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Nitrogen stress and the expression of asparagine synthetase in roots and nodules of soybean ( Glycine max )
Author(s) -
Antunes Flávia,
Aguilar Miguel,
Pineda Manuel,
Sodek Ladaslav
Publication year - 2008
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1111/j.1399-3054.2008.01092.x
Subject(s) - xylem , asparagine synthetase , nitrogen fixation , glycine , asparagine , biology , nitrate , root nodule , gene expression , shoot , gene , botany , biochemistry , enzyme , genetics , amino acid , ecology , bacteria
The difficulty of assaying asparagine synthetase (AS) (EC 6.3.5.4) activity in roots of soybean has been circumvented by measuring expression of the AS genes. Expression of three soybean asparagine synthetase (SAS) genes ( SAS1 , SAS2 and SAS3 ) was observed in roots of non‐nodulated soybean plants cultivated on nitrate. Expression of these genes was reduced to very low levels within days after submitting the plants to a N‐free medium. The subsequent return to a complete medium (containing nitrate) restored expression of all three AS genes. Roots of nodulated plants, where symbiotic nitrogen fixation was the exclusive source of N (no nitrate present), showed very weak expression of all three AS genes, but on transfer to a nitrate‐containing medium, strong expression of these genes was observed within 24 h. In nodules, all three genes were expressed in the absence of nitrate. Under conditions that impair nitrogen fixation (nodules submerged in aerated hydroponics), only SAS1 expression was reduced. However, in the presence of nitrate, an inhibitor of N 2 fixation, SAS1 expression was maintained. High and low expressions of AS genes in the roots were associated with high and low ratios of Asn/Asp transported to the shoot through xylem. It is concluded that nitrate (or one of its assimilatory products) leads to the induction of AS in roots of soybean and that this underlies the variations found in xylem sap Asn/Asp ratios. Regulation of nodule AS expression is quite different from that of the root, but nodule SAS1 , at least, appears to involve a product of N assimilation rather than nitrate itself.