Premium
5‐Aminoimidazole‐4‐carboxamide riboside activates nitrate reductase in darkened spinach and pea leaves
Author(s) -
Huber Steven C.,
Kaiser Werner M.
Publication year - 1996
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.1399-3054.1996.tb06692.x
Subject(s) - spinacia , dephosphorylation , biochemistry , okadaic acid , spinach , phosphatase , metabolite , in vivo , riboside , pisum , nitrate reductase , phosphorylation , chemistry , biology , enzyme , microbiology and biotechnology , chloroplast , gene
Nitrate reductase (NR) activity is modulated in vivo by phosphorylation (inactivation)/dephosphorylation (activation) in response to light/dark signals. The dephosphorylation of phospho‐NR in vitro, catalyzed by endogenous protein phosphatases, is known to be stimulated by 5′‐AMP suggesting that this metabolite may be an important regulator of the activity of NR, e.g. under anoxia. To determine whether 5′‐AMP might be a regulatory metabolite in vivo, excised spinach ( Spinacia oleracea ) and pea ( Pisum sativum ) leaves were provided 5‐aminoimidazole‐4‐carboxamide riboside (AICAR) via the transpiration stream, and the apparent phosphorylation status of NR was assessed by assay of activity in the presence of free Mg 2+ . NR was activated in darkened spinach leaves in a time‐ and concentration‐dependent manner when leaves were fed AICAR; there was also an accumulation of nitrite in treated leaves in the dark. The activation by AICAR could be blocked by several type 2A protein phosphatase inhibitors (microcystin‐LR, okadaic acid and cantharidin), and was not the result of a reduction of kinase activity by lack of ATP because cellular adenylates were unaffected. It was confirmed that AICAR‐P, but not AICAR, mimicked 5′‐AMP in the activation of phospho‐NR in vitro. Our results are consistent with the notion that AICAR is converted to the monophosphorylated derivative, which accumulates in cells and acts as a structural analog of 5′‐AMP. Our results suggest that a rise in cytosolic [5′‐AMP] may be sufficient to activate NR in vivo. AICAR should be a useful compound for identifying AMP‐regulated processes in plant systems.