Aminosäuretransport in Raps unter besonderer Berücksichtigung des Entwicklungsstadiums der Pflanze und der Stickstoffdüngung

Abstract

Oilseed rape (Brassica napus L.) is the most important oil crop plant in Europe. Despite a very efficient uptake of nitrogen (N) from the soil, oilseed rape has a low nitrogen use efficiency and needs substantial fertilizer applications. The plant sheds its leaves early during flowering. In this Ph.D thesis the question was addressed whether inefficient remobilization of amino-N from senescing leaves is a cause of the inefficient nitrogen use. The main transport form of reduced N in plants are amino acids. They are translocated from source organs to sink organs via the phloem. This necessitates an active uptake of amino acids into the sieve elements and companion cells of the phloem. Several classes of transport proteins functioning as secondary active amino acid-proton-symporters have been identified from Arabidopsis and other plant species. The amino acid permeases (AAPs) have a very broad substrate specificity and probably are responsible for phloem loading of all neutral and acidic amino acids. Two AAP cDNAs were cloned from oilseed rape and called BnAAP1 and BnAAP6 with reference to their closest Arabidopsis homologues. The functionality of the transporters was demonstrated by heterologous expression in Xenopus oocytes. It turned out that BnAAP1 and BnAAP6 both are able to transport amino acids even in the low substrate concentrations which are usually found in the apoplast. In their substrate specificities and organ-specific expression profiles the transporters mostly resembled their Arabidopsis counterparts. In senescing leaves, no decrease in the expression of BnAAP1 or BnAAP6 on the mRNA level was observed. On the contrary, BnAAP1 expression even increased with leaf age. Nonaqeous fractionation of oilseed rape leaves showed amino acid concentrations in all subcellular compartments to be rather high. Excess amino acids were stored mainly in the vacuole. Although amino acid concentrations decreased with leaf age in all compartments, senescing leaves still contained amounts of amino-N similar to mature source leaves in other species. Together with the high amino acid concentrations in oilseed rape phloem sap which had previously been reported, these data indicate that amino-N remobilization from the leaves is highly efficient and the reasons for the low nitrogen use efficiency must lie elsewhere. However, a comparative analysis of nitrogen metabolism in 45 double haploid lines deriving from a single crossing showed a high variability, indicative of a genetic determination of nitrogen use efficiency. As a possible way to genetically improve nitrogen use efficiency in oilseed rape, delay of leaf senescence past the induction of flowering is suggested.