Phi exhibits toxicity at high concentrations but exerts beneficial effects on plants at low concentrations (Estrada-Ortiz et al., 2016; Trejo-Téllez and Gómez-Merino, 2018). The inhibitory effects of Phi on Plants occur via two mechanisms. First, the accumulation and toxic effects of Phi are likely associated with reduced Pi assimilation and the inability to metabolize Phi or convert it Pi within the cells (Thao and Yamakawa, 2009; Ticconi et al., 2001). Second, Phi attenuates the phosphate starvation response (PSR) in cells, reducing the plant’s sensitivity to P deficiency (Ticconi et al., 2001). Phi suppresses the activity of key enzymes implicated in the PSR, including ATPase and PPI-dependent phosphofructokinase. This inhibition diminishes the PSR, particularly in plants suffering from inorganic phosphate (Pi) deficiency.

In Pi-deficient soils, Phi may be perceived by plants as Pi, thereby inhibiting the activation of P starvation responses that are critical for maintaining plant growth and function (McDonald et al., 2001a). However, Phi cannot participate in Pi-related biological processes due to its inability to be oxidized to Pi (Achary et al., 2017; Danova-Alt et al., 2008). As a result, Phi stress leads to arrested plant growth and the development of and toxic symptoms (Havlin and Schlegel, 2021; Singh et al., 2003; Thao et al., 2008a; Ticconi et al., 2001; Zambrosi et al., 2011). Plants can absorb Phi due to its structural similarity to Pi (McDonald et al., 2001b). However, Pi antagonistically inhibits the uptake of Phi in plants (Jost et al., 2015; Pratt et al., 2009; Trejo-Téllez and Gómez-Merino, 2018). In Phytophthora, Pi and Phi compete for the binding sites of Pi transporters (Griffith et al., 1989). Under Pi-deficient conditions, Phi accumulates in the cytoplasm, but when Pi is supplied, Phi is rapidly expelled from the cell (Achary et al., 2017). The presence of Pi enhances the concealed role of Phi in the vacuole. Under adequate Pi conditions, plants tolerate moderate concentrations of Phi without adverse effects (Thao and Yamakawa, 2009). However, under low-Pi conditions, Phi blocks the Pi starvation-induced transduction signaling pathway, weakening the cellular response to P deficiency and impairing P utilization even when Pi is sufficient (Singh et al., 2003; Ticconi et al., 2001; Vinas et al., 2020).

Phi functions in metabolic interruption and growth inhibition by suppressing phosphorylation and competing for the Pi-binding sites of phosphorylase (Achary et al., 2017). Phi stress causes defective phenotypes in plants, such as stunted growth of primary roots, yellowing of young leaves, and patchy anthocyanin accumulation in old leaves (Hirosse et al., 2012; Manna et al., 2015; Thao et al., 2008a, b). In common bean (Phaseolus vulgaris), Phi inhibits growth, resulting in poor grain filling under Pi-deficient conditions (Ávila et al., 2013). Similarly, in potato (Solanum tuberosum), Phi significantly reduces shoot length, root length, total root length, root volume, root tips, and fresh biomass (Dormatey, 2022). Phi also inhibits photosynthesis and the TCA cycle in the aboveground parts of alfalfa (Medicago sativa) seedlings (Li et al., 2022). The growth rate, length, and dry weight of sweet potato (Ipomoea batatas) are significantly reduced by excessive Phi application (Hirosse et al., 2012). In addition, the shoot dry weight and P concentration of spinach (Spinacia oleracea) decrease as the Pi: Phi ratio decrease from 100:0 to 0:100 (Thao et al., 2008a). Phi also significantly inhibits root growth and root hair development in both onion (Allium cepa) (Carswell et al., 1996) and spinach and reduces biomass and chlorophyll content in citrus (Citrus spp.) (Zambrosi et al., 2011).

