First RNA-seq approach to study fruit set and parthenocarpy in zucchini (Cucurbita pepo L.)

Parthenocarpy in Cucurbitaceae: Advances for Economic and Environmental Sustainability

Parthenocarpy is an important agricultural trait that not only produces seedless fruits, but also increases the rate of the fruit set under adverse environmental conditions. The study of parthenocarpy in Cucurbitaceae crops has considerable implications for cultivar improvement. This article provides a comprehensive review of relevant studies on the parthenocarpic traits of several major Cucurbitaceae crops and offers a perspective on future developments and research directions.

Identification of Candidate Genes Involved in the Determinism of Pollen Grain Aperture Morphology by Comparative Transcriptome Analysis in Papaveraceae

In the last decade, certain genes involved in pollen aperture formation have been discovered. However, those involved in pollen aperture shape remain largely unknown. In Arabidopsis, the interaction during the tetrad development stage of one member of the ELMOD protein family, ELMOD_E, with two others, MCR/ELMOD_B and ELMOD_A, can change the morphology of apertures from colpus (elongated) to pore (round). Here, comparative transcriptome analysis is used to identify candidate genes involved in the determination of pollen aperture morphology in Papaveraceae (order Ranunculales). Furthermore, the role of ELMOD genes in the genetic determinism of aperture shape was tested by comparative analysis of their expression levels using RNA-seq data and RT-qPCR. Two pairs of species belonging to two different subfamilies were used. Within each pair, one species has colpate pollen and the other porate (Fumarioideae-Dactylicapnos torulosa, 6-colpate, and Fumaria bracteosa, pantoporate; Papaveroideae-Eschsholzia californica, 5-7 colpate, and Roemeria refracta, 6-porate). The transcriptomes were obtained at the tetrad stage of pollen development. A total of 531 DEGs were found between the colpate and porate pollen species groups. The results from RNA-seq and RT-qPCR indicate that pollen aperture shape is not determined by the relative expression levels of ELMOD family genes in Papaveraceae. However, genes related to callose wall formation or cytoskeleton organisation were found, these processes being involved in pollen aperture formation. In addition, transcriptomes from anthers with pollen during the tetrad stage of three species (D. torulosa, R. refracta, and F. bracteosa) were obtained for the first time. These data will be available for further studies in the field of floral evolution and development.

Genome-wide identification and characterization of parthenocarpic fruit set-related gene homologs in cucumber (Cucumis sativus L.)

Cucumber (Cucumis sativus L.), a major horticultural crop, in the family Cucurbitaceae is grown and consumed globally. Parthenocarpy is an ideal trait for many fruit and vegetables which produces seedless fruit desired by consumers. The seedlessness occurs when fruit develops without fertilization which can be either natural or induced. So far, a limited number of genes regulating parthenocarpic fruit set have been reported in several fruit or vegetable crops, most of which are involved in hormone biosynthesis or signalling. Although parthenocarpic cucumber has been widely used in commercial production for a long time; its genetic basis is not well understood. In this study, we retrieved thirty five parthenocarpy fruit-set related genes (PRGs) from bibliomic data in various plants. Thirty-five PRG homologs were identified in the cucumber genome via homology-based search. An in silico analysis was performed on phylogenetic tree, exon-intron structure, cis-regulatory elements in the promoter region, and conserved domains of their deduced proteins, which provided insights into the genetic make-up of parthenocarpy-related genes in cucumber. Simple sequence repeat (SSR) sequences were mined in these PRGs, and 31 SSR markers were designed. SSR genotyping identified three SSRs in two polymorphic genes. Quantitative real-time PCR of selected genes was conducted in five cucumber lines with varying degrees of parthenocarpic fruit set capacities, which revealed possible association of their expression with parthenocarpy. The results revealed that homologs CsWD40 and CsPIN-4 could be considered potential genes for determination of parthenocarpy as these genes showed parental polymorphism and differential gene expression in case of parthenocarpic and non-parthenocarpic parents.

