Regulation of Development of Leucine Uptake Activity by Glutamine in the Scutellum of Germinating Barley Grain

Abstract

Scutella from ungerminated grains of barley (Hordeum vulgare L. cv Pirkka) take up leucine at a slow rate, which increases rapidly during germination. When endosperms were removed from the gains after imbibition for 4 hours or after germination for 12 or 72 hours, the increase in the rate of leucine uptake was greatly accelerated during subsequent incubation of the embryos or scutella. These increases were rapidly inhibited by cordycepin and cycloheximide, suggesting that protein synthesis, probably synthesis of the carrier protein, was required for the development of the uptake activity.In separated embryos or scutella, the increases in the leucine uptake activity were inhibited by glutamine. The inhibitions caused by glutamine and cycloheximide were not additive, suggesting that glutamine did not interfere with the function of the carrier but repressed its synthesis. Glutamine did not inhibit the simultaneous increase in peptide uptake; in this respect, its effect was specific for leucine uptake, which appears to be due to a general amino acid uptake system. Some other protein amino acids also inhibited the increase in leucine uptake without inhibiting the increase in peptide uptake. However, these effects were smaller than that of glutamine.These results suggest that the transfer of leucine (and other amino acids) from the endosperm to the seedling in a germinating barley grain is regulated at the uptake step by repression of the synthesis of the amino acid carrier protein by glutamine and-possibly to a lesser extent-by some other amino acids taken up from the endosperm.The hydrolysis of reserve proteins in the starchy endosperm of a germinating barley grain results in a mixture of free amino acids and short oligopeptides (7,13,14,20). The amino acids and peptides are taken up into the scutellum by separate transport systems (6,11,(20)(21)(22). We have previously studied some properties of amino acid uptake into the scutellum using leucine as the substrate (24). Leucine is rapidly taken up by an active transport system(s), the rate of uptake being highest near pH 5, the pH of the starchy endosperm (14).The uptake of leucine is slow in scutella separated from ungerminated grains, but increases about 10-fold during the first 3 d ofgermination of whole grains (24). This increase is followed by a lag phase of about 1 d, after which there is another increase.Removal of the embryonic axis has no effect on the initial ' Supported by the Research Council for Natural Sciences of which T. S. is a Research Fellow. The work was also supported by a grant from the Emil Aaltonen foundation to S. N.2 Abbreviations: CHI, cycloheximide; Sar, sarcosine; COR, cordycepin; AIB, a-amino-iso-butyric acid.increase in the rate of uptake (24). On the other hand, removal of the endosperm either after imbibition for 4 h or after germination for 3 d enhances the rate ofthe increase during subsequent incubation. In both cases, the increase is completely prevented by CHI2 and markedly slowed down by glutamine. It was therefore su...

Hormonal Regulation of Organic and Phosphoric Acid Release by Barley Aleurone Layers and Scutella

“…When isolated embryos were incubated with de-embryonated half grains, the amount of acidity in the incubation medium decreased (Table 111). The importance of protons for the uptake of amino acids and peptides by barley scutellum has been shown by severa1 investigators (Sopanen et al, 1980;Nyman et al, 1983;Sopanen and Vaisanen, 1985;Vaisanen and Sopanen, 1986;Higgins and Payne, 1987;Salmenkallio and Sopanen, 1989;Hardy and Payne, 1991), and we presume, by analogy with proton co-transport mechanisms described for sugar uptake in other plant tissues, that protons are essential for the uptake of sugars by the scutellum (Bush, 1993). The loss of acids from the endosperm during post-germinative growth and after incubation of embryos with half grains could therefore be accounted for by uptake at the scutellar epithelium.…”

“…At least two peptide carriers (Salmenkallio and Sopanen, 1989;Hardy and Payne, 1991) and four distinct amino acid carriers (Sopanen et al, 1980;Nyman et al, 1983;Sopanen and Vaisanen, 1985;Vaisanen and Sopanen, 1986;Higgins and Payne, 1987) have been identified in the barley scutellum, and a11 depend on a proton gradient, acid on the endosperm side, to drive amino acid uptake into the scutellum. Less is known about the mechanism of sugar uptake by the scutellum, but by analogy with the uptake of sugars at the plasma membrane of other higher plants, a protoncoupled system is likely to operate (Bush, 1993).…”

Amino Acid and Peptide Uptake in the Scutella of Germinating Grains of Barley, Wheat, Rice, and Maize

“…The free amino acids are taken up into barley scutellum by at least four separate uptake systems (or groups of systems). Two of these amino acid uptake systems have a broad specificity (8,16,17), one is specific for proline (19), and one is specific for basic amino acids (12).…”

Uptake of Glutamine by the Scutellum of Germinating Barley Grain

“…Previously, some properties of the uptake of amino acids into the scutellum were studied using leucine as substrate (18,26). The uptake of leucine is separate from the uptake of peptides and is due to active transport.…”

“…In a germinating barley grain, the uptake ofglutamine deserves special attention for three reasons: (a) glutamine is the most abundant amino acid in the storage proteins of barley, comprising about 30% of the amino acid residues in the hordeins (8); (b) it appears to be the most important amino acid in the regulation of amino acid uptake into the scutellum (18); and (c) it also seems to be an important compound for 'long-distance' translocation of nitrogen in cereals (21,29).…”