Edna Ben-Izhak Monselise

Low-frequency electromagnetic fields induce a stress effect upon higher plants, as evident by the universal stress signal, alanine

15N NMR analysis reveals alanine production in Duckweed plants exposed to low intensity sinusoidally varying magnetic fields (SVMF) at 60 and 100Hz, and fed by 15N-labeled ammonium chloride. Alanine does not accumulate in the absence of SVMF. Addition of vitamin C, a radical scavenger, reduced alanine production by 82%, indicating the roll of free radicals in the process. Alanine accumulation in plants and animals in response to exposure to a variety of stress conditions, including SVMF, is a general phenomenon. It is proposed that alanine is a universal first stress signal expressed by cells.

Different ammonium-ion uptake, metabolism and detoxification efficiencies in two Lemnaceae

Abstract15N-Nuclear magnetic resonance spectroscopy was used to follow nitrogen assimilation and amino-acid production in Wolffia arrhiza (L.) Hork. ex. Wimmer, clone Golan exposed to 4.0 mM 15NH4Cl solutions for 24 h. The main 15N-labelled metabolites were asparagine and glutamine, as well as substantial amounts of unreacted, intracellular NH4+. These results were compared with those of a previous study on Lemna gibba L. clone Hurfeish (Monselise et al., 1987, New Phytol. 10, 341–345) with regard to NH4+uptake, assimilation and detoxification efficiencies. Both species, grown under continuous white light, were capable of preferential uptake of NH4+in the presence of nitrate. Relative growth rates indicate that both species tolerate increased levels of NH4+, up to 10−2 mol · 1−1, with L. gibba showing a slightly greater tolerance. No 15N-labelled free NH4+was detectable in L. gibba, while in W. arrhiza excess NH4+was found within the cells. This fact indicates that L. gibba is more efficient in detoxification than W. arrhiza, presumably because of inability of W. arrhiza to regenerate the “NH4+traps”, glutamate and aspartate, rapidly enough. This is also evident from the observation that addition of α-ketoglutarate to the medium caused nearly complete assimilation of intracellular NH4+in W. arrhiza. In both plants, addition of α-ketoglutarate increased both NH4+uptake and assimilation. Addition of l-methionine dl-sulfoximine, an inhibitor of glutamine synthetase inhibited NH4+assimilation, while addition of azaserine, an inhibitor of glutamate synthase, resulted in 15N incorporation into the glutamine-amide position only. These results are consistent with the glutamine synthetase-glutamate synthase pathway being the major route of NH4+assimilation in the two plants under the conditions used.

Ultraviolet radiation induces stress in etiolated Landoltia punctata, as evidenced by the presence of alanine, a universal stress signal: a 15N NMR study

Analysis with (15) N NMR revealed that alanine, a universal cellular stress signal, accumulates in etiolated duckweed plants exposed to 15-min pulsed UV light, but not in the absence of UV irradiation. The addition of 10 mm vitamin C, a radical scavenger, reduced alanine levels to zero, indicating the involvement of free radicals. Free D-alanine was detected in (15) N NMR analysis of the chiral amino acid content, using D-tartaric acid as solvent. The accumulation of D-alanine under stress conditions presents a new perspective on the biochemical processes taking place in prokaryote and eukaryote cells.

15N NMR SPECTROSCOPIC STUDY OF AMMONIUM ION ASSIMILATION BY SPIRODELA OLIGORRHIZA—LEMNACEAE—AS AFFECTED BY LIGHT AND CARBON SUPPLY IN GREEN AND EXTOLIATED PLANTS

ABSTRACT Nitrogen assimilation and amino acid production in Spirodela oligorrhiza plants exposed to 30 mM 15NH4Cl was studied using l5N NMR spectroscopy. Green and etiolated plants were studied under different light regimes and in the presence of added carbon, either as sucrose or as α-ketoglutarate. Etiolated plants are capable of ammonium assimilation and, as in green plants, this occurs via the glutamine synthetase/glutamine oxoglutarate amine transferase (GS/GOGAT) and the aspartate aminotransferase/asparagine synthetase pathways. The major assimilation products in both etiolated and green plants were glutamine and asparagine. Thus our results confirm that N-amides are key detoxification products when plants are exposed to external ammonium ion, and act as storage reservoirs or sinks for assimilated ammonium. In plants grown under continuous light, ammonium ion was taken up and assimilated to completion. L-methionine DL-sulfoximine, a GS inhibitor, inhibited ammonium ion assimilation but not its uptak...