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

Abstract

lSN.Nuclea r 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 15NH4C1 solutions for 24 h. The main 15N_labelled metabolites were asparagine and glutamine, as well as substantial amounts of unreacted, intracellular NH~. These results were compared with those of a previous study on Lemna 9ibba L. clone Hurfeish (Monselise et al., 1987, New Phytol. 10, 341-345) with regard to NH2 uptake, assimilation and detoxification efficiencies. Both species, grown under continuous white light, were capable of preferential uptake of NH2 in the presence of nitrate. Relative growth rates indicate that both species tolerate increased levels of NH +, up to 10 -2 mol" 1-1, with L. 9ibba showing a slightly greater tolerance. No 15N-labelled free NH2 was detectable in L. 9ibba, while in W. arrhiza excess NH2 was found within the cells. This fact indicates that L. 9ib -ba is more efficient in detoxification than W. arrhiza, presumably because of inability of W. arrhiza to regenerate the "NH + traps", glutamate and aspartate, rapidly enough. This is also evident from the observation that addition of a-ketoglutarate to the medium caused nearly complete assimilation of intracellular NH + in W. arrhiza. In both plants, addition of a-ketoglutarate increased both NH2 uptake and assimilation. Addition of L-methionine DL-sulfoximine, an inhibitor of glutamine synthetase inhibited NH + 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 * This article is dedicated to Professor Bernhard Schrader on the occasion of his 60th birthday Abbreviations: AZA = azaserine (O-diazoacetyl-L-serine) ; GOGAT=glutamine oxoglutarate amine transferase=glutamate synthase E.C. 1.4.7. and E.C. 1.4.1.13. ; GS = glutamine synthetase E.C. 6.3.1.2. ; a-KG = c~-ketoglutarate = 2-oxoglutarate; MSO = Lmethionine DL-sulphoximine; NMR -nuclear magnetic resonance; RGR = relative growth rate major route of NH2 assimilation in the two plants under the conditions used.

Analysis of 13NH4+ Efflux in Spruce Roots (A Test Case for Phase Identification in Compartmental Analysis)

“…At the enzyme level, it is known that MSO can act as an inhibitor not only of Gln synthetase but also of glutamate synthase (Gauthier, 1983;Takashi et al, 1983) as well as Asn synthetase (Monselise and Kost, 1993). Our results (Tables IV and VI) show that the rates of unidirectional influx and efflux of NH,+ were clearly altered by MSO.…”

“…More subtle explorations of the identity of compartment I11 were achieved by exposures to MSO and a-KG. MSO is a known inhibitor of Gln synthetase (Wedler and Horn, 1976;Meister, 1981;Monselise and Kost, 1993), the primary NH,+-assimilating enzyme located in the cytoplasm. Significant increases in cytoplasmic NH4+ concentrations (up to 90 mM) after MSO pretreatment in root cells of other plant systems have been observed by NMR (Lee and Ratcliffe, 1991) and inferred from labeling kinetics (Fentem et al, 1983a(Fentem et al, , 1983b.…”

“…Although this was most certainly attributable to enhanced rates of NH,+ assimilation, a depression of NH,+ influx also contributed to the effect (Table IV). It is well established that the availability of carbon skeletons to the roots, both endogenous in the roots and supplied via the shoot, affects rates of N uptake (Michael et al, 1970;Monselise and Kost, 1993;Rideout et al, 1994). In our study, an additional "regulatory" role of a-KG was evident in an effect on the partitioning of NH4+ assimilation products between root and shoot.…”

Carbon, Nitrogen, and Nutrient Interactions in Beta vulgaris L. as Influenced by Nitrogen Source, NO3- versus NH4+

“…The assimilation of Gln requires a substantial consumption of fixed C in the form of a-KG (GOGAT reaction) which is obtained from photosynthetically fixed triose-P entering the glycolytic and Krebs cycle pathways. In fact, the availability of a-KG may be a limiting factor in N assimilation as indicated by the studies of Magalhaes et al (1992) and Monselise and Kost (1993).…”

Bio-recovery of N and P from an anaerobic digester effluent: The potential of duckweed (<i>Lemna minor</i>)

“…They attributed ammonia toxicity to the effect of NH 3 only. However, other workers claim both forms of ammonia-N at high concentrations are toxic to plants (Litav & Lehrer, 1978;Monselise & Kost, 1993). In another study, Oron et al (1984) found that total NH 4 -N level of 200 mg l -1 in domestic waste water was unfavourable to duckweed growth at pH 7 but Koles et al (1987) reported the death of duckweed when the ammonium nitrogen concentration in the growth medium was above 50 mg l -1 .…”