The rare sugar D-tagatose protects plants from downy mildews and is a safe fungicidal agrochemical

The study investigates whether the rare sugar D-tagatose can function as an eco-friendly agrochemical to control plant diseases, with emphasis on downy mildews. Rare sugars are monosaccharides scarcely present in nature; the Izumoring concept enabled their enzymatic production. Prior rare sugars such as D-allulose and D-allose can induce plant defense responses or growth effects, typically via phosphorylation at C-6 in plant cells. Given D-tagatose’s established safety as a food ingredient and commercial availability, the authors hypothesize that D-tagatose may control plant pathogens, potentially by interfering with pathogen carbohydrate metabolism rather than activating host defenses. The purpose is to define its disease control spectrum and elucidate its mode of action using Arabidopsis–Hyaloperonospora arabidopsidis as a model.

Previous work showed D-allulose and D-allose exert antihyperglycemic and anti-aging effects in animals and in C. elegans, and in plants they can induce disease tolerance via transient ROS production and PR gene activation, with effects dependent on phosphorylation at the C-6 position by plant hexose kinases. Plant defense induction by these sugars involves pathways linked to hormones like gibberellin signaling and results in lesion mimic formation. Downy mildews possess mannan-rich cell walls and genome-scale models suggest fructose/mannose metabolism is central to oomycete survival. Disruption of phosphomannose isomerase impairs pathogenic fungi such as Cryptococcus neoformans and Aspergillus nidulans, implicating mannose metabolism in fungal development. D-tagatose has been detected in natural contexts (heated milk, plant exudates) and is GRAS by FDA and evaluated by WHO/FAO, but its fungicidal properties and mechanisms in plants had not been reported prior to this study.

The authors conducted pot and field trials to determine effective concentrations (w/v) of D-tagatose that reduce disease severity by ≥50% across multiple host–pathogen pairs including oomycetes (downy mildews, Phytophthora spp., Pythium spp.), ascomycetes (powdery mildews, Botrytis, Alternaria, Colletotrichum), and basidiomycetes (rusts, Rhizoctonia). Pot trials involved foliar sprays of 0.5–10% D-tagatose with a sticker; severity was scored 7 days post-inoculation. Preventive (1–7 days before inoculation) and curative (12–72 h post-inoculation) timings were compared against fungicides (PBZ, ASM, metalaxyl). Field trials on grapevine, cucumber, Chinese cabbage, onion, and spinach used 1% or 5% D-tagatose vs registered fungicides (cyazofamid, chlorothalonil, mancozeb), with disease severity rated per JPPA guidelines. In the model Arabidopsis–Hyaloperonospora arabidopsidis Noco2 system, seedlings were treated with 0–100 mM D-tagatose (or 1% w/v) prior to inoculation. Disease symptoms, hyphal growth (trypan blue staining), and asexual/sexual reproduction (conidiospore counts from seedlings, conidiophores and oospores per cotyledon) were quantified. The effect of deoxygenated D-tagatose at C-6 (6-deoxy-D-tagatose) was tested. In vitro assays measured conidiospore germination and germ tube length in microtiter plates with 0–400 mM D-tagatose or D-mannitol controls. To assess host defense activation, cucumber PR/defense gene expression (POX, LOX, Hrf, 4CL, CCoAMT) was evaluated by RT-qPCR after 1% D-tagatose with/without Pseudoperonospora cubensis inoculation. In rice, microarray analyses compared PR gene expression after 0.5 mM D-tagatose, D-allulose, D-allose, or D-glucose. In Arabidopsis, RNA-seq profiled defense and signaling pathway genes after inoculation with/without 1% D-tagatose. To probe pathogen carbohydrate metabolism, crude enzyme extracts from H. arabidopsidis conidiospores were incubated with D-tagatose and analyzed by HPLC (ABEE labeling) to detect D-tagatose 6-phosphate. Candidate sugar kinases (fructokinase LC500344, glucokinase LC500564, xylulose kinase LC500562, ribokinase LC500561) and phosphomannose isomerase (PMI; LC500563) from H. arabidopsidis were cloned and heterologously expressed in E. coli. Enzyme activities were characterized by HPLC and spectrophotometric coupled assays. Kinetic parameters for fructokinase with D-fructose and D-tagatose were determined, and competitive inhibition by D-tagatose vs D-fructose was analyzed (Dixon plots). The effect of D-tagatose 6-phosphate (12.5–37.5 mM) on PMI bifunctional activity (conversion between F6P ↔ M6P and F6P ↔ G6P) was quantified by HPLC peak areas. Statistical analyses used Tukey–Kramer tests (p<0.05).

