Ethylene plays a dual role in sex determination and fruit shape in cucurbits

Genome-wide identification of the OVATE family proteins and functional analysis of BhiOFP1, BhiOFP5, and BhiOFP18 during fruit development in wax gourd (Benincasa hispida)

OVATE family proteins (OFPs) are transcriptional regulators in plants. They have a common domain called the OVATE domain and control the development of leaves, fruits, and flowers in plants. Although the OFP gene family has been widely explored in the plant kingdom, the identification and characterization of this family have not yet been performed in wax gourd. In this study, we conducted a genome-wide investigation of the OFP gene family and identified 18 OFP (BhiOFP) genes in wax gourd. Next, we discovered their evolutionary relationships, conserved motifs, gene structures, cis-acting elements, and expression patterns. The BhiOFP genes were irregularly distributed on nine chromosomes, with only two BhiOFP genes containing introns. The BhiOFP gene promoters contained cis-acting elements in response to phytohormones and environmental signals. The majority of BhiOFP genes were derived from whole-genome duplication events. Expression analysis demonstrated that the BhiOFP genes showed disparate modes of expression and some of them were highly expressed in fruits. Overexpression of BhiOFP1, BhiOFP5, and BhiOFP18 in Arabidopsis resulted in dwarf plants, small rosette leaves, and shortened siliques, while the BhiOFP1 overexpression plants displayed a more severe phenotype. In summary, our study systematically analyzed the wax gourd OFP gene family, facilitated the functional research of BhiOFP1, BhiOFP5, and BhiOFP18, and offered a theoretical foundation for the improvement of wax gourd varieties with appropriate fruit length.

Ethylene in fruits: beyond ripening control

Fruits are a rich source of nutrients, minerals, and dietary fibers for both humans and animals. While the gaseous phytohormone ethylene is well-known for its role in controlling fruit ripening, there is growing evidence that ethylene also plays crucial roles in regulating other developmental processes of fruits, such as sex determination, fruit set, and fruit growth. In this review, we aim to revisit these findings from various species like cucumber, melon, tomato, rice, maize, and more. These studies not only enhance our understanding of ethylene's function in fruits but also highlight the potential for manipulating ethylene to improve crops. Furthermore, we discuss recent studies that show the ethylene precursor ACC (1-AMINOCYCLOPROPANE-1-CARBOXYLATE), and the ethylene signaling components EIN2 (ETHYLENE INSENSITIVE2) and EIN3 (ETHYLENE INSENSITIVE3) have ethylene-independent function in specific conditions. This phenomenon, combined with findings of dosage-dependent ethylene functions in certain conditions, highlights the importance of analyzing mutants with completely blocked ethylene pathways in different species at specific developmental stages and tissue types. Overall, this review offers a timely and essential summary of ethylene's role in sex determination, fruit formation, and fruit growth, which could be beneficial for horticulture crop breeding.

Genome-wide identified VvOFP genes family and VvOFP4 functional characterization provide insight into fruit shape in grape

Ovate Family Proteins (OFPs) are emerging as novel transcriptional regulators of fruit shape. Despite their established role in various species, their involvement in regulating grape fruit shape remains understudied. This study encompassed a comprehensive evaluation of 16 grape OFP genes in total at the whole genome level. Phylogenetic and synteny analyses established a close relationship between grape VvOFP genes and their tomato counterparts. Expression profiling post-treatment with gibberellic acid (GA3) and thidiazuron (TDZ) revealed that certain OFP genes responded to these regulators, with VvOFP4 showing peak expression three days post-anthesis. Functional assays via overexpression of VvOFP4 in tobacco and tomato altered the morphology of both vegetative and reproductive organs, including leaves, stamens, and fruits/pods. Paraffin sections of transgenic tobacco stems and tomato fruits demonstrated that VvOFP4 overexpression modifies cell dimensions, leading to changes in organ morphology. Additionally, treatments with GA3 and TDZ similarly influenced the shape of grape pulp cells and thereby the overall fruit morphology. These findings suggest that the VvOFP4 gene plays a crucial role in fruit shape determination by modulating cell shape and presents a potential target for future grape breeding programs aimed at diversifying fruit shapes.

Harbinger transposon insertion in ethylene signaling gene leads to emergence of new sexual forms in cucurbits

In flowering plants, the predominant sexual morph is hermaphroditism, and the emergence of unisexuality is poorly understood. Using Cucumis melo (melon) as a model system, we explore the mechanisms driving sexual forms. We identify a spontaneous mutant exhibiting a transition from bisexual to unisexual male flower, and identify the causal mutation as a Harbinger transposon impairing the expression of Ethylene Insensitive 2 (CmEIN2) gene. Genetics and transcriptomic analysis reveal a dual role of CmEIN2 in both sex determination and fruit shape formation. Upon expression of CmACS11, EIN2 is recruited to repress the expression of the carpel inhibitor, CmWIP1. Subsequently, EIN2 is recruited to mediate stamina inhibition. Following the sex determination phase, EIN2 promotes fruit shape elongation. Genome-wide analysis reveals that Harbinger transposon mobilization is triggered by environmental cues, and integrates preferentially in active chromatin, particularly within promoter regions. Characterization of a large collection of melon germplasm points to active transpositions in the wild, compared to cultivated accessions. Our study underscores the association between chromatin dynamics and the temporal aspects of mobile genetic element insertions, providing valuable insights into plant adaptation and crop genome evolution.