Parthenocarpy-related genes induced by naphthalene acetic acid in oil palm interspecific O × G [Elaeis oleifera (Kunth) Cortés × Elaeis guineensis Jacq.] hybrids

Parthenocarpy is the development without fertilization of seedless fruits. In the oil palm industry, the development of parthenocarpic fruits is considered an attractive option to increase palm oil production. Previous studies have shown the application of synthetic auxins in Elaeis guineensis, and interspecific O×G hybrids (Elaeis oleifera (Kunth) Cortés × E. guineensis Jacq.) induces parthenocarpy. The aim of this study was to identify the molecular mechanism through transcriptomics and biology system approach to responding to how the application of NAA induces parthenocarpic fruits in oil palm O×G hybrids. The transcriptome changes were studied in three phenological stages (PS) of the inflorescences: i) PS 603, pre-anthesis III, ii) PS 607, anthesis, and iii) PS 700, fertilized female flower. Each PS was treated with NAA, Pollen, and control (any application). The expression profile was studied at three separate times: five minutes (T0), 24 hours (T1), and 48 h post-treatment (T2). The RNA sequencing (RNA seq) approach was used with 27 oil palm O×G hybrids for a total of 81 raw samples. RNA-Seq showed around 445,920 genes. Numerous differentially expressed genes (DEGs) were involved in pollination, flowering, seed development, hormone biosynthesis, and signal transduction. The expression of the most relevant transcription factors (TF) families was variable and dependent on the stage and time post-treatment. In general, NAA treatment expressed differentially more genes than Pollen. Indeed, the gene co-expression network of Pollen was built with fewer nodes than the NAA treatment. The transcriptional profiles of Auxin-responsive protein and Gibberellin-regulated genes involved in parthenocarpy phenomena agreed with those previously reported in other species. The expression of 13 DEGs was validated by RT-qPCR analysis. This detailed knowledge about the molecular mechanisms involved in parthenocarpy could be used to facilitate the future development of genome editing techniques that enable the production of parthenocarpic O×G hybrid cultivars without growth regulator application.

Chromosomal fragment deletion in APRR2-repeated locus modulates the dark stem color in Cucurbita pepo

Cp4.1LG15g03420 (CpDsc-1), which encodes a two-component response regulator-like protein (APRR2) in the nucleus, influences dark green stem formation in Cucurbita pepo by regulating the chlorophyll content. Stem color is an important agronomic trait in zucchini (Cucurbita pepo) for robust seeding and high yield. However, the gene controlling the stem color has not been characterized. In this study, we identified a single locus accounting for the dark green stem color of C. pepo (CpDsc-1). Genetic analysis of this trait in segregated populations derived from two parental lines (line 296 with dark green stems and line 274 with light green stems) revealed that stem color was controlled by a single dominant gene (dark green vs. light green). In bulked segregant analysis, CpDsc-1 was mapped to a 2.09-Mb interval on chromosome 15. This region was further narrowed to 65.2 kb using linkage analysis of the F₂ population. Sequencing analysis revealed a 14 kb deletion between Cp4.1LG15g03420 and Cp4.1LG15g03360; these two genes both encoded a two-component response regulator-like protein (APRR2). The incomplete structures of the two APRR2 genes and abnormal chloroplasts in line 274 might be the main cause of the light green phenotype. Gene expression pattern analysis showed that only Cp4.1LG15g03420 was upregulated in line 296. Subcellular localization analysis indicated that Cp4.1LG15g03420 was a nuclear gene. Furthermore, a co-dominant marker, G4563 (93% accuracy rate), and a co-segregation marker, Fra3, were established in 111 diverse germplasms; both of these markers were tightly linked with the color trait. This study provided insights into chlorophyll regulation mechanisms and revealed the markers valuable for marker-assisted selection in future zucchini breeding.

Gene Expression in Zucchini Fruit Development

The study of fruit development in zucchini via gene expression has proven to be applicable in breeding programs. Phenotypic and transcriptomic studies of fruit set and parthenocarpy have been previously developed and some relevant genes have been reported. From these studies, three genotypes (MUCU-16, Whitaker, and Cavili) and six genes (CpAUX22, CpIAA4, CpIAMT-1, CpPIN5, CpCYCD6-1, and CpEXPLB1) were selected. The expression of these genes was analyzed in each genotype under three different treatments (pollination, auxin-treatment and non-treatment) during one week post anthesis. Also, a phenotyping analysis was conducted. The different nature of the samples and the genes selected allowed associations between different fruit traits and fruit development stages. There was a rapid response of CpAUX22 and CpIAA4 to the auxin treatment. Also, these genes and the CpIAMT-1 became more overexpressed in pollinated samples over time. The CpPIN5 gene increased its expression over time in all genotypes while CpCYCD6-1 was overexpressed in the early stages of fruit development in all samples. The CpEXPLB1 was highly up-regulated in non-treated samples, suggesting a relationship with fruit abortion. The overexpression of CpAUX22 and the non-overexpression of CpEXPLB1 in early stages may be associated with fruit growth in zucchini.

