The rare sugar D-tagatose is a safe natural product used as a commercial food ingredient. Here, we show that D-tagatose controls a wide range of plant diseases and focus on downy mildews to analyze its mode of action. It likely acts directly on the pathogen, rather than as a plant defense activator. Synthesis of mannan and related products of D-mannose metabolism are essential for development of fungi and oomycetes; D-tagatose inhibits the first step of mannose metabolism, the phosphorylation of D-fructose to D-fructose 6-phosphate by fructokinase, and also produces D-tagatose 6-phosphate. D-Tagatose 6-phosphate sequentially inhibits phosphomannose isomerase, causing a reduction in D-glucose 6-phosphate and D-fructose 6-phosphate, common substrates for glycolysis, and in D-mannose 6-phosphate, needed to synthesize mannan and related products. These chain-inhibitory effects on metabolic steps are significant enough to block initial infection and structural development needed for reproduction such as conidiophore and conidiospore formation of downy mildew.

External control of gene expression is crucial in synthetic biology and biotechnology research and applications, and is commonly achieved using inducible promoter systems. The E. coli rhamnose-inducible rhaBAD promoter has properties superior to more commonly used inducible expression systems, but is marred by transient expression caused by degradation of the native inducer, l-rhamnose. To address this problem, 35 analogues of l-rhamnose were screened for induction of the rhaBAD promoter, but no strong inducers were identified. In the native configuration, an inducer must bind and activate two transcriptional activators, RhaR and RhaS. Therefore, the expression system was reconfigured to decouple the rhaBAD promoter from the native rhaSR regulatory cascade so that candidate inducers need only activate the terminal transcription factor RhaS. Rescreening the 35 compounds using the modified rhaBAD expression system revealed several promising inducers. These were characterized further to determine the strength, kinetics, and concentration-dependence of induction; whether the inducer was used as a carbon source by E. coli; and the modality (distribution) of induction among populations of cells. l-Mannose was found to be the most useful orthogonal inducer, providing an even greater range of induction than the native inducer l-rhamnose, and crucially, allowing sustained induction instead of transient induction. These findings address the key limitation of the rhaBAD expression system and suggest it may now be the most suitable system for many applications.

Only d-allose, among various rare monosaccharides tested, induced resistance to Xanthomonas oryzae pv. oryzae in susceptible rice leaves with defence responses: reactive oxygen species, lesion mimic formation, and PR-protein gene expression. These responses were suppressed by ascorbic acid or diphenylene iodonium. Transgenic rice plants overexpressing OsrbohC, encoding NADPH oxidase, were enhanced in sensitivity to d-allose. d-Allose-mediated defence responses were suppressed by the presence of a hexokinase inhibitor. 6-Deoxy-d-allose, a structural derivative of d-allose unable to be phosphorylated, did not confer resistance. Transgenic rice plants expressing Escherichia coli AlsK encoding d-allose kinase to increase d-allose 6-phosphate synthesis were more sensitive to d-allose, but E. coli AlsI encoding d-allose 6-phosphate isomerase expression to decrease d-allose 6-phosphate reduced sensitivity. A d-glucose 6-phosphate dehydrogenase-defective mutant was also less sensitive, and OsG6PDH1 complementation restored full sensitivity. These results reveal that a monosaccharide, d-allose, induces rice resistance to X. oryzae pv. oryzae by activating NADPH oxidase through the activity of d-glucose 6-phosphate dehydrogenase, initiated by hexokinase-mediated conversion of d-allose to d-allose 6-phosphate, and treatment with d-allose might prove to be useful for reducing disease development in rice.

The enantiomers of tagatose are converted to L-DGJ [a noncompetitive inhibitor of human lysosome α-galactosidase A (α-Gal A), K(i) 38.5 μM] and DGJ [a competitive inhibitor of α-Gal A, K(i) 15.1 nM] in 66% yield. L-DGJ and DGJ provide the first examples of pharmacological chaperones that (a) are enantiomeric iminosugars and (b) have synergistic activity with implications for the treatment of lysosomal storage disorders and other protein deficiencies.