André Fer

Physiological Changes in a Root Hemiparasitic Angiosperm, Thesium humile (Santalaceae), before and after Attachment to the Host Plant (Triticum vulgare)

Summary Thesium humile , an obligate root hemiparasitic angiosperm, can live in an autotrophic-like way during several weeks before attachment to the host. About 6 weeks after germination, a decreasing growth rate and symptoms of senescence of the seedling are observed. Before attachment, Thesium is able to take up water and inorganic ions from the soil, but its mineral content is characterized by very high Na and low P levels. During the same period, rates of net photosynthesis and dark respiration as well as RuBPc and PEPc activities decrease continuously. The expression of a photosynthetic gene like psbA also decreases and the amount of transcripts of the rbcL gene is very low. After attachment to the host, Thesium recovers a more physiological balance by deriving inorganic ions from the host xylem sap. Increasing growth rate, photosynthetic carbon fixation, carboxylase capacities and expression of a photosynthetic gene like rbcL are observed and become fairly similar to those found in non-parasitic C 3 plants. A strong K accumulation in leaves also occurred and that might be an explanation for the observed anomalous stomatal behaviour. Before attachment, the poorly developed root system of Thesium seems unable to selectively absorb inorganic ions from the soil and to maintain a physiological mineral balance. Consequential to the very high Na and the low P contents in the seedling of the parasite, metabolic perturbations affecting also gene expression might result.

Characteristics of the mannitol pathway in a root hemiparasitic species, Thesium humile Vahl. (Santalaceae)

Summary 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 CO 2 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 (M1Pase). 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.

Trophic relations of the dwarf mistletoe Arceuthobium oxycedri with its host Juniperus oxycedrus

Summary A basipetal transfer of 14 C-labelled assimilates is demonstrated from the host needles towards the aerial shoots of the parasite that acts as sink here. However, the endophytic system (= haustorium) of the parasite accumulates almost no radioactivity. The transferred substances are essentially soluble carbohydrates and amino acids (mainly asparagine). In the experimental conditions, the amount of carbon obtained from the host by the parasite may be estimated to be approximatively equal to the amount that it can fix from atmospheric CO 2 . The comparative study of mineral elements by X-ray microanalysis shows that the endophytic tissues of the parasite are richer in S, P and chiefly K than the close neighbouring host tissues, and consequently that the [K]/[Ca] ratio is higher in the parasite. It thus seems that mineral elements as well as organic compounds can be withdrawn by the endophytic system from both xylem and phloem of the host.

Mannitol metabolism in darkness in the leaves of the hemiparasitic angiosperm, Thesium humile

Abstract Thesium humile and wheat were submitted to a prolonged dark period following photosynthesis. Measurements of leaf respiratory rates indicated that the plants were still in a good physiological state at the 100 th h of darkness and that the parasitic weed exhibited a rapid decrease in respiratory metabolism during the first 6 h of the treatment. After 100 h in darkness, T. humile showed a much stronger decrease in mannitol content than in sucrose and starch. Mannitol utilization as a respiratory substrate in darkness was demonstrated by supplying leaves with [ 14 C]mannitol. The highest rate of hexitol metabolism occurred in the young leaves, but this metabolism was not totally restricted to growing organs. In the young leaves, [ 14 C]mannitol utilization led to 14 CO 2 release and 14 C-incorporation into sucrose, hexose, soluble anionic and cationic substances and insoluble material. Moreover, the sucrose pool exhibited a rapid turnover. Enzymes responsible for these carbon fluxes were then sought. Mannitol dehydrogenase, sucrose-phosphate synthase, sucrose synthase and neutral invertase activities were detected in the young leaves. Carbon metabolism and respiratory rate are clearly controlled by the mannitol oxidizing enzyme. These results could be used in order to define a chemical control method against the mannitol-producing parasitic weeds.

Activity and immunocytochemical localization of glutamine synthetase inLathraea clandestins L.

SummaryThe activity of glutamate synthetase (GS) was determined in the different organs ofLathraea clandeslina L., a holoparasitic Scrophulariaceae. It was very low throughout the plant but levels were slightly higher in the scale leaves. Immunoprecipitation reactions carried out with immune serums raised against the isoforms GS1 or GS2 of the enzyme showed that, in the scale leaves, isoenzyme GS1 was present, but the existence of small amounts of GS2 remained in question on account of possible cross reactions. On the other hand, the study of intracellular localization of GS in the scale leaves by indirect immunofluorescence, using the same antibodies anti-GS1 and anti-GS2, clearly demonstrated the occurrence of two GS forms: a GS1 isoenzyme located in the cytoplasm of glandular and parenchymatous cells and a GS2-type isoenzyme only detected in the stroma of the large amyloplasts present in the outer parenchyma. This amyloplastidial isoenzyme seems to be a peculiar GS form, distinct from both GS1 and GS2.