Applications of Cytokinins in Horticultural Fruit Crops: Trends and Future Prospects

BAP regulates lateral bud outgrowth to promote tillering in Paphiopedilum callosum (Orchidaceae)

BACKGROUND

Paphiopedilum orchids have a high ornamental value, and flower abundance is a key horticultural trait. Most Paphiopedilum plants exhibit weak tillering ability, with their tiller buds often entering a dormant state post-formation. Tiller production plays a crucial role in enhancing flower abundance and is potentially regulated by plant hormones. However, the effect of hormones on tillering in Paphiopedilum plants is still unclear.

RESULTS

In this study, we investigated the promotion of tillering in P. callosum through exogenous root irrigation of benzylaminopurine (BAP). We observed a dose-dependent promotion of tiller production by BAP, with the strongest effect observed at a concentration of 400 mg/L. By comparing the expression of key genes in P. 'SCBG Yingchun' (with strong tiller ability) and P callosum (with weak tiller ability), we found that BAP promotes tillering by interacting with abscisic acid (ABA). This interaction involves down-regulation of the ABA degradation gene PcCYP707A, leading to a reduction in ABA content, and the subsequent down-regulation of dormancy-associated genes (PcDRMH1, PcSVP) to release bud dormancy. Additionally, BAP promotes sustained outgrowth of tiller buds by increasing the level of indole-3-acetic acid (IAA) through up-regulation of the IAA synthesis gene PcYUC2 and the transport gene PcPINIC.

CONCLUSIONS

Our results indicated that the application of BAP promotes lateral bud outgrowth and increases tiller production in P. callosum. Through transcriptome analysis, we found that the BAP-promotion of tillering involves not only changes in endogenous IAA, ABA, and CTKs content but is also associated with the regulation of metabolism-related genes and dormancy-associated genes. This study presents the first comprehensive report of BAP-promoted tillering in P. callosum, providing a foundational basis for further mechanistic studies on tiller development in Paphiopedilum species and other non-model plants.

Multifunctional Role of Cytokinin in Horticultural Crops

Cytokinins (CKs) are a class of phytohormones identified in the early 1960s and are mainly responsible for stimulating cell division. Following the discovery, research to help understand the pluralistic roles of CKs in plant growth and stress biology increased. With their fascinating ability, CKs serve as an important element in regulating the defense-growth trade-off. Herein, we demonstrate how the CK fine-tuning the organogenesis of different parts of horticultural plants is discussed. CK's role in tailoring reproductive biology (flowering, sex differentiation, fruit set, and fruit attributes) has been presented. An extensive explanation of the CK-mediated response of horticultural crops to abiotic (temperature, drought, and salinity) and biotic stresses (fungal, bacterial, and nematodes) is provided. Finally, we posit the unexplored roles of CKs and highlight the research gaps worth addressing.

Fungal Biocontrol Agents in the Management of Postharvest Losses of Fresh Produce-A Comprehensive Review

Postharvest decay of vegetables and fruits presents a significant threat confronting sustainable food production worldwide, and in the recent times, applying synthetic fungicides has become the most popular technique of managing postharvest losses. However, there are concerns and reported proofs of hazardous impacts on consumers' health and the environment, traceable to the application of chemical treatments as preservatives on fresh produce. Physical methods, on the other hand, cause damage to fresh produce, exposing it to even more infections. Therefore, healthier and more environmentally friendly alternatives to existing methods for managing postharvest decays of fresh produce should be advocated. There is increasing consensus that utilization of biological control agents (BCAs), mainly fungi, represents a more sustainable and effective strategy for controlling postharvest losses compared to physical and chemical treatments. Secretion of antifungal compounds, parasitism, as well as competition for nutrients and space are the most common antagonistic mechanisms employed by these BCAs. This article provides an overview of (i) the methods currently used for management of postharvest diseases of fresh produce, highlighting their limitations, and (ii) the use of biocontrol agents as an alternative strategy for control of such diseases, with emphasis on fungal antagonists, their mode of action, and, more importantly, their advantages when compared to other methods commonly used. We therefore hypothesize that the use of fungal antagonists for prevention of postharvest loss of fresh produce is more effective compared to physical and chemical methods. Finally, particular attention is given to the gaps observed in establishing beneficial microbes as BCAs and factors that hamper their development, particularly in terms of shelf life, efficacy, commercialization, and legislation procedures.

