes showing instable monoecy or partial andromonoecy, i.e. an incomplete conversion of female into bisexual flowers, have been detected. Given that in melon and cucumber andromonoecy is the result of reduction of ethylene production in female floral buds, caused by mutations in the ethylene biosynthesis genes CmACS7 and CsACS2; we have cloned and characterized two related C. pepo genes, CpACS27A and CpACS27B. The molecular structure of CpACS27A and its specific expression in the carpels of female flowers during earlier stages of flower development suggests that this gene is the Cucurbita ortholog of CmACS7 and CsACS2. CpACS27B is likely to be a paralogous pseudogene since it has not been found to be expressed in any of the analyzed tissues. CpACS27A was sequenced in Bolognese (Bog) and Vegetable Spaghetti (Veg), two monoecious inbred lines whose F2 was segregating for partial andromonoecy. The Bog allele of CpACS27A carried a missense mutation that resulted in a substitution of the conserved serine residue in position 176 by an alanine. Segregation analysis indicated that this mutant variant is necessary but not sufficient to confer the andromonoecious phenotype in squash. In concordance with its involvement in stamen arrest, a reduction in CpACS27A expression has been found in bisexual flower buds at earlier stages of development. This reduction in CpACS27A expression was concomitant with a downregulation of other ethylene biosynthesis and signaling genes during earlier and later stages of ovary development. The role of CpACS27A is discussed regarding the regulation of ethylene biosynthesis and signaling genes in the control of andromonoecy-associated traits, such as the delayed maturation of corolla and stigma as well as the parthenocarpic development of the fruit.

Tapetum and middle layer control male fertility in Actinidia deliciosa

BACKGROUND AND AIMS

Dioecism characterizes many crop species of economic value, including kiwifruit (Actinidia deliciosa). Kiwifruit male sterility occurs at the microspore stage. The cell walls of the microspores and the pollen of the male-sterile and male-fertile flowers, respectively, differ in glucose and galactose levels. In numerous plants, pollen formation involves normal functioning and degeneration timing of the tapetum, with calcium and carbohydrates provided by the tapetum essential for male fertility. The aim of this study was to determine whether the anther wall controls male fertility in kiwifruit, providing calcium and carbohydrates to the microspores.

METHODS

The events occurring in the anther wall and microspores of male-fertile and male-sterile anthers were investigated by analyses of light microscopy, epifluorescence, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL assay) and transmission electron microscopy coupled with electron spectroscopy. The possibility that male sterility was related to anther tissue malfunctioning with regard to calcium/glucose/galactose provision to the microspores was also investigated by in vitro anther culture.

KEY RESULTS

Both tapetum and the middle layer showed secretory activity and both degenerated by programmed cell death (PCD), but PCD was later in male-sterile than in male-fertile anthers. Calcium accumulated in cell walls of the middle layer and tapetum and in the exine of microspores and pollen, reaching higher levels in anther wall tissues and dead microspores of male-sterile anthers. A specific supply of glucose and calcium induced normal pollen formation in in vitro-cultured anthers of the male-sterile genotype.

CONCLUSIONS

The results show that male sterility in kiwifruit is induced by anther wall tissues through prolonged secretory activity caused by a delay in PCD, in the middle layer in particular. In vitro culture results support the sporophytic control of male fertility in kiwifruit and open the way to applications to overcome dioecism and optimize kiwifruit production.

Insight into the role of sugars in bud burst under light in the rose

Bud burst is a decisive process in plant architecture that requires light in Rosa sp. This light effect was correlated with stimulation of sugar transport and metabolism in favor of bud outgrowth. We investigated whether sugars could act as signaling entities in the light-mediated regulation of vacuolar invertases and bud burst. Full-length cDNAs encoding two vacuolar invertases (RhVI1 and RhVI2) were isolated from buds. Unlike RhVI2, RhVI1 was preferentially expressed in bursting buds, and was up-regulated in buds of beheaded plants exposed to light. To assess the importance of sugars in this process, the expression of RhVI1 and RhVI2 and the total vacuolar invertase activity were further characterized in buds cultured in vitro on 100 mM sucrose or mannitol under light or in darkness for 48 h. Unlike mannitol, sucrose promoted the stimulatory effect of light on both RhVI1 expression and vacuolar invertase activity. This up-regulation of RhVI1 was rapid (after 6 h incubation) and was induced by as little as 10 mM sucrose or fructose. No effect of glucose was found. Interestingly, both 30 mM palatinose (a non-metabolizable sucrose analog) and 5 mM psicose (a non-metabolizable fructose analog) promoted the light-induced expression of RhVI1 and total vacuolar invertase activity. Sucrose, fructose, palatinose and psicose all promoted bursting of in vitro cultured buds under light. These findings indicate that soluble sugars contribute to the light effect on bud burst and vacuolar invertases, and can function as signaling entities.

Golgi-mediated synthesis and secretion of matrix polysaccharides of the primary cell wall of higher plants

The Golgi apparatus of eukaryotic cells is known for its central role in the processing, sorting, and transport of proteins to intra- and extra-cellular compartments. In plants, it has the additional task of assembling and exporting the non-cellulosic polysaccharides of the cell wall matrix including pectin and hemicelluloses, which are important for plant development and protection. In this review, we focus on the biosynthesis of complex polysaccharides of the primary cell wall of eudicotyledonous plants. We present and discuss the compartmental organization of the Golgi stacks with regards to complex polysaccharide assembly and secretion using immuno-electron microscopy and specific antibodies recognizing various sugar epitopes. We also discuss the significance of the recently identified Golgi-localized glycosyltransferases responsible for the biosynthesis of xyloglucan (XyG) and pectin.