A review of knowledge on the mechanisms of action of the rare sugar d-tagatose against phytopathogenic oomycetes

d-tagatose is a rare monosaccharide, naturally present at low concentrations in some fruits and dairy products. d-tagatose is "generally recognized as safe" and is used as a low-calorie sweetener in the food industry. It is able to inhibit the growth of numerous microorganisms, such as phytopathogenic oomycetes responsible for important crop diseases. Thanks to the negligible effects on human health and the environment, d-tagatose has been proposed as a sustainable product for crop protection. This review describes the current knowledge on modes of action of d-tagatose against phytopathogenic oomycetes and its potential uses in agriculture. d-tagatose can negatively affect the growth of phytopathogenic oomycetes by inhibiting key enzymes of sugar metabolism, such as β-glucosidase in Phytophthora infestans, and fructokinase and phosphomannose isomerase in Hyaloperonospora arabidopsidis. Moreover, d-tagatose affects sugar content, causes severe mitochondrial alterations, and inhibits respiration processes with the accumulation of reactive oxygen species in P. infestans, but not in P. cinnamomi. Differential effects of d-tagatose are associated with a global gene downregulation in P. infestans and with an efficient transcriptional reprogramming of multiple metabolic processes in P. cinnamomi. d-tagatose displays possible species-specific effects in Phytophthora spp. and nutritional properties on some plant-associated microorganisms. However, inhibitory effects are reversible and P. infestans growth can be restored in the absence of d-tagatose. Further functional studies are discussed in this review, in order to promote the use of d-tagatose for sustainable crop protection.