Phi has been shown to enhance plant growth and development (Gómez-Merino et al., 2022; Gómez-Merino and Trejo-Téllez, 2015; Thao and Yamakawa, 2009; Trejo-Téllez and Gómez-Merino, 2018). Several hypotheses have been proposed regarding its functional mechanism in promoting plant growth (Gómez-Merino et al., 2022). Phi influences sugar metabolism, modulates plant hormones levels, and impacts secondary metabolite synthesis by inducing the shikimic acid pathway (Lovatt and Mikkelsen, 2006). The shikimic acid pathway is responsible for the biosynthesis of aromatic amino acids such as phenylalanine, tyrosine, and tryptophan. These amino acids serve as precursors for a diverse array of secondary metabolites, including pigments, alkaloids, hormones, and cell wall components, which are crucial for plant growth and development (Gómez-Merino and Trejo-Téllez, 2015). Phi treatment enhances the levels of free amino acids, proteins, sugars, and anthocyanins in strawberry leaves (Estrada-Ortiz et al., 2013). Phi promotes plant growth by upregulating genes associated with the biosynthesis and signaling pathways of abscisic acid (ABA), salicylic acid (SA), and jasmonic acid (JA) (Pérez-Zavala et al., 2024). Moreover, Phi improves fruit quality by promoting the synthesis of ascorbic acid and anthocyanins (Gómez-Merino and Trejo-Téllez, 2015). Notably, when combined coupled with metal ions, Phi facilitates faster and more nutrient delivery within plants, thereby exhibiting significant fertilizer effects (Havlin and Schlegel, 2021; Herrera-Estrella and Lopez-Arredondo, 2016; Lovatt and Mikkelsen, 2006).

Studies have reported Phi enhances crop flowering, yield, quality, fruit size, and soluble matter content, functioning effectively as a bio-stimulant (Gómez-Merino et al., 2022; Gómez-Merino and Trejo-Téllez, 2015; Martínez, 2016). Currently, Phi is widely utilized to supplement plant nutrition and optimize agricultural productivity for growers (Havlin and Schlegel, 2021). For instance, it enhances the yield and quality of rice (Oryza sativa) (Martínez, 2016), soybean (Glycine max) (Carmona et al., 2018), strawberry (Fragaria × ananassa) (Estrada-Ortiz et al., 2013; Glinicki et al., 2010), onion bulb (Allium cepa) (Monsalve et al., 2012), squash (cucurbita pepo) (Omar et al., 2020), celery (Apium graveolens), peach (Prunus persica), and sweet orange (Citrus sinensis) and improves flowering in potato and tomato (Solanum lycopersicum) (Lovatt and Mikkelsen, 2006; Rickard, 2000). Foliar spray application of 1,775-3,550 g/hm² potassium (K) Phi increased rice yield by 5%-10% (Martínez, 2016). Seed treatment with K- and Mn-Phi increased seedling emergence by up to 29% compared to control (Carmona et al., 2018). In addition, fertigation with either sole Phi (consisting of 6.7% Phi of P) or Phi combined NPK increased strawberry shoot and root growth (Glinicki et al., 2010). Hydroponic application of 20%-30% Phi improved fruit quality by increasing anthocyanin concentrations in strawberry (Estrada-Ortiz et al., 2013). Notably, a nutrient solution containing 50% H₃PO₄ and 50% H₃PO₃ enhanced biomass, leaf area, and total P content in lettuce, tomato, and banana (Musa paradisiaca) (Bertsch et al., 2009). Foliar spray application of 5 mL/L potassium Phi efficiently increased the weight of first-class onion bulbs and total bulb weight (Monsalve et al., 2012). Potatoes treated with potassium Phi have a shorter interval between planting and germination and have increased leaf area and weight (Tambascio et al., 2014). Therefore, the optimal concentration of Phi, in combination with other essential nutrient ions, represents a promising bio-stimulant and a transformative enhancer of crop production and quality in modern agricultural systems.

Notably, the availability of Pi to plants is a crucial determinant of Phi toxicity. Co-application of Pi and Phi synergistically improved P absorption, confirming that Phi toxicity is proportional to Pi utilization (Bertsch et al., 2009). In contrast, exclusive Phi application inhibited growth, causing leaf wilting and root degeneration (Bertsch et al., 2009; Gómez-Merino and Trejo-Téllez, 2015). The effects of Phi on the metabolism of beets and lettuce varied different under hydroponic conditions. Specifically, Phi caused positive responses, including increased biomass and nutrient content, under sufficient-P conditions and at concentrations is less than 0.25 mM (Estrada-Ortiz et al., 2016).

