Control of nitrate uptake by phloem-translocated glutamine in Zea mays L. seedlings

The putative role of glutamine, exported from leaves to roots, as a negative feedback signal for nitrate uptake was investigated in Zea mays L. seedlings. Glutamine (Gin) was supplied by immersion of the tip-cut leaves in a concentrated solution. Nitrate (NO3-) uptake was measured by its depletion in amino acid-free medium. The treatment with Gin resulted in a strong inhibition of nitrate uptake rate, accompanied by a significant enrichment of amino compounds in root tissue. The effect of N-availability on NO3- uptake was determined in split-root cultures. The plants were subjected to complete or localized N supply. inducible NO3- uptake systems were also induced in N-deprived roots when the opposite side of the root system was supplied with KNO3. The inhibitory effect of Gin was unaffected by localized N supply on one side of the split-root. The potential role of Gin in the shoot-to-root control of NO3- uptake is discussed.