Arsenic exposure alters activity behaviour of key nitrogen assimilatory enzymes in growing rice plants

Abstract

Rice seedlings when grown in sand cultures for 5-20 days under 25 and 50 μM As2O3 in the medium showed a marked decline in growth when compared to controls. Increased absorption of arsenic from the medium, against the concentration gradient was observed. Greater localization of absorbed arsenic was noted in roots than in shoots. Rice plants grown for 20 days with 50 μmol 1-1 arsenic in the medium accumulated upto 370 μmol arsenic kg-1 dry weight in roots. Increasing levels of As2O3 in situ caused a marked decline in the activities of the nitrate assimilatory enzymes nitrate reductase (NR), nitrite reductase (NiR) and glutamine synthetase (GS), whereas an increase in the activities of alanine and aspartate aminotransferases was observed. The activities of animating (NADH-GDH) and deaminating (NAD+-GDH) glutamate dehydrogenases increased at moderately toxic level (25 μM) of As 2O3 whereas a higher As level of 50 7mu;M was inhibitory to the enzymes. Addition of 1 M proline in the reaction medium caused significant restoration in As-led loss of NR and GS activities. NR and GS extracted from arsenic exposed seedlings showed higher Km values compared to the enzymes extracted from control-grown seedlings, whereas GDHs extracted from As-stressed seedlings showed a decrease in Km. Results suggest that inhibition in the activities of N assimilatory enzymes accompanied with decreased affinity of the enzymes towards their substrates would eventually lead to a marked suppression of N assimilation and impaired growth of rice seedlings in As polluted environment. © 2004 Kluwer Academic Publishers.