However, Phi takes approximately four months to be fully oxidized to Pi in the natural environment (McDonald et al., 2001a), making it an ideal candidate for use as a slow-release fertilizer. Despite the challenges associated with promoting Phi in agricultural production, it holds significant potential to increase crop yields and health, address the challenges of feeding a growing global population, and minimize the negative impacts of agriculture on human health and the environment (Han et al., 2021). Therefore, Phi-based fertilizers are poised for development and could serve as a viable alternative to conventional P fertilizers in the future.

Phi effectively improves plant tolerance to diverse abiotic stresses, including UV radiation, water deficit, and heat shock (Gómez-Merino and Trejo-Téllez, 2015; Trejo-Téllez and Gómez-Merino, 2018; Xi et al., 2020). Phi activates defense responses against pathogens and modulates primary metabolism to help plants cope with abiotic stress. Furthermore, dual-channel Phi transport reduces energy consumption during nutrient transport, thereby providing additional energy to mitigate the effects of abiotic stress. This induces the accumulation of proteins associated with cell wall formation in plants subjected to abiotic stress (Trejo-Téllez and Gómez-Merino, 2018). In potatoes, Phi induces systemic defense response, including an increase in the levels of phytoantibiotics and chitinase, as well as enhanced activities of peroxidase and polyphenol oxidase (Lobato et al., 2011). Phi also improves the plant’s tolerance to UV stress by activating the antioxidant system and inducing the natural defense response of plants (Soledad et al., 2015). Pre-treatment with Phi enhanced potato resistance to UV-B, as evidenced by increased chlorophyll accumulation and elevated expression of the D1 polypeptide-encoding gene (psbA) of the photosystem II in the chloroplast, which serves as a photosynthetic protection protein (Soledad et al., 2015). Application of 0.25 mM Phi in hydroponic solutions increased P and chlorophyll concentrations in lettuce (Lactuca sativa) (Estrada-Ortiz et al., 2016). Phi also enhanced pathogen tolerance by improving oxidative levels, including SOD, POX, CAT, and APX activities, along with higher concentrations of antioxidant metabolites such as phenolics, flavonoids, and proline and proteins and carbohydrates in potato leaves (Mohammadi et al., 2020). In alfalfa Phi treatment increased the abundance of heat shock protein (HSP), mitochondrial alternative oxidase (AOX), and pathogenicity-related protein (PR protein) as well as enhanced DNA repair through enzymatic and non-enzymatic oxidative systems (Li et al., 2022).

Pretreatment with a Phi solution before planting effectively activates the defense response in fruits, increasing the content of ascorbic acid and anthocyanins (Estrada-Ortiz et al., 2013; Moor et al., 2009). The synthesis and accumulation of anthocyanins play a key role in responding to nutrient deficiency and pathogen infection (Routray and Orsat, 2011). Anthocyanins act as light attenuators, reducing photo-oxidative damage to leaves by shielding chloroplasts from excessive high-energy ions and scavenging reactive oxygen species (Zheng et al., 2021). Additionally, Phi promotes the synthesis of antioxidant enzymes and metabolites (Ávila et al., 2013). Low concentrations of Phi increase catalase activity, while medium and high levels of Phi significantly reduce enzyme activity under P deficiency (Ávila et al., 2013). Plants experiencing either Pi deficiency or excessive Phi exhibit reduced growth and diminished tolerance to toxicity (Trejo-Téllez and Gómez-Merino, 2018). These complex phenomena are attributed to the underlying overlapping of signaling pathways and their intricate interactions (Gómez-Merino and Trejo-Téllez, 2015).

Phi is highly effective in controlling a wide range of phytopathogens, including pathogenic bacteria, oomycetes, fungi, and nematodes (Grant et al., 1992; Lobato et al., 2010). Phi or chemical formulations containing Phi-active ingredients are used to control various plant pathogens by activating the plant defense system (Thao et al., 2008b). Although Phi has demonstrated a beneficial role in enhancing plant tolerance to biotic stress, the underlying mechanisms of its response remain unclear. Based on various experimental studies, numerous discussions have focused on how plants respond to biotic stress in the presence of Phi. (1) Phi directly targets pathogens and induces plants to produce inhibitors of toxic substances (Thao et al., 2008b). For example, Phi-induced disease resistance proteins rely on the salicylic acid (SA) pathway to elicit defensive effects (Havlin and Schlegel, 2021). While it is primarily hypothesized that Phi acts indirectly on plant diseases, it can also directly enhance plant health by controlling specific fungi on cultivated or wild plants. Overall, Phi serves as an initiator of multiple plant defense responses (Havlin and Schlegel, 2021) and operates through a complex mechanism to prevent oomycete infections. (2) Phi directly increases cell wall thickness, preventing further invasion and expelling pathogens from plant tissues (Carswell et al., 1996). It inhibits hyphae growth and spore germination, and suppresses or modulates membrane metabolism and phosphorylation reactions of pathogens. Phi also acts indirectly by activating plant defense responses (Daniel and Guest, 2005). By disrupting cellular sugar metabolism, Phi induces a state of sugar deprivation within plant cells, which stimulates the expression of chitinase genes, leading to the continuous degradation of newly synthesized chitin at the tips of mycelia (Andreu et al., 2006; Yang et al., 2006). Notably, the complexity of this defense mechanism prevents pathogens from developing resistance to these inhibitory effects.

