Are phloem amino acids involved in the shoot to root control of NO-3 uptake in Ricinus communis plants?

Abstract

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 Gin and Ser were rapidly lowered by 50%, while those of Val, Phe, Leu, and lie 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.