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.

The MADS-box gene FveSEP3 plays essential roles in flower organogenesis and fruit development in woodland strawberry

Flower and fruit development are two key steps for plant reproduction. The ABCE model for flower development has been well established in model plant species; however, the functions of ABCE genes in fruit crops are less understood. In this work, we identified an EMS mutant named R27 in woodland strawberry (Fragaria vesca), showing the conversion of petals, stamens, and carpels to sepaloid organs in a semidominant inheritance fashion. Mapping by sequencing revealed that the class E gene homolog FveSEP3 (FvH4_4g23530) possessed the causative mutation in R27 due to a G to E amino acid change in the conserved MADS domain. Additional fvesep3CR mutants generated by CRISPR/Cas9 displayed similar phenotypes to fvesep3-R27. Overexpressing wild-type or mutated FveSEP3 in Arabidopsis suggested that the mutation in R27 might cause a dominant-negative effect. Further analyses indicated that FveSEP3 physically interacted with each of the ABCE proteins in strawberry. Moreover, both R27 and fvesep3CR mutants exhibited parthenocarpic fruit growth and delayed fruit ripening. Transcriptome analysis revealed that both common and specific differentially expressed genes were identified in young fruit at 6–7 days post anthesis (DPA) of fvesep3 and pollinated wild type when compared to unpollinated wild type, especially those in the auxin pathway, a key hormone regulating fruit set in strawberry. Together, we provided compelling evidence that FveSEP3 plays predominant E functions compared to other E gene homologs in flower development and that FveSEP3 represses fruit growth in the absence of pollination and promotes fruit ripening in strawberry.

Physicochemical characterization of bunches and oil composition in a backcross progeny (caiaué x oil palm) x oil palm

Hybrids between caiaué and oil palm (IEH OxG) are explored in genetic improvement with the main objective of associating the high palm oil productivity of oil palm with resistance of caiaué to Bud Rot disease. In addition, caiaué has better oil quality and greater resistance to some pests and diseases than oil palm and these characteristics are transmitted to the hybrid between species. Due to fertility problems, IEH OxG plantations require assisted pollination, which raises the production costs. Backcrossings using oil palm as recurrent parent (BC OGxG) are being evaluated as a solution to restore the fertility of IEH OxG, however, the species contrast to physicochemical characteristics of the bunch and to oil composition, therefore, there is a need to observe how these characteristics vary in BC OGxG. Physicochemical characteristics of the bunch and fatty acid composition of the oil produced by 90 plants from an BC OGxG progeny were analysed. A large variation was observed in the progeny, with mean values of 9.0 kg for bunch weight, 46.0% of parthenocarpic fruits in the bunch weight, 72.2% of mesocarp in normal fruit and 86.5% in parthenocarpic fruit, 37.6% of oil in the humid mesocarp of normal fruits and 35.9% in parthenocarpic fruits and 16.4% of oil content in the freshfruits bunch (FFB). The unsaturated fatty acids oleic (52.5%) and linoleic (10.8%) and the saturated fatty acids palmitic (30.7%) and stearic (4.4 %) stood out in the oil composition. In the progeny BC OGxG is possible to select individuals for high OCB and for a greater or lesser percentage of saturated or unsaturated fatty acids in the oil composition.

Induced Genetic Variations in Fruit Trees Using New Breeding Tools: Food Security and Climate Resilience

Fruit trees provide essential nutrients to humans by contributing to major agricultural outputs and economic growth globally. However, major constraints to sustainable agricultural productivity are the uncontrolled proliferation of the population, and biotic and abiotic stresses. Tree mutation breeding has been substantially improved using different physical and chemical mutagens. Nonetheless, tree plant breeding has certain crucial bottlenecks including a long life cycle, ploidy level, occurrence of sequence polymorphisms, nature of parthenocarpic fruit development and linkage. Genetic engineering of trees has focused on boosting quality traits such as productivity, wood quality, and resistance to biotic and abiotic stresses. Recent technological advances in genome editing provide a unique opportunity for the genetic improvement of woody plants. This review examines application of the CRISPR-Cas system to reduce disease susceptibility, alter plant architecture, enhance fruit quality, and improve yields. Examples are discussed of the contemporary CRISPR-Cas system to engineer easily scorable PDS genes, modify lignin, and to alter the flowering onset, fertility, tree architecture and certain biotic stresses.

Downstream of GA4, PbCYP78A6 participates in regulating cell cycle-related genes and parthenogenesis in pear (Pyrus bretshneideri Rehd.)