In agricultural production, Phi has been shown to effectively control pathogens in over 20 crops, including rice (Huang et al., 2020; Martínez, 2016), corn (Dias-Arieira et al., 2012), wheat (Oka et al., 2007), soybean (Dias-Arieira et al., 2012; Silva et al., 2011), common bean (Fagundes-Nacarath et al., 2018), potato (Borza et al., 2014; Burra et al., 2014; Lobato et al., 2011; MaChinandiarena et al., 2012), tomato (Su et al., 2022), strawberry (Marin et al., 2023), and grape (Speiser et al., 2000). The diseases controlled mainly include downy mildew, late blight, root rot, white mold, dieback, bacterial wilt, canker and others. It is worth noting that Phi exhibits varying effects against different pathogens across different crops. Table 1 outlines the optimal concentrations of Phi for controlling various crop diseases.

Phi effectively reduced potato tuber disease symptoms caused by Phytophthora, Fusarium wilt, and Rhizoctonia solani (Lobato et al., 2010). Copper phosphite (CuPhi) significantly resisted four pathogens, including Phytophthora infestans, Fusarium solani, Rhizoctonia solani, and Streptomyces scabies. Similarly, calcium Phi (CaPhi) and potassium Phi (KPhi) exhibited comparable capability in defending against pathogens (Lobato et al., 2010). In potatoes, 1% and 0.67% potassium Phi inhibited the growth of Streptomyces by nearly 80% and 60%, respectively (Lobato et al., 2010). Potassium Phi has been used in combination with the biological control agent Bacillus amyloliquefaciens OPF8 (strain F8) to control bacterial wilt in tomatoes (Su et al., 2022). A 0.05% concentration of KPhi significantly inhibited the growth of Ralstonia solanacearum (Su et al., 2022). Similarly, 0.05% K-Phi enhanced plants’ resistance to B. amyloliquefaciens F8 (Su et al., 2022). Martínez (2016) reported that potassium Phi, when combined with fungicides, strobilurin, and triazole, effectively inhibited rice stem rot caused by Nakataea oryzae (Martínez, 2016). In a large-scale field trial, Liljeroth et al. (2016) found that combining KPhi with half-dose fluoscymidone (0.2 L/hm²) was more effective in protecting against late blight in potatoes (Liljeroth et al., 2016). Phi is highly effective in controlling avocado (Persea americana) root rot and stem rot. However, long-term use of Phi resulted in some degree of resistance. For example, the roots of young Lupinus angustifolius seedlings were more susceptible to colonization by the Phi-resistant Phytophthora cinnamomi strain, which produced more cysts and zoospores compared to sensitive strains (Hunter et al., 2023). KPhi inhibited Phytophthora cinnamomi spore formation on Banksia grandis and Eucalyptus marginata in greenhouses (Wilkinson et al., 2001). Moreover, potassium Phi effectively reduced the production of zoospores on susceptible plants (Wilkinson et al., 2001).

Phi is mainly applied through foliar spraying. Other application methods include root irrigation, drip irrigation, hydroponic nutrient solution mixing, and immersion treatment (Achary et al., 2017). Foliar spraying of KPhi on winter wheat effectively reduced the occurrence of snow plum leaf blight and yellow sickle pathogens (Hofgaard et al., 2010). Phi spraying also significantly reduced the incidence and severity of scab on walnut leaves and fruits (Clive H. Bock et al., 2012). In addition, foliar spraying of Phi effectively mitigated the severity of late blight on potato tubers (Singh et al., 2003) and decreased the damage caused by fungal downy mildew in soybeans (Silva et al., 2011). Dias-Arieira et al. (2012) reported that potassium Phi effectively reduced the population of short-bodied nematodes (Pratylenchus brachyurus) by stimulating plant defense mechanisms, including phytoalexin production (Dias-Arieira et al., 2012).

