Nitrate addition alleviates ammonium toxicity without lessening ammonium accumulation, organic acid depletion and inorganic cation depletion in Arabidopsis thaliana shoots.
When ammonium is the sole nitrogen (N) source, plant growth is suppressed compared with the situation where nitrate is the N source. This is commonly referred to as ammonium toxicity. It is widely known that a combination of nitrate and ammonium as N source alleviates this ammonium toxicity (nitrate-dependent alleviation of ammonium toxicity), but the underlying mechanisms are still not completely understood. In plants, ammonium toxicity is often accompanied by a depletion of organic acids and inorganic cations, and by an accumulation of ammonium. All these factors have been considered as possible causes for ammonium toxicity. Thus, we hypothesized that nitrate could alleviate ammonium toxicity by lessening these symptoms. We analyzed growth, inorganic N and cation content and various primary metabolites in shoots of Arabidopsis thaliana seedlings grown on media containing various concentrations of nitrate and/or ammonium. Nitrate-dependent alleviation of ammonium toxicity was not accompanied by less depletion of organic acids and inorganic cations, and showed no reduction in ammonium accumulation. On the other hand, shoot growth was significantly correlated with the nitrate concentration in the shoots. This suggests that nitrate-dependent alleviation of ammonium toxicity is related to physiological processes that are closely linked to nitrate signaling, uptake and reduction. Based on transcript analyses of various genes related to nitrate signaling, uptake and reduction, possible underlying mechanisms for the nitrate-dependent alleviation are discussed.
Department of Biological Sciences, Graduate School of Science, The University of Tokyo, Tokyo, Japan. takushi@biol.s.u-tokyo.ac.jp
Takushi
T
Hachiya
Chihiro K
CK
Watanabe
Masaru
M
Fujimoto
Toshiki
T
Ishikawa
Kentaro
K
Takahara
Maki
M
Kawai-Yamada
Hirofumi
H
Uchimiya
Yukifumi
Y
Uesono
Ichiro
I
Terashima
Ko
K
Noguchi
D000596:Amino Acids / Q000096:biosynthesis; D017360:Arabidopsis / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism*; D018533:Biomass; D002021:Buffers; D002264:Carboxylic Acids / Q000378:metabolism*; D002412:Cations; D002952:Citric Acid Cycle / Q000187:drug effects; D003470:Culture Media; D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D017343:Genes, Plant / Q000235:genetics; D006019:Glycolysis / Q000187:drug effects; D006863:Hydrogen-Ion Concentration / Q000187:drug effects; D009566:Nitrates / Q000494:pharmacology*; D009584:Nitrogen / Q000494:pharmacology; D018520:Plant Shoots / Q000187:drug effects / Q000254:growth & development; D000644:Quaternary Ammonium Compounds / Q000378:metabolism* / Q000633:toxicity*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D014020:Tissue Extracts
D000596:Amino Acids; D002021:Buffers; D002264:Carboxylic Acids; D002412:Cations; D003470:Culture Media; D009566:Nitrates; D000644:Quaternary Ammonium Compounds; D012333:RNA, Messenger; D014020:Tissue Extracts; D009584:Nitrogen