Hormonal interactions underlying parthenocarpic fruit formation in horticultural crops

In some horticultural crops, such as Cucurbitaceae, Solanaceae, and Rosaceae species, fruit set and development can occur without the fertilization of ovules, a process known as parthenocarpy. Parthenocarpy is an important agricultural trait that can not only mitigate fruit yield losses caused by environmental stresses but can also induce the development of seedless fruit, which is a desirable trait for consumers. In the present review, the induction of parthenocarpic fruit by the application of hormones such as auxins (2,4 dichlorophenoxyacetic acid; naphthaleneacetic acid), cytokinins (forchlorfenuron; 6-benzylaminopurine), gibberellic acids, and brassinosteroids is first presented. Then, the molecular mechanisms of parthenocarpic fruit formation, mainly related to plant hormones, are presented. Auxins, gibberellic acids, and cytokinins are categorized as primary players in initiating fruit set. Other hormones, such as ethylene, brassinosteroids, and melatonin, also participate in parthenocarpic fruit formation. Additionally, synergistic and antagonistic crosstalk between these hormones is crucial for deciding the fate of fruit set. Finally, we highlight knowledge gaps and suggest future directions of research on parthenocarpic fruit formation in horticultural crops.

Genome-Wide Identification of the ARF Gene Family and ARF3 Target Genes Regulating Ovary Initiation in Hazel via ChIP Sequencing

Hazel (Corylus spp.) is an economically important nut species with a unique biological characteristic of ovary differentiation and development initiating from the ovary primordium after pollination. Auxin participates in ovary initiation and has an essential impact on hazel fruit yield and quality. The regulation of auxin in ovary development is thought to be related to auxin response factors (ARFs); however, its detailed regulatory mechanism remains unclear. The spatiotemporal expression pattern of C. heterophylla ARF3 (ChARF3) was accessed via ARF gene family member identification and expression abundance analysis as well as immunohistochemistry. ChARF3 target genes were identified via chromatin immunoprecipitation followed by next-generation sequencing (ChIP-Seq). In total, 14 ChARF members containing at least B3 and Auxin_resp domains were found to be distributed on 9 of 11 chromosomes, and the protein molecular weights were predicted to range from 70.93-139.22 kD. Among eight differentially expressed ChARFs, ChARF3 showed the most significant differences over four ovary developmental stages. Immunohistochemical analysis revealed that ChARF3 was expressed in the ovary primordium and funiculus, integument, endosperm, radicle, and cotyledon indicating its potential regulatory roles in ovary differentiation and development. In total, 3,167 ChARF3 target genes were identified through ChIP-Seq in four ovary developmental stages and were significantly enriched in the biosynthesis of secondary metabolites (ko01110), phenylpropanoid biosynthesis (ko00940), and phytohormone signal transduction (ko04075). ChARF3 was hypothesized to be involved in the regulation of auxin-induced genes and the transcription factors MADS, AP2/ERF, TCP, FT, and LFY. These results suggest that ChARF3 may regulate ovary initiation and ovule development by mediating genes related to auxin biosynthesis and transport, cell division and proliferation, and flower and fruit development. This study provides new insights into the molecular mechanism of hazel yield formation.