In recent years, Phi-based fungicides have dominated the market. Notably, Bayer Crop Science has developed two globally recognized brands, Aliette and Fosetyl-Al, both of which feature Phi as their active ingredient. Other manufacturers also offer Phi-based fungicides that contain potassium, ammonium (NH₄ ⁺), sodium (Na), and aluminum (Al) under various commercial brands (Achary et al., 2017). In 2023, the global market value of potassium Phi fungicides was reported to be $107 million (Market Reports World Global Potassium Phosphite Market—Market Reports World). Given Phi’s disease prevention properties, its market competitiveness has significantly increased, and it is anticipated that Phi fertilizers will be extensively utilized in future agricultural practices.

In nature, the transformation of Phi to Pi through Phi oxidation and Pi reduction depends on the dominant microbial populations. However, Pi reduction is less commonly reported compared to Phi oxidation. Phi oxidation is an energy-efficient process, and some microorganisms oxidize Phi to Pi, subsequently using it as a source of P for cellular uptake (Ewens et al., 2021; Liu et al., 2023). The pathways of Phi oxidation include assimilatory phosphite oxidation (APO) and dissimilatory phosphite oxidation (DPO) (Figueroa and Coates, 2017; Liu et al., 2023).

Phi is oxidized into Pi by APO microorganisms and subsequently metabolized by microbial cells (Ewens et al., 2021; Metcalf and Wolfe, 1998; White and Metcalf, 2004). To date, more than 20 microorganisms, including proteobacteria, firmicutes, and cyanobacteria, have been isolated and shown to possess APO capability under laboratory conditions (Liu et al., 2023). Genetic and biochemical studies of some of these organisms have revealed several enzymes capable of oxidizing Phi. Currently, three APO enzymes have been characterized: C-P lyase, alkaline phosphatase (BAP), and phosphite dehydrogenase (ptxD). C-P lyases, found in Escherichia coli, are able to metabolize Phi into phosphonates (Seweryn et al., 2015), although the exact reaction mechanism remains unknown. Inferencing the known mechanism by which methylphosphonate is degraded by C-P lyase, the oxidation of Phi to Pi may involve cleavage of free radical P-H bonds ( Figure 3 ). Phi is also oxidized by the bacterial BAP enzyme in E. coli. BAP, a periplasmic protein encoded by the phoA gene ( Figure 3 ), is involved in the hydrolysis of Pi for P acquisition during Pi starvation (Yang and Metcalf, 2004). In addition, BAP oxidizes Phi to Pi and produces a hydrogen molecule in vitro (Yang and Metcalf, 2004).

In 1998, Metcalf and Wolf discovered that Pseudomonas stutzeri WM88 oxidized Phi to Pi (Metcalf and Wolfe, 1998). Subsequently, Costas et al. (2001) identified phosphite dehydrogenase (PTDH) from Pseudomonas stutzeri WM88, which can oxidize Phi (Costas et al., 2001). The PTDH enzyme complex contains five genes, ptxA, ptxB, ptxC, ptxD, and ptxE, that are involved in hypophosphite metabolism and belong to the ptx gene family. The proteins encoded by ptxABC are associated with the uptake and transport of Phi (Bisson et al., 2017), while ptxD encodes PTDH and while ptxE encodes a transcriptional regulatory factor (White and Metcalf, 2004). The PtxD enzyme oxidizes Phi in vitro using oxidized nicotinamide adenine dinucleotide (NAD⁺) as the only cofactor, producing Pi and reduced NAD (NADH) ( Figure 3 ) (White and Metcalf, 2004). It only uses NAD⁺ as its coenzyme for catalysis and exhibits an extremely low affinity for NADP⁺ (White and Metcalf, 2004). The reaction catalyzed by PTDH proceeds in a continuous and orderly manner: PTDH first binds to NAD⁺, undergoes a conformational change, and forms a pocket-like structure that binds to the Phi substrate. The product, orthophosphate, is released first, followed by NADH once the catalytic reaction is complete.

