Transport, Metabolism, and Redistribution of Xylem-Borne Amino Acids in Developing Pea Shoots

Abstract

Amino acid metabolism and transport was investigated in the leaves of 3-week-old nonnodulated seedlings of Pisum sativum L. Xylem sap entering the shoot contained nitrate (about 5 millimolar), and amino compounds (11 millimolar) of which 70% was asparagine plus glutamine; aspartate and homoserine were also present. Mature leaves showed stable nitrogen levels and incoming nitrogen was redistributed to growing leaves. Younger leaves, still enclosed in the stipules, showed negligible rates of transpiration, suggesting that most of their nitrogen must arrive in the phloem."C-Labeled amides and amino acids were supplied to detached shoots through the xylem, and metabolism and redistribution were followed over 12 hours in light. Asparagine entering mature leaves was reexported directly to young leaves, with relatively little metabolic conversion. Substantial amounts of glutamine were converted to glutamate, which was exported (with unchanged amide) with little further conversion. The pattern of redistribution was confirmed when "C-labeled amino acids were applied directly to the under surface of mature leaves. Labeled compounds were found in the phloem exudate from treated leaves, and the composition resembled the pattern of labeling in the compounds arriving in the young developing leaves. from mature to developing leaves was not followed in detail, nor were labeled compounds recovered in the developing leaves identified. Export from the mature leaves of label from amino acids (applied as tracers to the leaf surface) has been demonstrated in several plants (1 1, 20, 22).In P. sativum seedlings, the xylem sap contains a mixture of nitrate and organic nitrogenous compounds, mainly asparagine and glutamine, but also homoserine, asparate, and a few other amino acids (4,25). Experiments with lWN-labeling showed that pea leaves utilize the nitrogen from nitrate and from asparagine and glutamine for further amino acid metabolism (5). In these pea seedlings, the young leaves of the vegetative apex are enclosed within the stipules of the next oldest, expanding leaves, and there is little transpirational flow to the apical leaves. Young pea leaves are therefore a useful system in which to study redistribution of nitrogen from the mature leaves.Uptake and redistribution of 14C-labeled amides, the major nitrogenous compounds in the xylem of pea (25), was followed in mature, expanding, and immature leaves of nonnodulated plants over 12 h in the light, after supply to the shoot in the transpiration stream. Movement of amino acids from mature leaves to immature leaves and other parts of the plant was investigated by application of 14C-labeled amino acids to the lower surface of mature leaves.Nitrogen required for growth of plant shoots is transported from the roots via the xylem as a mixture of nitrate and organic nitrogen, the proportions varying with species and conditions (17). Mature leaves (vigorously transpiring) receive most of this xylemborne nitrogen (8,19). Nitrogen levels stabilize in fully-expanded leaves (4) an...

Use of Arabidopsis mutants and genes to study amide amino acid biosynthesis.

“…Because it is relatively inert and has a high nitrogen:carbon ratio (compared with glutamine), asparagine is an ideal compound for nitrogen transport and storage, especially under conditions in which the carbon supply is limited (Lea and Miflin, 1980;Urquhart and Joy, 1981;Sieciechowicz et al, 1988;Lam et al, 1994). This hypothesis is supported by the observation that asparagine levels are elevated in darkgrown or dark-adapted pea plants (Urquhart and Joy, 1982). Similarly, asparagine levels in Arabidopsis are also dramatically modulated by light (Schultz, 1994).…”

Distribution and Metabolism of Xylem-Borne Ureido and Amino Compounds in Developing Soybean Shoots

“…Therefore, the amides and their dicarboxylic amino acids provide the major sources of N for growth of the foliage, with the dicarboxylic amino acids predominantly supplying mature leaves via the transpiration stream, and the amides extensively undergoing direct xylem-to-phloem transfer to supply young developing leaves. In general, studies on pea ( 19) and cottonwood (5,23) …”

Nitrogen assimilation in Citrus based on CitEST data mining

“…This can explain the reduced AS activity in sick plants. Both the asparagine content in phloem exudates and AS activity were induced when light-grown plants were dark-adapted (Urquhart and Joy, 1982). The first striking observation of AS gene expression in pea and Arabidopsis was the high level of AS mRNA in dark-grown or dark-adapted plants (Tsai and Coruzzi, 1990).…”