Journal of Experimental Botany, Vol. 54, No. 383, pp. 789-799,
February 1, 2003
© 2003 Oxford University Press
Characterization of nitrogen relationships between Sorghum bicolor and the root-hemiparasitic angiosperm Striga hermonthica (Del.) Benth. using K15NO3 as isotopic tracer
Received 24 June 2002; Accepted 22 October 2002
1 Groupe de Physiologie et Pathologie Végétales, Faculté des Sciences et Techniques, BP 92208, 2, rue de la Houssinière, F-44322 Nantes Cedex 3, France
2 LABERCA, École Nationale Vétérinaire de Nantes, Route de Gachet, BP50707, F-44307 Nantes Cedex 3, France
3 Laboratoire d’Analyse Isotopique et Electrochimique de Métabolismes, CNRS UMR6006, Faculté des Sciences et Techniques, BP 92208, 2, rue de la Houssinière, F-44322 Nantes Cedex 3, France
4 To whom correspondence should be addressed. Fax: +33 2 51 12 56 12. E-mail: philippe.simier{at}svt.univ-nantes.fr
5 Present address: Unité de Nutrition Azotée des Plantes, INRA de Versailles, Route de St Cyr, 78026 Versailles Cedex, France.
Abbreviations: AA(s): amino acid(s); AS, asparagine synthetase; GC-MS, gas chromatography–mass spectrometry; DM, dry matter; FAA(s), free amino acid(s); FW, fresh weight; GABA, 
-butyric acid; HPLC, high performance liquid chromatography; IRMS, isotopic ratio mass spectrometry; NR, nitrate reductase; WAE, weeks after emergence; WAS, weeks after sowing.
The role of the host in the nitrogen nutrition of Striga hermonthica (Del.) Benth. (Scrophulariaceae) parasitic on Sorghum bicolor cv. SH4 Arval has been investigated using 15N-nitrate as the tracer. It is shown that, when nitrate is absorbed only by the roots of the host plant, a rapid transfer of nitrogen to the parasite can be detected. The xylem sap of S. hermonthica contained approximately equal amounts of nitrate and amino acids, mostly glutamine and asparagine. Infection altered the free amino acid profile of the host tissues, leading notably to a large increase in asparagine and a decrease in glutamine. The haustoria of S. hermonthica, although rich in nitrate, showed a low concentration of free amino acids, particularly lacking in asparagine and glutamine. The roots of S. hermonthica, in contrast, were rich in both asparagine and glutamine while, in the shoots, asparagine constituted 80% of the total FAA pool. Asparagine was also found to be the primary 15N-enriched amino acid in the shoots of S. hermonthica while, interestingly, it was glutamate that was most strongly enriched in the roots. It is concluded that nitrogen nutrition in S. hermonthica is based on a supply of both nitrate and amino acids from the host. This implies a non-specific transfer in the transpiration stream. Nitrate reduction probably occurs mainly in the leaves of the parasite. Assimilation also occurs in S. hermonthica and excess nitrogen is stored as the non-toxic nitrogen-rich compound, asparagine. This specific trait of nitrogen metabolism of the parasite is discussed in relation to the effect of nitrogen fertilization on reducing infestation.
Key words: Asparagine, nitrate, nitrogen nutrition, parasitic weeds, sorghum, Striga.
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