1 Introduction

Pitaya (Hylocereus spp.) is a fruit of high nutritional value and tremendous economic potential. It contains a high content of dietary fiber, vitamins (particularly vitamin C), minerals, and bioactive molecules like betaine and flavonoids with excellent antioxidant properties. Because of its health quality, international market demand for Pitaya consumption has increased (Mou et al., 2022). Economically, its price quotation, flexibility under varied climatic conditions, and low capital investment in planting make it a cash crop of interest. Despite all these merits, there are still issues such as inconsistency in fruit quality and postharvest losses, which show that further study of its maturation processes is necessary (Xie et al., 2021).

As Pitaya ripens, the texture is modified, sugar builds up, and the action of antioxidants is dramatically changed. The fruit softens largely because two enzymes break down the cell wall. Two of the major enzymes whose action softens the fruit are polygalacturonidase (PG) and pectin methyl esterase (PME) (Zhang et al., 2021). During the process of fruit ripening, the metabolism of sugar is changed. Levels of glucose, fructose, and sucrose rise. This is controlled by key enzymes like sucrose phosphate synthase (SPS) and convertase (INV). Additionally, antioxidant metabolism also plays a role in fruit quality maintenance. Both enzymatic antioxidants (like APX, CAT, and SOD) and non-enzymatic antioxidants (like vitamin C, flavonoids, and phenols) participate. The antioxidants all play a role together in the process of ripening to reduce oxidative stress (Wei et al., 2019; Xie et al., 2022).

This study investigates the physiological and molecular regulatory mechanisms of softening, sugar content accumulation and antioxidant metabolism during Pitaya ripening, and identifies key genes, metabolic pathways and their interactions to provide theoretical guidance and practical recommendations. Studies on its mechanism of softening can delay early softening of the fruit and improve storage stability; study on sugar deposition can maximize quality of flavor and nutrition; studies on antioxidant metabolism can improve antioxidant capacity of the fruit and extend shelf life. Thorough understanding of these control mechanisms will unveil valuable targets for molecular breeding and gene editing, enable new storage-resistant and high-quality varieties to be bred, and optimize post-harvest management practices to improve the industrial sustainability and market competitiveness of Pitayas.

2 Genetic Regulation of Pitaya Fruit Softening

2.1 Role of cell wall degradation in fruit softening

Breakdown of the cell wall is a chief operation for Pitayas softening, which is breakdown of complicated carbohydrates into simpler forms. This breakdown is promoted by a variety of cell wall alteration enzymes, modifying cell wall organization, thus fruit tissue softening. The cell wall degradation is a synergistic process involving the combined action of a variety of enzymes such as pectinase, cellulase and hemicellulase, which ultimately result in texture modification in fruit ripening (Chen et al., 2022).

2.2 Function of key cell wall-modifying enzymes

Some of the principal enzymes responsible for fruit softening include pectin methyl esterase (PME), polygalacturonidase (PG), and cellulase (CEL). Santos et al. (2019) showed that these enzymes play a crucial role in the degradation of the main components of the primary cell wall such as pectin, cellulose and hemicellulose. Its activity is stringently regulated during fruit softening during fruit ripening so that the fruit becomes softened at the appropriate time, thereby improving quality palatable and seed dispersal. These genes' expression is generally enhanced during the softening of the fruit, which indicates the function of the enzymes in softening.

2.3 Role of ethylene and other hormones in softening regulation

One of the key hormones for fruit softening and ripening of several fleshy fruits, including Pitaya, is ethylene. Ethylene, as a molecule of ethylene, can induce the expression of cell wall-breakdown-related genes and related mature genes. In addition to ethylene, abscisic acid (ABA) and auxin also play a role in fruit ripening regulation. These hormones convey complex network messages to control the duration and quantity of fruit softening to the desired level, hence optimizing fruit quality and shelf life (Bianchetti et al., 2024).

2.4 Softening-related genes and their signaling networks

Fruit softening is regulated by a cascade of hormone- and environment-sensitive gene networks. WRKY, Dof and MYB transcription factors were shown to be critical regulators of gene expression during fruit ripening. For example, WRKY transcription factor HpWRKY3 regulates sugar metabolism gene expression, indirectly affecting osmotic balance during fruit softening (Li et al., 2022). Apart from that, Liu et al. (2021) found that Dof transcription factors (such as HpDof1.7 and HpDof5.4) are involved in the transcriptional activation of sugar metabolism genes, which subsequently affect the process of fruit softening. The transcription factors belong to a complex signaling regulation system where hormone signals and developmental signals are integrated to refine the timing and process of fruit softening.

