Journal of Experimental Botany, Vol 49, 1371-1379, Copyright © 1998 by Oxford University Press
P Tillard, L Passama and A Gojon
The putative role of phloem amino acids as negative feedback signals for
root NO3- uptake was investigated in
Ricinus communis L. The
NO3--grown plants were subjected to N-deficiency due
either to complete N-deprivation, or to localized N-deprivation on one side
of a split-root system. In comparison with controls, complete N-deprivation
resulted in a transient increase in
15NO3- influx, and in
profound changes in downward phloem transport of amino acids. Total amino
acid concentration in the phloem sap decreased by 40%, but responses
markedly differed between the individual amino acids. Concentrations of Gln
and Ser were rapidly lowered by 50%, while those of Val, Phe, Leu, and Ile
displayed a marked increase. Localized N-deprivation on one side of the
split root system also resulted in the up-regulation of
15NO3- influx in the roots
still supplied with NO3-. However, the amino acid
composition of the phloem sap directed to these roots was not modified by
the treatment, and remained similar to that in N-sufficient control plants.
Only amino acid transport to the N-deprived roots was affected as observed
in response to complete N-deprivation. The results from split-root plants
indicate that the response of root NO3- influx to
N-deficiency is controlled by shoot-borne regulatory signals, and provide a
case study where these signals are not related to a qualitative change or a
significant decrease in downward phloem transport of amino
acids.Keywords: N-deficiency,
NO3- uptake, phloem amino acids, Ricinus
communis, whole plant signalling
ARTICLES
Are phloem amino acids involved in the shoot to root control of NO3- uptake in Ricinus communis plants?
Biochimie et Physiologie Moléculaire des Plantes, ENSA-M/INRA/UM2/CNRS URA 2133, Place Viala, F-34060 Montpellier Cedex 1, France; Corresponding author
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