A long noncoding RNA functions in pumpkin fruit development through S-adenosyl-L-methionine synthetase

Long noncoding RNAs (lncRNAs) play important roles in various biological processes. However, the regulatory roles of lncRNAs underlying fruit development have not been extensively studied. The pumpkin (Cucurbita spp.) is a preferred model for understanding the molecular mechanisms regulating fruit development because of its variable shape and size and large inferior ovary. Here, we performed strand-specific transcriptome sequencing on pumpkin (Cucurbita maxima "Rimu") fruits at 6 developmental stages and identified 5,425 reliably expressed lncRNAs. Among the 332 lncRNAs that were differentially expressed during fruit development, the lncRNA MSTRG.44863.1 was identified as a negative regulator of pumpkin fruit development. MSTRG.44863.1 showed a relatively high expression level and an obvious period-specific expression pattern. Transient overexpression and silencing of MSTRG.44863.1 significantly increased and decreased the content of 1-aminocyclopropane carboxylic acid (a precursor of ethylene) and ethylene production, respectively. RNA pull-down and microscale thermophoresis assays further revealed that MSTRG.44863.1 can interact with S-adenosyl-L-methionine synthetase (SAMS), an enzyme in the ethylene synthesis pathway. Considering that ethylene negatively regulates fruit development, these results indicate that MSTRG.44863.1 plays an important role in the regulation of pumpkin fruit development, possibly through interacting with SAMS and affecting ethylene synthesis. Overall, our findings provide a rich resource for further study of fruit-related lncRNAs while offering insights into the regulation of fruit development in plants.

Transcriptomics reveal useful resources for examining fruit development and variation in fruit size in Coccinia grandis

Introduction

The Cucurbitaceae family comprises many agronomically important members, that bear nutritious fruits and vegetables of great economic importance. Coccinia grandis, commonly known as Ivy gourd, belongs to this family and is widely consumed as a vegetable. Members of this family are known to display an impressive range of variation in fruit morphology. Although there have been studies on flower development in Ivy gourd, fruit development remains unexplored in this crop.

Methods

In this study, comparative transcriptomics of two Ivy gourd cultivars namely "Arka Neelachal Kunkhi" (larger fruit size) and "Arka Neelachal Sabuja" (smaller fruit size) differing in their average fruit size was performed. A de novo transcriptome assembly for Ivy gourd was developed by collecting fruits at different stages of development (5, 10, 15, and 20 days after anthesis i.e. DAA) from these two varieties. The transcriptome was analyzed to identify differentially expressed genes, transcription factors, and molecular markers.

Results

The transcriptome of Ivy gourd consisted of 155205 unigenes having an average contig size of 1472bp. Unigenes were annotated on publicly available databases to categorize them into different biological functions. Out of these, 7635 unigenes were classified into 38 transcription factor (TF) families, of which Trihelix TFs were most abundant. A total of 11,165 unigenes were found to be differentially expressed in both the varieties and the in silico expression results were validated through real-time PCR. Also, 98768 simple sequence repeats (SSRs) were identified in the transcriptome of Ivy gourd.

Discussion

This study has identified a number of genes, including transcription factors, that could play a crucial role in the determination of fruit shape and size in Ivy gourd. The presence of polymorphic SSRs indicated a possibility for marker-assisted selection for crop breeding in Ivy gourd. The information obtained can help select candidate genes that may be implicated in regulating fruit development and size in other fruit crops.

Striking a growth-defense balance: Stress regulators that function in maize development

Maize (Zea mays) cultivation is strongly affected by both abiotic and biotic stress, leading to reduced growth and productivity. It has recently become clear that regulators of plant stress responses, including the phytohormones abscisic acid (ABA), ethylene (ET), and jasmonic acid (JA), together with reactive oxygen species (ROS), shape plant growth and development. Beyond their well established functions in stress responses, these molecules play crucial roles in balancing growth and defense, which must be finely tuned to achieve high yields in crops while maintaining some level of defense. In this review, we provide an in-depth analysis of recent research on the developmental functions of stress regulators, focusing specifically on maize. By unraveling the contributions of these regulators to maize development, we present new avenues for enhancing maize cultivation and growth while highlighting the potential risks associated with manipulating stress regulators to enhance grain yields in the face of environmental challenges.