3 Regulatory Mechanisms of Sugar Accumulation in Pitaya Fruit

3.1 Changes in sugar content and their impact on fruit quality

The sugar content is an essential parameter that represents the quality of the fruit and affects the taste and consumers' preferences. Glucose prevails in Pitaya, followed by sucrose and fructose, and these all have significant functions to determine the sweetness of the fruit as well as overall flavor profiles (Li et al., 2024). During fruit ripening, the content of soluble sugar is significantly increased, which improves the sweetness and quality of the fruit for sale. Sugar deposition is closely linked to the expression of some genes and the activities of sugar metabolic enzymes, and they are critical to the formation of high-quality characteristics of the fruit (Wu et al., 2019).

3.2 Major sugars and their metabolic pathways

The major sugars of Pitaya are glucose, fructose and sucrose. The biochemical metabolism of the sugars involves some key enzymes like vacuole acid converting enzymes (VAI), neutral converting enzymes (NI), and sucrose synthase (SS). These enzymes' actions on sugar metabolism play a vital role in setting the storage of fruit sugar content, of which glucose is the most dominant sugar in mature Pitayas. Sugar degradation and synthesis are strictly regulated in order to keep the ideal equilibrium of sugar during fruit development and ripening (Ren et al., 2023).

3.3 Role of key sugar metabolic enzymes

Enzymes such as VAI, NI and SS play a role in the metabolism of fructose in bird's nest. VAI and NI hydrolyze sucrose into glucose and fructose, which enables these sugars to accumulate in the fruit (Liu et al., 2024). SS not only involves itself in the process of sugar synthesis, but also in catalyzing the process of sugar degradation, regulating dynamically sugar levels during fruit growth. Their expression is strictly connected to the expression of specific genes, which are regulated by a broad range of transcription factors (Li et al., 2018).

3.4 Transcription factors and signaling pathways involved in sugar accumulation

Transcription factors such as HpDof1.7, HpDof5.4 and HpWRKY3 were discovered to be the key controllers of fructose deposition in bird's nest by the study. The transcription factors can stimulate gene expression involved in sugar metabolism, such as HpSuSy1 and HpINV2, which play crucial roles in sugar conversion and deposition. Besides that, HpDof1.7 and HpDof5.4 also contribute to enhancing sugar transporter gene functions (such as HpTMT2 and HpSWEET14) and further increasing sugar accumulation. They are part of complex networks that control fine tuning of sugar metabolism, which ultimately affects the fruit quality (Jiang et al., 2023).

4 Antioxidant Metabolism During Pitaya Fruit Ripening

4.1 Accumulation and scavenging mechanisms of reactive oxygen species (ROS)

In ripening of Pitayas, elimination and build-up of reactive oxygen species (ROS) plays a crucial role in cell homeostasis. ROS is a normal by-product of cell metabolism, but when not properly regulated, can lead to oxidative damage. During fruit ripening, the balance between the production and scavenging of ROS is relevant to the normal maturation process. ROS regulation is generally performed by non-enzymatic and enzymatic antioxidant systems that work together to counteract oxidative stress and ensure fruit stability and quality (Li et al., 2023).

4.2 Dynamic changes of key antioxidant enzymes

Key antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) display dynamic variation in the ripening of the bird's nest. These enzymes play a role in the removal of ROS and protection of the fruit from oxidative injury. Overall, the activity of these enzymes is boosted at the early stage of fruit ripening to fight against rising ROS levels, which may decline when the fruit is fully ripening. This regulation of enzyme activity is crucial to oxidative balance and the formation of sensory qualities of the fruit (Qi et al., 2020).

