Influence of UVB irradiation on nitrate and ammonium assimilating enzymes in Spirodela polyrhiza

Abstract

The influence of UVB radiation on nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), NADH-dependent glutamate synthase (NADH-GOGAT) and ferredoxin-dependent glutamate synthase (Fd-GOGAT) was investigated in fronds and turions of Spirodela polyrhiza. When white light (4.3 W m −2) was supplemented with UVB (0.55 W m −2; λ max = 311 nm), a 40% transient decrease in the growth rate was observed after 12 h UVB irradiation. In addition, the activities of all enzymes investigated, except for the cytoplasmic isoform of GS, were substantially inhibited (NR, 80%; NiR, 32%; GS, 30%; NADH-GOGAT, 36%; Fd-GOGAT, 54% in green fronds). The extent of the inhibition in fronds as well as in turions is in accordance with the assumption that the light-induced increases of enzyme activities are prevented by simultaneous application of UVB. The uptake of nitrate was not influenced by white light or UVB. As shown by western analysis, a correlation between the enzyme activities and their protein amounts was found. Northern analysis revealed that UVB influenced the transcript accumulation of NR but not of Fd-GOGAT. No obvious correlation was observed between the mRNA and the protein amount. This suggests that UVB damage occurs at the transcriptional and post-transcriptional levels as well as at the protein level.