In the DPO process, Phi serves as an electron donor and energy source for microbial growth and carbon fixation (Helder et al., 2000). DPO microorganisms include Desulfotignum phosphitoxidans FiPS-3, Candidatus Phosphitivorax anaerolimi strain Phox-21 (Figueroa and Coates, 2017; Schink et al., 2002), and the Phosphitivorax strain Ca. P. anaerolimi F81, which is isolated from Danish wastewater (Figueroa et al., 2017). D. Phosphitoxidans strain FiPS-3 is a newly discovered sulfate-reducing bacterium isolated from marine sediments. It has a slow propagation rate, doubling its population in 72 to 80 hours. It uses Phi as the sole electron donor and CO₂ as the only carbon source. It can oxidize substrates, such as Phi, fumarate, pyruvate, glycine, glutamate, and maleate while reducing sulfate to sulfide (Poehlein et al., 2013; Schink et al., 2002). The five genes associated with D. Phosphitoxidans (ptdFCGHI) have only been identified in strain FiPS-3. PtdC is an inner membrane transporter facilitates Phi uptake, likely functioning as a Phi/Pi antiporter. PtdFGHI is potentially involved in energy conservation during DPO, but its function has not been experimentally confirmed (Figueroa et al., 2017; Figueroa and Coates, 2017; Poehlein et al., 2013).

Accumulation of Phi in plants significantly inhibits growth and can lead to death by interfering with the signaling pathways of PSR (Singh et al., 2003). The discovery of PTDH in microorganisms has enabled the use of Phi as an effective Phi-based fertilizer and herbicide (Achary et al., 2017; Heuer et al., 2017). The development of the ptxD/Phi system offers an efficient solution to the current challenges of P availability in soil and the evolution of herbicide-resistant weeds ( Table 2 ). Phi has been used as a broad-spectrum and non-selective herbicide. It also improves crop productivity (Achary et al., 2017) and reduces Pi input by 30% to 50% (Herrera-Estrella and Lopez-Arredondo, 2016).

To our knowledge, Lopez-Arredondo and Herrera-Estrella (2012) overexpressed the ptxD gene in Arabidopsis and tobacco (Nicotiana tabacum), enabling the transgenic plants to oxidize Phi to Pi. Phi provides P nutrition, thereby reducing Pi fertilizer use by 30%–50% while effectively inhibiting the growth of weeds, such as Brachypodium distachyon, Alexander grass (Brachiaria plantaginea), morning-glory (Ipomoea purpurea), and smooth pigweed (Amaranthus hybridus). Foliar application of Phi can effectively inhibit or even eradicate broad-leaved weeds (Lopez-Arredondo and Herrera-Estrella, 2012). The transgenic rice lines harboring the ptxD gene exhibited vigorous growth and normal development of root systems. Additionally, both the aboveground and underground biomass of these transgenic plants were significantly higher than those of the wild-type counterparts (Manna et al., 2016). Nahampun et al. (2016) introduced a codon-optimized ptxD gene into corn. The callus tissue of the transgenic corn could grow normally on the medium with Phi as the P source (Nahampun et al., 2016). Similarly, Pandeya et al. (2018) overexpressed the ptxD gene in cotton (Gossypium hirsutum), which grew normally with Phi as the sole P source (Pandeya et al., 2018). The transgenic cotton plants accumulated 330%–480% more biomass than those grown under Pi conditions when cultivated with 80 mg/kg or 120 mg/kg Phi. Phi also significantly inhibited the growth of the glyphosate-resistant weed Palmer amaranth (Amaranthus palmeri) (Table 2) (Pandeya et al., 2018). Xu et al. (2024) transformed ptxD into rapeseed (Brassica napus) ( Table 2 ). Notably, the transgenic rapeseed grew normally, while weeds were significantly inhibited when subjected to foliar fertilization with 200 mM K₂HPO₄ (Xu et al., 2024) ( Table 2 ).