4.3 Accumulation of non-enzymatic antioxidants

Non-enzymatic antioxidants such as vitamin C, phenolic compounds, and flavonoids gradually accumulate during Pitaya fruit ripening. Not only do they enhance the fruit's antioxidant activity, but they also play an important role in the ROS neutralization. Its accumulation tends to be closely associated with the development stage of the fruit, with advanced maturity, higher antioxidant content (Peng et al., 2022). The improvement of these antioxidants not only defends the fruit against oxidative stress, but also improves the nutritional value and marketability of the fruit (Ding et al., 2024).

4.4 Key antioxidant genes and their regulatory mechanisms

During Pitaya ripening, the regulation of major antioxidant genes is strictly regulated. They code for enzymatic and non-enzymatic antioxidants and are regulated by several transcription factors and environmental factors. For example, transcription factors like MYB and WRKY have been shown to be accountable for the regulation of genes related to antioxidant metabolism. These regulatory mechanisms ensure that antioxidant defense systems are able to adaptively modulate depending on the constantly fluctuating oxidative environment during fruit ripening (Durán-Soria et al., 2020).

5 Coordinated Regulation of Softening, Sugar Accumulation, and Antioxidant Metabolism

5.1 Influence of ethylene signaling on sugar metabolism

Ethylene plays a major role in sugar metabolism control during fruit ripening. Ethylene is a key hormone signal for controlling the sugar composition and softening potential of the fruit. Ethylene interacts synergistically with abscisic acid (ABA) in controlling major changes in fruit development, including sugar accumulation and cell wall rupture, both of which are integral to fruit ripening. The ethylene and sugar cross-talk is complex, with numerous transcription factors and metabolic genes participating to collectively regulate sugar content fluctuations in fruits (Li et al., 2017).

5.2 Interaction between cell wall degradation and sugar transport

During fruit ripening, cell wall breakdown is directly related to sugar transport and accumulation. During the maturation process, the enzymes such as convertases and sucrose synthase are mainly involved in the breakdown of complex carbohydrates to glucose and fructose, which are transported and accumulated inside the fruit. It has been found using research that transcription factors such as HpDof1.7 and HpDof5.4 can enhance the level of expression of genes associated with sugar metabolism and improve the accumulation of soluble sugars within Pitayas (Peng et al., 2022; Da Graca Tomas et al., 2023). Cell wall breakdown and the transport of sugar work together to contribute significantly to the formation of fruit texture and flavor (Figure 1).

5.3 Regulation of fruit softening and sugar accumulation by antioxidant metabolism

Antioxidant metabolism is essential in regulating fruit softening and sugar loading. Antioxidants (such as ascorbic acid) regulate the mechanism of fruit ripening by modulating oxidative stress and maintaining cellular integrity. Balancing antioxidant and sugar metabolism is essential to achieve optimal fruit quality because it controls the texture and sweetness of the fruit at the same time. Antioxidant metabolism genes like ascorbic acid biosynthesis and circulating genes help in fruit ripening to achieve this dynamic balance (Hua et al., 2018).

5.4 Integrated regulatory network of softening, sugar accumulation, and antioxidant metabolism

Yellow-skinned Pitaya fruit ripening is a coordinated regulatory network that coordinates fruit softening, sugar buildup, and antioxidant metabolism. The network is regulated by complicated interplays of transcription factors, phytohormones and metabolic routes. For example, HpWRKY3 and Dof transcription factors regulate sugar metabolism genes, while ethylene and ABA signaling routes are central to the overall maturation process (Xu et al., 2021). Furthermore, antioxidant metabolic processes also help control oxidative stress for stability of fruit quality during fruit ripening. Reveal of such a system-thinking regulatory mechanism could enable finding of the fruit ripening's genetic and biochemical mechanism and providing future strategies to improving fruit quality and extending shelf life (Durán-Soria et al., 2020; Li et al., 2023) (Figure 2).

6 Research Progress in Pitaya Ripening Regulation

6.1 Functional analysis and validation of ripening-related genes

Over the past few years, some of the key transcription factors participating in the regulation of bird's nest fructose accumulation and thereby fruit ripening have been identified by scientists. For example, the WRKY transcription factor HpWRKY3 induces the expression of sucrose metabolism-related genes HpINV2 and HpSuSy1, which results in fruit sugar accumulation. Similarly, Dof transcription factors HpDof1.7 and HpDof5.4 were discovered to induce the expression of sugar metabolism genes, leading to the accumulation of glucose and fructose (Vicente et al., 2007). These studies all show that transcription factors play a very significant role in the genetic control of Pitaya fruit ripening.