Identification of two postharvest ripening regulatory models in kiwifruit: based on plant hormones, physiology, and transcriptome analysis

Kiwifruit (Actinidia spp.), celebrated for its unique flavor and rich nutritional content, is a globally popular fruit. This fruit requires post-harvest ripening before consumption. However, the unpredictable ripening pace significantly impacts consumer acceptance and sales, thereby hindering the commercial growth of kiwifruit. To address this, understanding the key molecular mechanisms and metabolites governing postharvest ripening and senescence could offer valuable insights for developing storage strategies and breeding techniques in yellow-fleshed kiwifruits. We constructed two models that integrated these findings with existing theories. The first model suggests that, unlike the T6P-sucrose regulatory mechanism observed in plant leaves, the separation of harvested kiwifruit from the mother plant leads to an insufficient supply of T6P, which activates the SnRK1 kinase. This, in turn, inhibits the TOR kinase signaling pathway, regulating starch metabolism. The T6P-SnRK1-TOR-starch metabolism pathway plays a regulatory role during postharvest ripening, limiting excessive starch degradation that could accelerate aging and decay in yellow-fleshed kiwifruit. The second model highlights the role of abscisic acid (ABA), cytokinins (CKs), and ethylene in regulating the process, inducing the activation of ERFs and cell wall-degrading enzymes, promoting fruit postharvest softening. These findings indicate that at least two models, the T6P-SnRK1-TOR-starch metabolism model and the ABA-CKs-ethylene-cell wall degradation model, regulate postharvest fruit ripening, offering new insights into the artificial regulation of yellow-fleshed kiwifruit ripening speed.

Recent Advances in Postharvest Application of Exogenous Phytohormones for Quality Preservation of Fruits and Vegetables

The increasing global population has heightened the demand for food, leading to escalated food production and, consequently, the generation of significant food waste. Factors such as rapid ripening, susceptibility to physiological disorders, and vulnerability to microbial attacks have been implicated as contributing to the accelerated senescence associated with food waste generation. Fruits and vegetables, characterized by their high perishability, account for approximately half of all food waste produced, rendering them a major area of concern. Various postharvest technologies have thus been employed, including the application of phytohormone treatments, to safeguard and extend the storability of highly perishable food products. This review, therefore, explores the physicochemical properties and biological aspects of phytohormones that render them suitable for food preservation. Furthermore, this review examines the effects of externally applied phytohormones on the postharvest physiology and quality attributes of fresh produce. Finally, the review investigates the mechanisms by which exogenous phytohormones preserve food quality and discusses the associated limitations and safety considerations related to the use of these compounds in food applications.

How Do Plant Growth-Promoting Bacteria Use Plant Hormones to Regulate Stress Reactions?

Phytohormones play a crucial role in regulating growth, productivity, and development while also aiding in the response to diverse environmental changes, encompassing both biotic and abiotic factors. Phytohormone levels in soil and plant tissues are influenced by specific soil bacteria, leading to direct effects on plant growth, development, and stress tolerance. Specific plant growth-promoting bacteria can either synthesize or degrade specific plant phytohormones. Moreover, a wide range of volatile organic compounds synthesized by plant growth-promoting bacteria have been found to influence the expression of phytohormones. Bacteria-plant interactions become more significant under conditions of abiotic stress such as saline soils, drought, and heavy metal pollution. Phytohormones function in a synergistic or antagonistic manner rather than in isolation. The study of plant growth-promoting bacteria involves a range of approaches, such as identifying singular substances or hormones, comparing mutant and non-mutant bacterial strains, screening for individual gene presence, and utilizing omics approaches for analysis. Each approach uncovers the concealed aspects concerning the effects of plant growth-promoting bacteria on plants. Publications that prioritize the comprehensive examination of the private aspects of PGPB and cultivated plant interactions are of utmost significance and crucial for advancing the practical application of microbial biofertilizers. This review explores the potential of PGPB-plant interactions in promoting sustainable agriculture. We summarize the interactions, focusing on the mechanisms through which plant growth-promoting bacteria have a beneficial effect on plant growth and development via phytohormones, with particular emphasis on detecting the synthesis of phytohormones by plant growth-promoting bacteria.