Hirota et al. (2012) isolated a soluble and heat-resistant Phi dehydrogenase, PTDH-R, from Ralstonia sp. strain 4506, which exhibited a higher affinity for Phi and a catalytic efficiency nearly six times that of PTDH-P (Hirota et al., 2012). The stability of the two catalytic domains may increase when the 139^th amino acid of PTDH mutates from tyrosine to glutamine or phenylalanine, enhancing PtxD activity (Liu et al., 2021). Overexpression of the mutated ptxD_Q in Arabidopsis and rice effectively promoted plant growth when Phi was the sole P source (Liu et al., 2021). Enrique Asin-Garcia et al. (2022) integrated the Phi assimilation gene into the Pseudomonas putida KT2440 genome and knocked out the orthophosphate transporter gene to produce the PSAG-9 strain (Asin-Garcia et al., 2022). Notably, PSAG-9 acquired the ability to utilize Pseudomonas putida and could be cultured with Phi as the sole P source under non-sterile conditions (Asin-Garcia et al., 2022). These findings highlight the potential for Phi industrial application and release into the environment.

In 2004, Yang and Metcalf identified a BAP enzyme from E. coli phn mutants that was involved in Phi oxidation (Yang and Metcalf, 2004). Ram et al. (2019) transformed the codon-optimized phoA gene of Desulfotignum phosphitoxidans into rice. The results showed that transgenic rice grew healthily under 15 mM Phi, and the seeds germinated normally under 10 mM Phi (Ram et al., 2019). In addition to the transgenic rice, weeds such as Phyllanthus urinaria, Portulaca oleracea and Amaranthus sp. were arrested when sprayed with 100 mM potassium Phi (Ram et al., 2019) ( Table 2 ).

Traditional herbicides target specific enzymes by binding to their catalytic sites of the enzyme (Heap and Duke, 2018). However, a few mutations in the active site can significantly reduce the binding of the herbicide to its target site, leading to the rapid evolution of herbicide-resistant weeds. In contrast, the utilization of Phi significantly reduces the probability of plant mutations because the development of point mutations requires the replacement of amino acids in multiple target proteins within the cell, which is highly unlikely to occur. Dominant mutations in multiple targets in the cell are lethal to the entire plant and are, therefore, completely unfeasible (Achary et al., 2017). Moreover, Phi can be degraded by soil microorganisms with no residual effects on subsequent crop rotations, which benefits both the environment and human health (Achary et al., 2017). Therefore, the dual fertilization and weed control system, which allows plants to use Phi as the sole P source, can potentially slow the evolution of herbicide-resistant weeds. To date, no genetically modified crops expressing recombinant ptxD or BAP have been commercially released worldwide.

Phi-based fertilizers may reduce the occurrence of algae because Phi does not promote algal reproduction and has no toxic effects on algae due to their inability to metabolize Phi (Achary et al., 2017; Loera-Quezada et al., 2015, 2016). In the long term, Phi does not pose a threat to the species diversity of algae in aquatic ecosystems and can help maintain the equilibrium of the ecosystem (Loera-Quezada et al., 2015). Loera-Quezada et al. (2016) demonstrated that the introduction of ptxD into the nucleus of Chlamydomonas reinhardtii yielded transgenic lines capable of growing in media containing Phi as the sole P source, even under non-sterile conditions. These strains have a significant selective advantage over contaminating or competing species (Loera-Quezada et al., 2016). Changko et al. (2020) expressed the ptxD gene in the chloroplasts of Chlamydomonas reinhardtii, enabling it to grow in Phi-based media without any negative effects on its growth rate. However, the growth of Chlamydomonas reinhardtii in Phi media was severely hindered under contaminated microbial conditions (Changko et al., 2020).

Cutolo et al. (2020) transferred a mutated version of the ptxD gene into Chlamydomonas reinhardtii and used a mixture of NADP ⁺ and NAD⁺ to efficiently convert Phi into Pi, making this system an environmentally friendly alternative to antibiotic resistance genes for large-scale cultivation and application (Cutolo et al., 2020). Enrique Asin-Garcia et al. (2022) integrated the Phi assimilation gene into the Pseudomonas putida KT2440 genome and knocked out the orthophosphate transporter gene to produce the PSAG-9 strain (Asin-Garcia et al., 2022). PSAG-9 acquired the ability of Pseudomonas putida and could be cultured with Phi as the sole P source under non-sterile conditions (Asin-Garcia et al., 2022). These findings highlight the potential industrial applications of Phi and its release into the environment. Phi effectively inhibits the large-scale reproduction of green algae when used as a Pi fertilizer. Using Phi as a Pi fertilizer is important for protecting water bodies from P pollution because algae metabolize and utilize Phi, which is nearly non-toxic to algae and other aquatic organisms.