Nitrogen recycling from the xylem in rice leaves: dependence upon metabolism and associated changes in xylem hydraulics

摘要

Measurements of amino acids in the guttation fluid and in the xylem exudates of cut leaves from intact plants provideudevidence of the remarkable efficiency with which these nitrogenous compounds are reabsorbed from the xylem sap.udThis could be achieved by mechanisms involving intercellular transport and/or metabolism. Developmental changesudin transcripts and protein showed that transcripts for phosphoenolpyruvate carboxykinase (PEPCK) increased fromudthe base to the leaf tip, and were markedly increased by supplying asparagine. Supplying amino acids also increasedudthe amounts of protein of PEPCK and, to a lesser extent, of pyruvate, Pi dikinase. PEPCK is present in the hydathodes,udstomata and vascular parenchyma of rice leaves. Evidence for the role of PEPCK was obtained by using 3-mercaptopicolinicudacid (MPA), a specific inhibitor of PEPCK, and by using an activation-tagged rice line that had an increaseudin PEPCK activity, to show that activation of PEPCK resulted in a decrease in N in the guttation fluid and that treatmentudby MPA resulted in an increase in amino acids in the guttation fluid and xylem sap towards the leaf tip. Furthermore,udincreasing PEPCK activity decreased the amount of guttation fluid, whereas decreasing PEPCK activity increased theudamount of xylem sap or guttation fluid towards the leaf tip. The findings suggest the following hypotheses: (i) bothudmetabolism and transport are involved in xylem recycling and (ii) excess N is the signal involved in modulating xylemudhydraulics, perhaps via nutrient regulation of water-transporting aquaporins. Water relations and vascular metabolismudand transport are thus intimately linked.