Meta-Topolin as an Effective Benzyladenine Derivative to Improve the Multiplication Rate and Quality of In Vitro Axillary Shoots of Húsvéti Rozmaring Apple Scion

In vitro mass propagation of apple plants plays an important role in the rapid multiplication of genetically uniform, disease-free scions and rootstocks with desired traits. Successful micropropagation of apple using axillary shoot cultures is influenced by several factors, the most critical of which is the cytokinin included in the culture medium. The impact of medium composition from single added cytokinins on shoot proliferation of apple scion Húsvéti rozmaring cultured on agar-agar gelled Murashige and Skoog medium fortified with indole butyric acid and gibberellic acid was investigated. The optimum concentration for efficient shoot multiplication differs according to the type of cytokinin. The highest significant multiplication rate (5.40 shoots/explant) was achieved using 2.0 μM thidiazuron while the longest shoots (1.80 cm) were observed on the medium containing benzyladenine at a concentration of 2.0 μM. However, application of either thidiazuron or benzyladenine as cytokinin source in the medium resulted in shoots of low quality, such as stunted and thickened shoots with small leaves. In the case of benzyladenine riboside, the 8 μM concentration was the most effective in increasing the multiplication rate (4.76 shoots/explant) but caused thickened stem development with tiny leaves. In the present study, meta-topolin was shown to be the most effective cytokinin that could be applied to induce sufficient multiplication (3.28 shoots/explant) and high-quality shoots along with shoot lengths of 1.46 cm when it was applied at concentrations of 4 μM. However, kinetin was the least active cytokinin; it practically did not induce the development of new shoots. The superior cytokinin for in vitro axillary shoot development of apple scion Húsvéti rozmaring with high-quality shoots was the meta-topolin, but it may be different depending on the variety/genotype under study.

Strigolactones affect the yield of Tartary buckwheat by regulating endogenous hormone levels

BACKGROUND

As a newly class of endogenous phytohormones, strigolactones (SLs) regulate crop growth and yield formation by interacting with other hormones. However, the physiological mechanism of SLs affect the yield by regulating the balance of endogenous hormones of Tartary buckwheat is still unclear.

RESULTS

In this study, a 2-year field experiment was conducted on Tartary buckwheat (Jinqiao 2) to study the effects of different concentrations (0, 10, and 20 µmol/L) of artificial synthetic analogs of SLs (rac-GR24) and inhibitor of SL synthesis (Tis-108) on the growth, endogenous-hormone content, and yield of Tartary buckwheat. The main-stem branch number, grain number per plant, grain weight per plant, and yield of Tartary buckwheat continuously decreased with increased rac-GR24 concentration, whereas the main-stem diameter and plant height initially increased and then decreased. Rac-GR24 treatment significantly increased the content of SLs and abscisic acid (ABA) in grains, and it decreased the content of Zeatin (Z) + Zeatin nucleoside (ZR). Conversely, Tis-108 treatment decreased the content of SLs and ABA but increased the content of Z + ZR. Results of correlation analysis showed that the content of ABA and SLs, the ratio of SLs/(Z + ZR), SLs/ABA, and ABA/(Z + ZR) were significantly negatively correlated with the yield of Tartary buckwheat, and that Z + ZR content was significantly positively correlated with the yield. Regression analysis further showed that ABA/ (Z + ZR) can explain 58.4% of the variation in yield.

CONCLUSIONS

In summary, by adjusting the level of endogenous SLs in Tartary buckwheat, the balance of endogenous hormones in grains can be changed, thereby exerting the effect on yield. The results can provide a new agronomic method for the high-yield cultivation of Tartary buckwheat.

Occurrence of plant hormones in composts made from organic fraction of agri-food industry waste

Utilizing the organic fraction of agri-food industry waste for fertilization represents one approach to waste management, with composting emerging as a popular method. Composts derived from this waste may contain plant hormones alongside primary macronutrients. This study aimed to evaluate the content of plant hormones in composts crafted from the organic fraction of agri-food industry waste. The presence of these substances was ascertained using liquid chromatography-mass spectrometry (LC-MS) analysis, applied to extracted samples from three composts produced in a bioreactor and three obtained from companies. The results indicate the presence of 35 compounds, which belong to six types of plant hormones: auxins, cytokinins, gibberellins, brassinosteroids, abscisic acid, and salicylic acid, in composts for the first time. The highest amount of plant hormones was noted in buckwheat husk and biohumus extract (35 compounds), and the lowest in hemp chaff and apple pomace (14 compounds). Brassinosteroids (e.g., brassinolide, 28-homobrassinolide, 24-epicastasterone, 24-epibrassinolide, and 28-norbrassinolide) and auxins (e.g., indolilo-3-acetic acid) are dominant. The highest concentration of total phytohormones was reported in biohumus extract (2026.42 ng g-1 dry weight), and the lowest in organic compost (0.18 ng g-1 dry weight).

