, Volume 248, Issue 2, pp 437–449 | Cite as

Are phloem-derived amino acids the origin of the elevated malate concentration in the xylem sap following mineral N starvation in soybean?

  • Simone C. Vitor
  • Luciano do Amarante
  • Ladaslav Sodek
Original Article


Main conclusion

A substantial increase in malate in the xylem sap of soybean subjected to mineral N starvation originates mainly from aspartate, a prominent amino acid of the phloem.

A substantial increase in xylem malate was found when non-nodulated soybean plants were transferred to a N-free medium. Nodulated plants growing in the absence of mineral N and, therefore, dependent on symbiotic N2 fixation also contained elevated concentrations of malate in the xylem sap. When either nitrate or ammonium was supplied, malate concentrations in the xylem sap were low, both for nodulated and non-nodulated plants. Evidence was obtained that the elevated malate concentration of the xylem was derived from amino acids supplied by the phloem. Aspartate was a prominent component of the phloem sap amino acids and, therefore, a potential source of malate. Supplying the roots of intact plants with 13C-aspartate revealed that malate of the xylem sap was readily labelled under N starvation. A hypothetical scheme is proposed whereby aspartate supplied by the phloem is metabolised in the roots and the products of this metabolism cycled back to the shoot. Under N starvation, aspartate metabolism is diverted from asparagine synthesis to supply N for the synthesis of other amino acids via transaminase activity. The by-product of aspartate transaminase activity, oxaloacetate, is transformed to malate and its export accounts for much of the elevated concentration of malate found in the xylem sap. This mechanism represents a new additional role for malate during mineral N starvation of soybean, beyond that of charge balance.


Aspartate GC–MS Glycine max N starvation Organic acids 





Gamma-aminobutyric acid


Aspartate transaminase




Phosphoenolpyruvate carboxylase


Tricarboxylic-acid cycle



This research was supported by grants from the São Paulo State Research Foundation (FAPESP) [Grant number 2013/03325-8] and the National Council for Scientific and Technological Development (CNPq) [Grant number 303931/2009-4].

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • Simone C. Vitor
    • 1
  • Luciano do Amarante
    • 1
    • 2
  • Ladaslav Sodek
    • 1
  1. 1.Department of Plant Biology, Institute of BiologyUniversity of Campinas-UNICAMPCampinasBrazil
  2. 2.Department of BotanyFederal University of PelotasPelotasBrazil

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