Ethylene produced in carpel primordia controls CmHB40 expression to inhibit stamen development

Sex determination evolved to control the development of unisexual flowers. In agriculture, it conditions how plants are cultivated and bred. We investigated how female flowers develop in monoecious cucurbits. We discovered in melon, Cucumis melo, a mechanism in which ethylene produced in the carpel is perceived in the stamen primordia through spatially differentially expressed ethylene receptors. Subsequently, the CmEIN3/CmEIL1 ethylene signalling module, in stamen primordia, activates the expression of CmHB40, a transcription factor that downregulates genes required for stamen development and upregulates genes associated with organ senescence. Investigation of melon genetic biodiversity revealed a haplotype, originating in Africa, altered in EIN3/EIL1 binding to CmHB40 promoter and associated with bisexual flower development. In contrast to other bisexual mutants in cucurbits, CmHB40 mutations do not alter fruit shape. By disentangling fruit shape and sex-determination pathways, our work opens up new avenues in plant breeding.

Form and contour: breeding and genetics of organ shape from wild relatives to modern vegetable crops

Shape is a primary determinant of consumer preference for many horticultural crops and it is also associated with many aspects of marketing, harvest mechanics, and postharvest handling. Perceptions of quality and preference often map to specific shapes of fruits, tubers, leaves, flowers, roots, and other plant organs. As a result, humans have greatly expanded the palette of shapes available for horticultural crops, in many cases creating a series of market classes where particular shapes predominate. Crop wild relatives possess organs shaped by natural selection, while domesticated species possess organs shaped by human desires. Selection for visually-pleasing shapes in vegetable crops resulted from a number of opportunistic factors, including modification of supernumerary cambia, allelic variation at loci that control fundamental processes such as cell division, cell elongation, transposon-mediated variation, and partitioning of photosynthate. Genes that control cell division patterning may be universal shape regulators in horticultural crops, influencing the form of fruits, tubers, and grains in disparate species. Crop wild relatives are often considered less relevant for modern breeding efforts when it comes to characteristics such as shape, however this view may be unnecessarily limiting. Useful allelic variation in wild species may not have been examined or exploited with respect to shape modifications, and newly emergent information on key genes and proteins may provide additional opportunities to regulate the form and contour of vegetable crops.

Molecular and genetic regulations of fleshy fruit shape and lessons from Arabidopsis and rice

Fleshy fruit shape is an important external quality trait influencing the usage of fruits and consumer preference. Thus, modification of fruit shape has become one of the major objectives for crop improvement. However, the underlying mechanisms of fruit shape regulation are poorly understood. In this review we summarize recent progress in the genetic basis of fleshy fruit shape regulation using tomato, cucumber, and peach as examples. Comparative analyses suggest that the OFP-TRM (OVATE Family Protein - TONNEAU1 Recruiting Motif) and IQD (IQ67 domain) pathways are probably conserved in regulating fruit shape by primarily modulating cell division patterns across fleshy fruit species. Interestingly, cucumber homologs of FRUITFULL (FUL1), CRABS CLAW (CRC) and 1-aminocyclopropane-1-carboxylate synthase 2 (ACS2) were found to regulate fruit elongation. We also outline the recent progress in fruit shape regulation mediated by OFP-TRM and IQD pathways in Arabidopsis and rice, and propose that the OFP-TRM pathway and IQD pathway coordinate regulate fruit shape through integration of phytohormones, including brassinosteroids, gibberellic acids, and auxin, and microtubule organization. In addition, functional redundancy and divergence of the members of each of the OFP, TRM, and IQD families are also shown. This review provides a general overview of current knowledge in fruit shape regulation and discusses the possible mechanisms that need to be addressed in future studies.

Construction of a density mutant collection in bitter gourd via new germplasms innovation and gene functional study

Although a few studies have elucidated the creation of bitter gourd mutants, the suitable concentration and duration of ethyl methanesulfonate (EMS) mutagenesis have not been determined. In this study, mutant collection was conducted to create new germplasms and widen genetic diversity. By employing the seeds of the inbred line Y52 as the mutagenic material, EMS as the mutagen, and the suitable mutagenic conditions for bitter gourd seeds (EMS concentration 0.2%, mutagenic time 10 h), we mutated 10,000 seeds and acquired 3223 independent M₁ lines. For the randomly selected 1000 M₂ lines, 199 M₂ lines with visible phenotypes were found, and 167 M₂ lines were mutants of fruit shape, size, and tubercles. Furthermore, fourteen dwarf, eleven leaf color, five leaf shape, and eight meristem defect mutants were discovered in this mutant collection. In addition, three lines of 1253, 2284, and 3269 represented recessive mutants crossed with Y52. Furthermore, the yellow leaf lines of 2284 and 3269 were not mutated at the same gene locus. This study constructed a mutant collection through innovative new germplasms and provided valuable resources for bitter gourd breeding and functional gene research.