Mitteilungen der Gesellschaft für Pflanzenbauwissenschaften Band 23

Mitt. Ges. Pflanzenbauwiss. 23: 67–68 (2011)
Auxin-mediated ammonium toxicity in Arabidopsis roots
Huaiyu Yang, Jenny von der Fecht-Bartenbach and Uwe Ludewig
Institute of Crop Science, Nutritional Crop Physiology, University of Hohenheim, Stuttgart. E-Mail:
u.ludewig@uni-hohenheim.de
Introduction
Most plant species prefer the combined nutrition of NO3- and NH4 + , and suffer when
ammonium (the sum of NH4 + and NH3) is the predominant or exclusive mineral nitrogen
(N) source. Stunted root and shoot growth, reduced photosynthetic activity, and
in extreme cases chlorosis of leaves, are typical visible symptoms of so-called
ammonium toxicity. Reduced cation uptake results in mineral nutrient deficiencies of
K + , Mg2 + and Ca2 + and leads to a lowered osmotic potential and finally reduced water
uptake. High K + alleviated toxicity by inhibition of NH4 + uptake and stimulated carbon
and nitrogen assimilation in the roots. Plants can detoxify excess ammonium at least
to some extent metabolically, e.g. by glutamine synthetase (GS) or endogenous
glutamate dehydrogenase (GDH). With ammonium nutrition, excess protons need to
be excreted into the soil, which acidifies the rhizosphere and reduces root expansion,
a phenotype that can partially be suppressed by buffering the root medium.
Nutritional and cellular imbalances are integrated via plant hormones, such as
auxin and cytokinin, and the growth regulation by these hormones ultimately shape
roots and shoots. Auxin synthesis and distribution within the plant is mediated by a
complex transport network; the decrease in auxin at the root tips altered cell growth
and elongation. Previous reports identified connections of the root development with
its nitrate status, which may be integrated with the auxin concentration in the roots.
The root growth inhibition of maize by high nitrate involved reduced endogenous
auxin and enhanced cytokinin in root tips.
To investigate the growth defects associated with ammonium nutrition, a transcriptome
analysis and genetic approach was carried out with Arabidopsis seedlings. This
analysis confirmed many well-known physiological changes associated with
ammonium nutrition and identified novel links to cell wall metabolism and suggest an
altered control of root growth by auxin in ammonium stress that can be partially
overcome by mutants in the auxin distribution.
Material and Methods
Arabidopsis thaliana was grown with buffered 3 mM NH4SO4, 6 mM KNO3 or 1.5 mM
NH4NO3 as the nitrogen forms, RNA was isolated and microarray analysis with ATH1
chips was done. The expression estimates and statistical analysis used the rank
product method and the mapman software. Mutant seeds included the auxin-related
mutants rcn1, 35S::PID, DR5::GUS, aux1 and pin2.
Results and Discussion
As expected from the similar plant growth, there were only minor differences in the
transcriptome of plants grown on NH4NO3 and NO3 - . The cluster analysis suggested
that the primary and secondary metabolism, protein biosynthesis and degradation
pathways slightly differed between these conditions. These minor changes were in
accordance with the moderately altered amino acid profile and the different
ammonium content. By contrast, the ammonium content, amino acid and
transcriptional profile were markedly changed with pure NH4 + nutrition. A Rank