• D-tagatose effectively reduced disease severity by ≥50% in pot and/or field trials across diverse crop–pathogen combinations. For downy and powdery mildews, effective doses were low (0.5–1% w/v). Examples (Table 1): Pseudoperonospora cubensis on cucumber (0.5% pot and field), Plasmopara viticola on grapevine (1% pot; 3% field), Peronospora destructor on onion (1% field), Peronospora farinosa f. sp. spinaciae on spinach (1% field). Necrotrophs generally required higher concentrations (>5%).
• In cucumber downy mildew pot trials, 5% D-tagatose applied 5 days before inoculation prevented symptoms comparably to PBZ, ASM, or metalaxyl. No phytotoxicity was observed for D-tagatose, whereas D-allose and D-allulose inhibited leaf development.
• Preventive and curative activity: Applications 5–7 days before inoculation eliminated or strongly suppressed symptoms. Applied within 48 h post-inoculation, D-tagatose prevented symptoms; applications at ≥60 h post-inoculation were less effective. Residual efficacy persisted up to 7 days.
• Field trials: 1–5% D-tagatose suppressed downy mildew symptoms on grapevine, cucumber, Chinese cabbage, onion, and spinach to levels comparable to cyazofamid, chlorothalonil, or mancozeb.
• Arabidopsis–H. arabidopsidis Noco2: 1% (≈50 mM) D-tagatose suppressed symptoms and hyphal proliferation. Dose-dependent reduction in hyphal growth: leaves with 75–100% colonization fell from ~58% (mock) to ~7–10% at 25–100 mM. Conidiation was significantly reduced even at 2.5 mM and almost abolished at ≥25 mM; conidiophore and oospore formation were similarly suppressed.
• In vitro, 50 mM D-tagatose reduced conidial germination by ~47.6% vs control and inhibited germ tube elongation at ≥25 mM (no additional dose response up to 400 mM). D-mannitol had no effect. 6-deoxy-D-tagatose (50 mM) did not inhibit germination or conidiation, highlighting the necessity of C-6 phosphorylation.
• Host defense activation was minimal: In cucumber, 1% D-tagatose did not significantly induce PR/defense genes (POX, LOX, Hrf, 4CL, CCoAMT) within 48 h, with only minor LOX induction at 24 h post-inoculation. In rice, PR genes strongly induced by D-allulose or D-allose were not significantly induced by D-tagatose. Arabidopsis RNA-seq revealed no consistent up- or downregulation of PR, hormone-related, or MAMPs-response genes attributable to D-tagatose.
• Pathogen metabolism: Crude extracts from H. arabidopsidis phosphorylated D-tagatose to D-tagatose 6-phosphate (T6P). Heterologously expressed fructokinase (LC500344; 58 kDa) catalyzed phosphorylation of D-fructose (to F6P) and D-tagatose (to T6P) with kinetics: D-fructose Km 0.197 ± 0.044 mM, Vmax 3.48 ± 0.89 µmol mg−1 min−1; D-tagatose Km 52.67 ± 22.75 mM, Vmax 0.15 ± 0.04 µmol mg−1 min−1. D-tagatose competitively inhibited fructokinase with Ki 70.05 ± 5.14 mM; at 250 mM D-tagatose and 0.5 mM D-fructose, F6P production was inhibited by ~41% (≈13% at 50 mM D-tagatose).
• Phosphomannose isomerase (PMI; LC500563; 45 kDa) exhibited bifunctional PMI/PGI activity, producing M6P and G6P from F6P. T6P (12.5–37.5 mM) significantly inhibited PMI activity: M6P production reduced by 51.6–60.8% (p=8.85e−7 to 2.47e−7) and G6P by 19.3–38.2% (p=3.53e−4 to 2.25e−6).
• Mechanistic model: D-tagatose competitively inhibits fructokinase-mediated phosphorylation of D-fructose, and its product T6P inhibits PMI, thereby reducing glycolysis substrates (G6P, F6P) and M6P needed for mannan biosynthesis, leading to failure of infection structures and reproductive structures (conidiophores, conidia, oospores) in downy mildew.

The results support the hypothesis that D-tagatose protects plants by acting directly on pathogens rather than by eliciting host defenses. Its strong efficacy against downy mildews and powdery mildews, coupled with minimal host gene expression changes, indicates a pathogen-targeted metabolic disruption. Biochemical evidence identifies fructokinase and phosphomannose isomerase as sequential targets: D-tagatose competes with D-fructose for fructokinase, and the resulting D-tagatose 6-phosphate inhibits PMI’s bifunctional isomerase activity. This dual interference reduces key intermediates required both for energy metabolism (glycolysis) and for mannan-based cell wall biosynthesis, which is critical in oomycete development. These chain-inhibitory effects explain the pronounced inhibition of hyphal growth and asexual/sexual reproduction in H. arabidopsidis and align with observed preventive and early curative field efficacy. Because the inhibition is competitive and not absolute, dose plateaus observed at ≥25 mM are consistent with kinetic expectations near the Ki. The multi-site metabolic impact suggests a lower risk of resistance development compared with single-target fungicides and, combined with an excellent safety profile (GRAS), positions D-tagatose as a promising eco-friendly agrochemical.

This work demonstrates that D-tagatose is an effective and safe fungicidal agent against a broad spectrum of plant diseases, with notable potency against downy mildews and powdery mildews at low application rates and with no observable phytotoxicity. Using Arabidopsis–H. arabidopsidis as a model, the study elucidates a pathogen-directed mechanism: competitive inhibition of fructokinase and subsequent inhibition of phosphomannose isomerase by D-tagatose 6-phosphate, leading to reductions in glycolytic intermediates and mannan biosynthesis required for pathogen development. These findings open avenues for deploying rare sugars as novel agrochemicals and for formulation strategies that enhance efficacy while maintaining safety. Future research should refine formulations to reduce effective concentrations further, explore efficacy and mechanisms in other pathogen classes (especially powdery mildew with possibly differing targets), assess long-term resistance risk in field populations, and investigate ecological impacts of D-tagatose exudation and its role in shaping the plant microbiome.

• Mechanistic clarity is strongest for downy mildews; the precise targets in powdery mildews and other pathogen classes remain unresolved.
• In vitro inhibition of conidial germination and germ tube elongation was moderate compared with strong in planta suppression; variability in conidiospore physiological status may affect in vitro outcomes.
• Some pathogens (e.g., necrotrophs) required relatively high concentrations (>5–10%), which may limit practicality without optimized formulations.
• Host defense gene analyses did not reveal consistent transcriptional changes; while supporting a pathogen-targeted mode, subtle or transient host responses could have been missed.
• The equal-contribution superscript indicates author contribution rather than an affiliation; broader toxicological and environmental fate assessments beyond prior GRAS evaluations were not within the scope of this study.