Decoding the molecular mechanism of parthenocarpy in Musa spp. through protein-protein interaction network

Banana, one of the most important staple fruit among global consumers is highly sterile owing to natural parthenocarpy. Identification of genetic factors responsible for parthenocarpy would facilitate the conventional breeders to improve the seeded accessions. We have constructed Protein-protein interaction (PPI) network through mining differentially expressed genes and the genes used for transgenic studies with respect to parthenocarpy. Based on the topological and pathway enrichment analysis of proteins in PPI network, 12 candidate genes were shortlisted. By further validating these candidate genes in seeded and seedless accession of Musa spp. we put forward MaAGL8, MaMADS16, MaGH3.8, MaMADS29, MaRGA1, MaEXPA1, MaGID1C, MaHK2 and MaBAM1 as possible target genes in the study of natural parthenocarpy. In contrary, expression profile of MaACLB-2 and MaZEP is anticipated to highlight the difference in artificially induced and natural parthenocarpy. By exploring the PPI of validated genes from the network, we postulated a putative pathway that bring insights into the significance of cytokinin mediated CLAVATA(CLV)-WUSHEL(WUS) signaling pathway in addition to gibberellin mediated auxin signaling in parthenocarpy. Our analysis is the first attempt to identify candidate genes and to hypothesize a putative mechanism that bridges the gaps in understanding natural parthenocarpy through PPI network.

A comprehensive RNA-Seq-based gene expression atlas of the summer squash (Cucurbita pepo) provides insights into fruit morphology and ripening mechanisms

BACKGROUND

Summer squash (Cucurbita pepo: Cucurbitaceae) are a popular horticultural crop for which there is insufficient genomic and transcriptomic information. Gene expression atlases are crucial for the identification of genes expressed in different tissues at various plant developmental stages. Here, we present the first comprehensive gene expression atlas for a summer squash cultivar, including transcripts obtained from seeds, shoots, leaf stem, young and developed leaves, male and female flowers, fruits of seven developmental stages, as well as primary and lateral roots.

RESULTS

In total, 27,868 genes and 2352 novel transcripts were annotated from these 16 tissues, with over 18,000 genes common to all tissue groups. Of these, 3812 were identified as housekeeping genes, half of which assigned to known gene ontologies. Flowers, seeds, and young fruits had the largest number of specific genes, whilst intermediate-age fruits the fewest. There also were genes that were differentially expressed in the various tissues, the male flower being the tissue with the most differentially expressed genes in pair-wise comparisons with the remaining tissues, and the leaf stem the least. The largest expression change during fruit development was early on, from female flower to fruit two days after pollination. A weighted correlation network analysis performed on the global gene expression dataset assigned 25,413 genes to 24 coexpression groups, and some of these groups exhibited strong tissue specificity.

CONCLUSIONS

These findings enrich our understanding about the transcriptomic events associated with summer squash development and ripening. This comprehensive gene expression atlas is expected not only to provide a global view of gene expression patterns in all major tissues in C. pepo but to also serve as a valuable resource for functional genomics and gene discovery in Cucurbitaceae.

Fruit ripening in Lycium barbarum and Lycium ruthenicum is associated with distinct gene expression patterns

Goji berries have been used as food and medicine for millennia. Due to their high morphological similarity, fruits of two distinct species belonging to the family Solanaceae, Lycium barbarum (LB) and Lycium chinense (Chinese boxthorn), are usually marketed together as goji berries, but nearly 90% of all commercially available goji berries belong to the former species. A third closely related species, a wild perennial thorny shrub native to north‐western China, Lycium ruthenicum (LR; known as Russian box thorn, and its fruit as black wolfberry), has become a popular choice for combating soil desertification and for alleviating soil salinity/alkalinity due to its high resistance to the harsh environment of saline deserts. Despite the phylogenetic closeness of LB and LR, their fruits are very different. To identify the genes involved in these distinct phenotypes, here we studied expression patterns of 22 transcriptional regulators that may be crucial drivers of these differences during five developmental stages. BAM1 may contribute to higher sugar content in LB. High expression of BFRUCT in ripe LR is likely to be an evolutionary adaptation to fruit ripening in an arid environment. Two arogenate dehydratase paralogues, CHS and LDOX, are probably crucial elements of the mechanism by which LR accumulates much higher levels of anthocyanin. DXS2 (carotenoid accumulation in LB) and CCD4 (carotenoid degradation in ripe LR fruit) may be crucial drivers behind the much higher content of carotenoids in LB. EIL3 and ERF5 are two transcription factors that may contribute to the higher abiotic stress resilience of LR. GATA22‐like appears to have more important roles in growth than ripening in LB fruit and vice versa in LR. HAT5‐like exhibited opposite temporal patterns in two fruits: high in the 1st stage in LB and high in the 5th stage in LR. PED1 was expressed at a much lower level in LR. Finally, we hypothesise that the poorly functionally characterised SCL32 gene may play a part in the increased resistance to environmental stress of LR. We suggest that BAM1, BFRUCT, EIL3, ERF5, ADT paralogues (for functional redundancy), PED1, GATA22‐like, HAT5‐like and SCL32 warrant further functional studies.

