Xylem Transport and Storage of Amino Acids by S.W. Australian Mistletoes and their Hosts

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Abstract
Amino acid composition of xylem (tracheal) sap and ethanolic extracts of shoots of mistletoes (Amyema spp. and Lysiana casuarinae) and their hosts were compared, using material collected in their native habitats. Data indicated that certain host xylem solutes were transferred directly to the parasite xylem, while others were either not absorbed or were metabolized prior to transfer. Certain solutes were major constituents of parasite xylem, but undetected or only in trace amount in the host. Shoot amino acid pools of parasites differed markedly from those of hosts. The mistletoe, Amyema preissii, exhibited differential storage and transport of arginine when parasitizing three different species, but accumulated proline on only two of these hosts. Host- specific amino acids (djenkolic acid in Acacia saligna, and tyramine in Acacia acuminata) were transported and accumulated in relatively large amounts by the parasite, but were not detected in other associations. Proline was the major solute of Amyema linophyllum parasitizing Casuarina obesa, but arginine predominated in Lysiana casuarinae on the same host. However, when L. csuarinae parasitized A. linophyllum, in turn parasitic on C. obesa, the Lysiana accumulated equal amounts of proline and arginine and more asparagine than when directly on the Casuarina. Xylem feeding of 15N-labelled aspartic acid or 13N-(amide labelled) asparagine to cut shoots or whole haustoria-bearing plants of the mistletoe A. preissii resulted in 68–73% of the 15N of aspartate and 24–30% of that of asparagine appearing in ethanol-soluble shoot amino compounds other than the fed solute. 15N labelling patterns of detached shoots were not noticeably different from that of whole plants suggesting that the haustorium had relatively little effect on processing incoming solutes. Alanine, glutamine, and arginine were principal recipients of 15N from aspartate, alanine and glutamine in the case of fed asparagine. It is estimated that 24% of the carbon requirements for dry matter accumulation in Amyema linophyllm were met by intake of xylem sap solutes from its host Casuarina obesa.

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