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The metabolic flux phenotype of heterotrophic A rabidopsis cells reveals a complex response to changes in nitrogen supply
Author(s) -
Masakapalli Shyam K.,
Kruger Nicholas J.,
Ratcliffe R. George
Publication year - 2013
Publication title -
the plant journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.058
H-Index - 269
eISSN - 1365-313X
pISSN - 0960-7412
DOI - 10.1111/tpj.12142
Subject(s) - pentose phosphate pathway , ammonium , flux balance analysis , nitrogen cycle , metabolic pathway , flux (metallurgy) , biology , heterotroph , nitrate , arabidopsis , biochemistry , metabolic network , nitrogen , biomass (ecology) , chemistry , metabolism , botany , glycolysis , gene , ecology , genetics , organic chemistry , bacteria , mutant
Summary The extent to which individual plants utilise nitrate and ammonium, the two principal nitrogen sources in the rhizosphere, is variable and many species require a balance between the two forms for optimal growth. The effects of nitrate and ammonium on gene expression, enzyme activity and metabolite composition have been documented extensively with the aim of understanding the way in which plant cells respond to the different forms of nitrogen, but ultimately the impact of these changes on the organisation and operation of the central metabolic network can only be addressed by analysing the fluxes supported by the network. Accordingly steady‐state metabolic flux analysis was used to define the metabolic phenotype of a heterotrophic A rabidopsis thaliana cell culture grown in M urashige and S koog and ammonium‐free media, treatments that influenced growth and biomass composition. Fluxes through the central metabolic network were deduced from the redistribution of label into metabolic intermediates and end products observed when cells were labelled with [1‐ 13 C]‐, [2‐ 13 C]‐ or [ 13 C 6 ]glucose, in tandem with 14 C‐measurements of the net accumulation of biomass. Analysis of the flux maps showed that: (i) flux through the oxidative pentose phosphate pathway varied independently of the reductant demand for biosynthesis, (ii) non‐plastidic processes made a significant and variable contribution to the provision of reducing power for the plastid, and (iii) the inclusion of ammonium in the growth medium increased cell maintenance costs, in agreement with the futile cycling model of ammonium toxicity. These conclusions highlight the complexity of the metabolic response to a change in nitrogen nutrition.