Transcriptomic changes associated with husk scald incidence on pomegranate fruit peel during cold storage

Pomegranate fruit is valued for its social, economic, aesthetic and health benefits. The fruit rapidly loses quality after harvest due to continued metabolic responses and physiological disorders under sub-optimal conditions. The incidence of physiological disorder such as husk scald manifests during storage and commercial shipping, which affects the appearance and limits marketability. Despite the importance of pomegranate husk scald, little information is available about the origin and molecular mechanisms. Therefore, the aim of this study was to investigate the scald incidence of pomegranate fruit at molecular level using RNA-Seq (Ion Proton™ Next Generation Sequencing) by analyzing peel transcriptomic changes. The RNA-seq analysis generated 98,441,278 raw reads. 652 Differentially Expressed Genes (DEGs) with a fold change of > |2|, a p value ≤ 0.05 and a false discovery rate (FDR) of <0.05 were identified between healthy and scald fruit peels. An analysis of the gene ontologies of these DEGs revealed the 432 genes were assigned with molecular functions, 272 as cellular components and 205 as part of biological processes. In this analysis, genes (Pgr023188 and Pgr025081) that encode uncharacterized protein and gene (Pgr007593) that encodes glycosyltransferase showed significantly highest fold changes. Genes (Pgr003448, Pgr006024 and Pgr023696) involved in various iron binding and oxidoreductase activities were significantly suppressed. This is the first transcriptome analysis of pomegranate fruit peel related to husk scald development. Results obtained from this study will add valuable information on husk scald related changes on pomegranate fruit at genomic level and provide insight on other related physiological disorders.

Whole-genome resequencing of Cucurbita pepo morphotypes to discover genomic variants associated with morphology and horticulturally valuable traits

Cucurbita pepo contains two cultivated subspecies, each of which encompasses four fruit-shape morphotypes (cultivar groups). The Pumpkin, Vegetable Marrow, Cocozelle, and Zucchini Groups are of subsp. pepo and the Acorn, Crookneck, Scallop, and Straightneck Groups are of subsp. ovifera. Recently, a de novo assembly of the C. pepo subsp. pepo Zucchini genome was published, providing insights into its evolution. To expand our knowledge of evolutionary processes within C. pepo and to identify variants associated with particular morphotypes, we performed whole-genome resequencing of seven of these eight C. pepo morphotypes. We report for the first time whole-genome resequencing of the four subsp. pepo (Pumpkin, Vegetable Marrow, Cocozelle, green Zucchini, and yellow Zucchini) morphotypes and three of the subsp. ovifera (Acorn, Crookneck, and Scallop) morphotypes. A high-depth resequencing approach was followed, using the BGISEQ-500 platform that enables the identification of rare variants, with an average of 33.5X. Approximately 94.5% of the clean reads were mapped against the reference Zucchini genome. In total, 3,823,977 high confidence single-nucleotide polymorphisms (SNPs) were identified. Within each accession, SNPs varied from 636,918 in green Zucchini to 2,656,513 in Crookneck, and were distributed homogeneously along the chromosomes. Clear differences between subspecies pepo and ovifera in genetic variation and linkage disequilibrium are highlighted. In fact, comparison between subspecies pepo and ovifera indicated 5710 genes (22.5%) with Fst > 0.80 and 1059 genes (4.1%) with Fst = 1.00 as potential candidate genes that were fixed during the independent evolution and domestication of the two subspecies. Linkage disequilibrium was greater in subsp. ovifera than in subsp. pepo, perhaps reflective of the earlier differentiation of morphotypes within subsp. ovifera. Some morphotype-specific genes have been localized. Our results offer new clues that may provide an improved understanding of the underlying genomic regions involved in the independent evolution and domestication of the two subspecies. Comparisons among SNPs unique to particular subspecies or morphotypes may provide candidate genes responsible for traits of high economic importance.