Carbon Acquisition and Metabolism in a Root Hemiparasitic Angiosperm, Thesium humile (Santalaceae) Growing on Wheat (Triticum vulgare)

Abstract

Thesium humile Vahl (Santalaceae) is a root-hemiparasite which causes economic losses in cereal crops in the Mediterranean countries. O2 exchange measurements showed that photosynthesis does occur in Thesium. Experiments using 14CO2 gave additional evidence of the photosynthetic capacity of T. humile at both preparasitic and parasitic stages; roots of T. humile, unlike those of non-parasitic plants, exhibited a low sink strength for photosynthates. The end product of photosynthesis in T. humile is mannitol which is also the main carbohydrate translocated in the phloem. Nevertheless, feeding host leaf with [3H]glucose clearly demonstrated that the parasite, in spite of its photosynthetic capacity, remained dependent upon its host for an additional supply of reduced carbon. Moreover, in isolated T. humile shoots supplied with [14C]sucrose via the transpiration stream, the absorbed sucrose was rapidly converted into mannitol. Thus the parasite would be able to convert sucrose derived from the host plant into mannitol. The present work gives clear evidence that mannitol can be regarded as the major soluble carbohydrate in the carbon metabolism and in the translocation of reduced carbon in T. humile. The mannitol synthesis pathway might be a metabolic target for the chemical control of this parasite in cereals, in which this polyol has never been detected.

Characterisation of Simple In Vitro Cultures of Striga hermonthica Suitable for Metabolic Studies

“…While metabolic profiling in plants is increasing (Fiehn et al, 2000), such studies have not been carried out so far on a parasitic higher plant, and information on biochemistry remains poor in S. hermonthica. Nevertheless, most of the metabolic studies carried out on S. hermonthica and some other root parasites have focused on the specific trait of mannitol accumulation (Nour et al, 1984;Stewart et al, 1984;Wegmann, 1986;Press, 1989;Fer et al, 1993;Press, 1995;Pageau et al, 1998). Unlike the situation in the infested plants, mannitol is a key compound in physiology and carbon metabolism in these root parasites.…”

Habitat requirements and host selectivity of Thesium species (Santalaceae)

“…This is in agreement with the conclusions of Lawrence & Kaye (2008), who found higher survival of Castilleja levisecta in treatments without any host added. Similarly, Fer et al . (1993, 1994) suggested in their studies that T. humile Vahl is able to germinate in the absence of host chemical stimulants.…”

“…are well‐known agricultural weeds (e.g. Fer et al ., 1993), whereas others are endangered (e.g. T. ebracteatum , T. rostratum and T. arvense ; Holub & Procházka, 2000).…”

Biological and Structural Aspects of Parasitism

“…Years ago, I was able to maintain Gaiadendron punctatum alive and vigorous for nearly a year, after which an accident terminated the effort. Thesium humile is able to grow without host contact for 1-3 months following germination, although at a low level of photosynthesis (Fer et al 1993). I have not been able to find any information on seedling survival of any other Santalales except for Comandra (Piehl 1965a), where it seems to have several environmental constraints; Barber (as cited in Pilger 1935) speaks of more than a year for Santalum album.…”

“…Santalum album shows superior growth performance and higher photosynthetic rates and foliar N concentrations when partnered with legumes rather than Eucalyptus. A different study was carried out by Fer et al (1993) on the parasitism of Thesium humile on Triticum, where it was shown that sucrose absorbed from the host is rapidly converted to mannitol. The latter also seems to be the end product of the parasite's own photosynthesis.…”