Involvement of calcium and calmodulin in protein and amino acid metabolism in rice roots under anoxia

Abstract

The prctreatment of rice roots for 1 h in aerobic conditions with the Ca2+-channel blockers ruthenium red (RR) and verapamil and the calmodulin (CaM) antagonists N-(6-aminohexyl)-5-chloro-1-naphtylenesulfonamide (W-7) and trifluoperazine, induced during 3 h of anoxia: (i) inhi-bition of amino acid and y-aminobutyric acid (Gaba) accumulation; (ii) a decline in the protein con-tent; (iii) a release of amino acids and K+ into the growth media. The calcium ionophore A23187 reversed these effects in RR-treated roots. Moreover, the aerobic pretreatment of rice roots with A23187 alone or CaCl2 increased the accumulation of amino acids and Gaba. These data indicate that the Ca2+/CaM complex is involved in the transduction of an anaerobic signal by inhibiting proteolysis and solute release, and activating the Ca2+/CaM-dependent glutamate decarboxylase. Key words: y-Aminobutyrate — Anoxia — Calcium — Calmodulin — Oryza saliva — Root. Anoxia induces marked changes in the metabolism of amino acids and proteins. The anaerobic response of plant tissues involves a decrease in the rate of protein synthesis (Bertani et al. 1981b) and an induction of a unique set of proteins, termed anaerobic polypeptides (Sachs et al. 1980). The lower demand for amino acids determined by the decline in the rate of protein synthesis has been sug-gested to be the cause for the anaerobic increase in the con-centration of free amino acids (Aurisano et al. 1995, Ber-tani and Brambilla 1982, Reggiani et al. 1988). The high concentration of free amino acids in plant tissues subject-ed to oxygen deficiency significantly contributes to the os-motic potential, thus compensating for the decrease in soluble hexose sugars employed in fermentative processes (Bertani et al. 1981a). Among the amino acids, alanine and y-aminobutyrate (Oaba) are accumulated at high levels in