Fungal-Mediated Biotransformation of the Plant Growth Regulator Forchlorfenuron by Cunninghamella elegans

The synthetic cytokinin forchlorfenuron (FCF), while seemingly presenting relatively low toxicity for mammalian organisms, has been the subject of renewed scrutiny in the past few years due to its increasing use in fruit crops and potential for bioaccumulation. Despite many toxicological properties of FCF being known, little research has been conducted on the toxicological effects of its secondary metabolites. Given this critical gap in the existing literature, understanding the formation of relevant FCF secondary metabolites and their association with mammalian metabolism is essential. To investigate the formation of FCF metabolites in sufficient quantities for toxicological studies, a panel of four fungi were screened for their ability to catalyze the biotransformation of FCF. Of the organisms screened, Cunninghamella elegans (ATCC 9245), a filamentous fungus, was found to convert FCF to 4-hydroxyphenyl-forchlorfenuron, the major FCF secondary metabolite identified in mammals, after 26 days. Following the optimization of biotransformation conditions using a solid support system, media screening, and inoculation with a solid pre-formed fungal mass of C. elegans, this conversion time was significantly reduced to 7 days-representing a 73% reduction in total reaction time as deduced from the biotransformation products and confirmed by LC-MS, NMR spectroscopic data, as well as a comparison with synthetically prepared metabolites. Our study provides the first report of the metabolism of FCF by C. elegans. These findings suggest that C. elegans can produce FCF secondary metabolites consistent with those produced via mammalian metabolism and could be used as a more efficient, cost-effective, and ethical alternative for producing those metabolites in useful quantities for toxicological studies.

Optimistic contributions of plant growth-promoting bacteria for sustainable agriculture and climate stress alleviation

Global climate change is the major cause of abiotic and biotic stresses that have adverse effects on agricultural productivity to an irreversible level, thus threatening to limit gains in production and imperil sustainable agriculture. These climate change-induced abiotic stresses, especially saline, drought, extreme temperature, and so on affect plant morphological, physiological, biochemical, and metabolic characteristics through various pathways and mechanisms, ultimately hindering plant growth, development, and productivity. However, overuse and other inappropriate uses of agrochemicals are not conducive to the protection of natural resources and the environment, thus hampering sustainable agricultural development. With the vigorous development of modern agriculture, the application of plant growth-promoting bacteria (PGPB) can better ensure sustainable agriculture, due to their ability to improve soil properties and confer stress tolerance in plants. This review deciphered the underlying mechanisms of PGPB involved in enhancing plant stress tolerance and performance under various abiotic and biotic stresses. Moreover, the recent advancements in PGPB inoculation techniques, the commercialization of PGPB-based technology and the current applications of PGPB in sustainable agriculture were extensively discussed. Finally, an outlook on the future directions of microbe-aided agriculture was pointed out. Providing insights into plant-PGPB interactions under biotic and abiotic stresses and offering evidence and strategies for PGPB better commercialization and implementation can inspire the development of innovative solutions exploiting PGPB under climatological conditions.

Advances of section drying in citrus fruit: The metabolic changes, mechanisms and prevention methods

Citrus fruit are consumed worldwide due to their excellent features, such as delicious taste and health-promoting compounds. However, section drying, a physiological disorder of citrus fruit, often occurs both in the preharvest and postharvest storage, causing a significant reduction in fruit quality and consumer acceptance. In this review, section drying of citrus fruit was divided into three types: granulation, vesicle collapse and both above. The main causes, metabolic changes and mechanisms of section drying were discussed, respectively. Furthermore, the prevention methods of section drying in citrus fruit, including preharvest and postharvest methods, were also summarized. Given the significant influence of section drying in citrus fruit production, the mechanisms and prevention methods of section drying are worth further exploration. A better understanding of section drying may provide guidance for the prevention of this disorder and future breeding of citrus fruit.

Auxin Coordinates Achene and Receptacle Development During Fruit Initiation in Fragaria vesca

In strawberries, fruit set is considered as the transition from the quiescent ovary to a rapidly growing fruit. Auxin, which is produced from the fertilized ovule in the achenes, plays a key role in promoting the enlargement of receptacles. However, detailed regulatory mechanisms for fruit set and the mutual regulation between achenes and receptacles are largely unknown. In this study, we found that pollination promoted fruit development (both achene and receptacle), which could be stimulated by exogenous auxin treatment. Interestingly, auxin was highly accumulated in achenes, but not in receptacles, after pollination. Further transcriptome analysis showed that only a small portion of the differentially expressed genes induced by pollination overlapped with those by exogenous auxin treatment. Auxin, but not pollination, was able to activate the expression of growth-related genes, especially in receptacles, which resulted in fast growth. Meanwhile, those genes involved in the pathways of other hormones, such as GA and cytokinin, were also regulated by exogenous auxin treatment, but not pollination. This suggested that pollination was not able to activate auxin responses in receptacles but produced auxin in fertilized achenes, and then auxin might be able to transport or transduce from achenes to receptacles and promote fast fruit growth at the early stage of fruit initiation. Our work revealed a potential coordination between achenes and receptacles during fruit set, and auxin might be a key coordinator.