6.2 Application of omics technologies in fruit ripening studies

Omics technology, especially transcriptomics, is especially significant in demonstrating genes involved in fructose metabolism in bird's nest. For example, in transcriptome analysis, a number of sugar and organic acid metabolism genes were identified through analysis, resulting in new findings regarding fruit development processes (Xu et al., 2024). In addition, omics technology also offers the potential to genome-wide screen transcription factors such as R2R3-MYB family proteins regulating fruit ripening through mechanisms such as regulating beet elutin biosynthesis. Coupling with multiomics approaches can further be capable of disclosing the complex network of Pitaya ripening control (Adaskaveg and Blanco‐Ulate, 2023).

6.3 Postharvest quality control and ripening regulation strategies

Post-harvest quality maintenance of Pitaya is mainly the management of sugar levels and fruit softening, to which consumer acceptance is most sensitive. Management of sugar metabolism enzymes (e.g., vacuole acid converting enzymes and sucrose synthase) is crucial in post-harvest sugar level maintenance. Also, the deeper understanding of ethylene and abscisic acid functions throughout the ripening process can allow the formulation of control measures for fruit ripening, in addition to shelf life extension. Such studies are also essential for postharvest handling practice optimization (Shi et al., 2022).

6.4 Future breeding strategies: enhancing fruit quality and extending shelf life

Future breeding of bird's nest fruit needs to improve quality (such as sugar content and antioxidant activity) and shelf life. Identification of genes and transcription factors involved in sugar accumulation and regulation of maturation is useful for gene improvement. In addition, utilization of omics data to comprehend the genetic mechanism of fruit ripening could be applied to breed new varieties with higher quality and improved resistance to environmental stress. These breeding practices are especially significant to meeting market demand and improving the economic benefits of the Pitaya industry (Huang, 2024).

7 Concluding Remarks

Pitaya fruit ripening is a complex physiological process, which is mainly reflected by the coordinated regulation of the accumulation of sugars, softening and antioxidant metabolism. Transcription factors HpWRKY3, HpDof1.7 and HpDof5.4 trigger glucose and fructose accumulation by activating sucrose metabolism-related genes such as HpINV2 and HpSuSy1. Additionally, during fruit coloration during pigment synthesis, R2R3-MYB transcription factor HuMYB1 regulates the fruit coloration process by inhibiting beet erythronin synthesis-related gene expression. These metabolic processes are regulated by key enzymes such as vacuole acid converting enzyme (VAI) and sucrose synthase (SS), which in turn regulate fruit sugar content and quality.

While much has been accomplished, gaps remain in pitaya fruit ripening research. Precisely what regulatory networks and how various transcription factors and metabolic pathways communicate with one another is still elusive. For instance, while functions of certain transcription factors in sugar metabolism are known, how they interact with other environmental and hormonal signals remains unknown. Additionally, the effect of abiotic stress factors on hormonal control of fruit ripening, with specific focus on the role of abscisic acid and ethylene, should be researched to determine how these factors control the quality of the fruit as a function of varying environmental conditions.

Physiological control of Pitaya (Hylocereus megalanthus) ripening has yet to be investigated. Gene control, metabolic control, environmental adaptation and smart agriculture can be investigated in the future. At the molecular level, how transcription factor control, non-coding RNA and epigenetic modification affect maturity can be researched, while at the same time how hormones such as ethylene and abscisic acid control fruit development. At the level of metabolic regulation, metabolomics can be combined with metabolomics to study the dynamics of sugar formation, acid and aromatic compounds, and gene editing to enhance the quality of flavor. In addition, studying the effect of pigment deposition, antioxidant activity and environmental factors on fruit maturity will improve the quality of fruit and storage tolerance. In the meantime, intelligent agricultural technology construction can perform accurate mature prediction and control by means of image recognition and nano delivery systems. Generally, a whole system of research from molecular mechanisms to production applications should be built in the future to ensure efficient development of the Pitaya industry.

Acknowledgments

The authors especially thank the Pitaya growers who assisted during the study and share their valuable experience to provide the necessary support for studying the physiological mechanisms of Pitaya fruit ripening.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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