Background Parthenocarpy results in traits attractive to both consumers and breeders, and it overcomes the obstacle of self-incompatibility in the fruit set of horticultural crops, including pear (Pyrus bretshneider). However, there is limited knowledge regarding the genetic and molecular mechanisms that regulate parthenogenesis. Results Here, in a transcriptional comparison between pollination-dependent fruit and GA4-induced parthenocarpy, PbCYP78A6 was identified and proposed as a candidate gene involved in parthenocarpy. PbCYP78A6 is similar to Arabidopsis thaliana CYP78A6 and highly expressed in pear hypanthia. The increased PbCYP78A6 expression, as assessed by RT-qPCR, was induced by pollination and GA4 exposure. The ectopic overexpression of PbCYP78A6 contributed to parthenocarpic fruit production in tomato. The PbCYP78A6 expression coincided with fertilized and parthenocarpic fruitlets development and the expression of fruit development-related genes as assessed by cytological observations and RT-qPCR, respectively. PbCYP78A6 RNA interference and overexpression in pear calli revealed that the gene is an upstream regulator of specific fruit development-related genes in pear. Conclusions Our findings indicate that PbCYP78A6 plays a critical role in fruit formation and provide insights into controlling parthenocarpy.

Forchlorfenuron Application Induced Parthenocarpic Fruit Formation without Affecting Fruit Quality of Cucumber

The plant growth regulator forchlorfenuron is often applied to promote fruit setting and development in cucumber production. However, the effect of forchlorfenuron on the appearance and nutritional quality of cucumber is unknown. In the present study, forchlorfenuron was applied to female flowers 1 day before anthesis and at the day of anthesis. The application of forchlorfenuron successfully induced parthenocarpic fruit formation in cucumber. In addition, cytokinin-responsive genes were upregulated under forchlorfenuron treatment. Fruit treated with forchlorfenuron did not differ from pollinated fruit in shape, texture and major nutrients, such as protein, total flavonoids and vitamin C, with the exception of the lower phenolic acid content. Overall, our finding suggests that forchlorfenuron could be vital in promoting the parthenocarpic fruit set without altering the quality of cucumber.

Transcriptomic and Metabolomic Studies Reveal Mechanisms of Effects of CPPU-Mediated Fruit-Setting on Attenuating Volatile Attributes of Melon Fruit

N1-(2-chloro-4-pyridyl)-N3-phenylurea (CPPU), a synthetic cytokinin-active compound, is widely applied to induce parthenocarpic fruit set and enhance melon fruit enlargement (Cucumis melo L.). CPPU may also influence fruit quality; however, the mechanisms through which this occurs remain unknown. We investigated the differences in volatile emissions between parthenocarpic fruit set by CPPU (C) and seeded fruit set by artificial pollination (P). Gas chromatography–mass spectrometry (GC–MS) analysis revealed that six volatile organic compounds (VOCs) emitted by the P-group fruits were not detected in C-group fruits. The relative abundances of another 14 VOCs emitted by the CPPU-treated fruits were less than those in the P-group fruits. RNA sequencing analysis indicated that a total of 1027, 994, and 743 differentially expressed genes (DEGs) were detected in the C20 (treatment with 20 mg·L–1 CPPU) vs. P, P-C20 (pollination followed by 20 mg·L−1 CPPU treatment) vs. P, and P-C20 vs. C20 treatments, respectively. Compared with the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases, the DEGs related to fatty acid degradation and metabolism, which contribute to volatile production, were enriched. In particular, DEGs such as carotenoid cleavage dioxygenase (CCD)-, lipoxygenase (LOX)-, alcohol dehydrogenase (ADH)-, and alcohol acyltransferase (ATT)-related genes were closely related to the formation of volatiles. In summary, our study provides a metabolic and transcriptomic atlas, reveals the impact of CPPU on VOCs, and enhances our understanding of the mechanisms of CPPU that contribute towards generally reducing the quality of melon fruit.

Downstream of GA4, PbCYP78A6 Regulates Parthenogenesis by Mediating Cell Cycle-Related Genes in Pear (Pyrus Bretshneider Rehd.)

Background: Parthenocarpy results in traits attractive to both consumers and breeders, and it overcomes the obstacle of self-incompatibility in the fruit set of horticultural crops, including pear (Pyrus bretshneider). However, there is limited knowledge regarding the genetic and molecular mechanisms that regulate parthenogenesis. Results: Here, in a transcriptional comparison between pollination-dependent and GA4-induced parthenocarpy, PbCYP78A6 was identified and proposed as a candidate gene involved in parthenocarpy. PbCYP78A6 is similar to Arabidopsis thaliana CYP78A6 and is highly expressed in pear hypanthia. The increased PbCYP78A6 expression, as assessed by RT-qPCR, was induced by pollination and GA4 exposure. The ectopic overexpression of PbCYP78A6 contributed to parthenocarpic fruit production in tomato. The PbCYP78A6 expression coincided with fertilized and parthenocarpic fruitlet development and the expression of fruit development-related genes as assessed by cytological observations and RT-qPCR, respectively. PbCYP78A6 RNA interference and overexpression revealed that the gene is an upstream regulator of fruit development-related genes in pear. Conclusions: Our findings indicate that PbCYP78A6 plays a critical role in cell proliferation and provide insights into controlling parthenocarpy.