Ethnobotanical Uses, Nutritional Composition, Phytochemicals, Biological Activities, and Propagation of the Genus Brachystelma (Apocynaceae)

The Brachystelma genus (family: Apocynaceae) consists of geophytes that are traditionally utilised among rural communities, especially in East Africa, southern Africa, West Africa, and northern and western India. Apart from being used as a food source, they are indicated as treatment for ailments such as colds, chest pains, and wounds. This review provides a critical appraisal on the ethnobotanical uses, nutritional value, phytochemical profiles, and biological activities of Brachystelma species. In addition, we assessed the potential of micropropagation as a means of ensuring the sustainability of Brachystelma species. An inventory of 34 Brachystelma species was reported as a source of wild food and traditional medicine (e.g., respiratory-related conditions, pains, and inflammation) across 13 countries, predominantly in Africa and Asia. Brachystelma circinnatum and Brachystelma foetidum were the most popular plants based on the high number of citations. Limited data for the nutritional content was only available for Brachystelma edulis and Brachystelma naorojii, as well as phytochemical profiles (based on qualitative and quantitative techniques) for five Brachystelma species. Likewise, a few Brachystelma species have evidence of biological activities such as antimicrobial, antioxidant, and acetyl cholinesterase (AChE) inhibitory effects. Extensive studies on Brachystelma togoense have resulted in the isolation of four compounds with therapeutic potential for managing different health conditions. As a means of contributing to the sustainability of Brachystelma species, micropropagation protocols have been devised for Brachystelma glabrum, Brachystelma pygmaeum, Brachystelma ngomense, and Brachystelma pulchellum. Nevertheless, continuous optimisation is required to enhance the efficiency of the micropropagation protocols for these aforementioned Brachystelma species. Despite the large number of Brachystelma with anecdotal evidence as food and medicine, a significant number currently lack empirical data on their nutritional and phytochemical profiles, as well as their biological activities. The need for new propagation protocols to mitigate the declining wild populations and ensure their sustainability remains pertinent. This is important should the potential of Brachystelma species as novel food and medicinal products be achieved.

Efficiency of a biological growth regulator in the cultivation of branched seedlings

The article presents the results of studies on the effectiveness of biological preparations Gibbersib - obtained on the basis of the Fusarium moniliforme strain - a polygibberellin preparation containing a set of gibberellic acids, as well as the preparation 6-Benzyladenine - a synthetic cytokinin intended to activate the vital activity of a plant associated with the growth and development of lateral shoots that improve the crowning of seedlings. According to the results of preparations testing with separate and combined use, the effectiveness of using each of the regulator separately, in comparison with the control, was confirmed. The greatest efficiency was obtained with the use of treatment with combined regulators, which makes it possible to enhance the branching of seedlings.

Abiotic Stress in Crop Species: Improving Tolerance by Applying Plant Metabolites

Reductions in crop yields brought about by abiotic stress are expected to increase as climate change, and other factors, generate harsher environmental conditions in regions traditionally used for cultivation. Although breeding and genetically modified and edited organisms have generated many varieties with greater abiotic stress tolerance, their practical use depends on lengthy processes, such as biological cycles and legal aspects. On the other hand, a non-genetic approach to improve crop yield in stress conditions involves the exogenous application of natural compounds, including plant metabolites. In this review, we examine the recent literature related to the application of different natural primary (proline, l-tryptophan, glutathione, and citric acid) and secondary (polyols, ascorbic acid, lipoic acid, glycine betaine, α-tocopherol, and melatonin) plant metabolites in improving tolerance to abiotic stress. We focus on drought, saline, heavy metal, and temperature as environmental parameters that are forecast to become more extreme or frequent as the climate continues to alter. The benefits of such applications are often evaluated by measuring their effects on metabolic, biochemical, and morphological parameters in a variety of crop plants, which usually result in improved yields when applied in greenhouse conditions or in the field. As this strategy has proven to be an effective way to raise plant tolerance to abiotic stress, we also discuss the prospect of its widespread implementation in the short term.