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Phloem‐derived γ ‐aminobutyric acid (GABA) is involved in upregulating nodule N 2 fixation efficiency in the model legume Medicago truncatula
Author(s) -
SULIEMAN SAAD,
SCHULZE JOACHIM
Publication year - 2010
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1111/j.1365-3040.2010.02214.x
Subject(s) - phloem , medicago truncatula , amino acid , nitrogen fixation , biology , biochemistry , legume , downregulation and upregulation , botany , food science , bacteria , symbiosis , genetics , gene
Nitrogen fixation in legumes is downregulated through a whole plant N feedback mechanism, for example, when under stress. This mechanism is probably triggered by the impact of shoot‐borne, phloem‐delivered compounds. However, little is known about any whole‐plant mechanism that might upregulate nitrogen fixation, for example, under N deficiency. We induced emerging N‐deficiency through partial excision of nodules from Medicago truncatula plants. Subsequently, the activity and composition of the remaining nodules and shifts in concentration of free amides/amino acids in the phloem were monitored. Furthermore, we mimicked these shifts through artificial feeding of γ ‐aminobutyric acid (GABA) into the phloem of undisturbed plants. As a result of increased specific activity of nodules, N 2 fixation per plant recovered almost completely 4–5 d after excision. The concentration of amino acids, sugars and organic acids increased strongly in the upregulated nodules. A concomitant analysis of the phloem revealed a significant increase in GABA concentration. Comparable with the effect of nodule excision, artificial GABA feeding into the phloem resulted in an increased activity and higher concentration of amino acids and organic acids in nodules. It is concluded that GABA might be involved in upregulating nodule activity, possibly because of its constituting part of a putative amino acid cycle between bacteroids and the cytosol.