CHARACTERISTICS OF THE MANNITOL PATHWAY IN A ROOT HEMIPARASITIC SPECIES, THESIUM HUMILE VAHL (SANTALACEAE)

The root hemiparasite, Thesium humile Vahl., causes damage in cereal crops in Mediterranean countries. It is studied here growing on wheat, from which it differs by the main accumulated soluble carbohydrate. Thesium contains effectively large amounts of mannitol while sucrose is typically accumulated in the host. The mannitol pathway may be thus a metabolic target for a specific chemical control of this hemiparasite. Identification and characterization of intermediary metabolites and enzymes required for mannitol production are related in this paper Mannitol biosynthesis from CO2 fixation occurred in aerial parts and especially in the young leaves. Fructose-6-P (F6P), resulting from the triose-phosphate (TP) conversion, is at first converted to mannose-6-P (M6P) with the mannose-6-P isomerase (M6PI). With the NADP-dependent mannose-6-P:mannitol-1-P oxydoreductase (M6PR), M6P is then reduced to mannitol-1-P (M-1P), which is finally dephosphorylated with the mannitol-1-phosphate phosphatase (M-1Pase). The values of optimum pH of these enzymes agree with a cytosolic location of the mannitol biosynthesis. Mannitol production is greatly regulated by temperature, notably concerning the M-1Pase. No correlation between mannitol export and increased mannitol synthesis has been observed in the expanding leaves. Activities are compared with those measured in other mannitol-synthesizing parasitic genera, such as Orobanche and Osyris. M6PI, and M-1Pase then appear as the limiting enzymes of the mannitol pathway in these plants. The role of the phosphate-pentose pathway (PPP) in the regeneration of the cytosolic NADPH required for mannitol biosynthesis is also discussed. These results allow to envisage further assays to inhibit the mannitol pathway in Thesium leaves, in order to develop a specific chemical control of this hemiparasitic weed in the future.