Root-derived cytokinins as long-distance signals for NO3--induced stimulation of leaf growth

Leaf growth of many plant species shows rapid changes in response to alterations of the form and the level of N supply. In hydroponically-grown tomato (Lycopersicon esculentum L.), leaf growth was rapidly stimulated by NO3- application to NH4+ precultured plants, while NH4+ supply or complete N deprivation to NO3- precultured plants resulted in a rapid inhibition of leaf growth. Just 10 mu M NO3- supply was sufficient to stimulate leaf growth to the same extent as 2 mM. Furthermore, continuous NO3- supply induced an oscillation of leaf growth rate with a 48 h interval. Since changes in NO3- levels in the xylem exudate and leaves did not correlate with NO3--induced alterations of leaf growth rate, additional signals such as phytohormones may be involved. Levels of a known inhibitor of leaf growth, abscisic acid (ABA), did not consistently correspond to leaf growth rates in wild-type plants. Moreover, leaf growth of the ABA-deficient tomato mutant flacca was inhibited by NH4+ without an increase in ABA concentration and was stimulated by NO3- despite its excessive ethylene production. These findings suggest that neither ABA nor ethylene are directly involved in the effects of N form on leaf growth. However, under all experimental conditions, stimulation of leaf growth by NO3- was consistently associated with increased concentration of the physiologically active forms of cytokinins, zeatin and zeatin riboside, in the xylem exudate. This indicates a major role for cytokinins as long-distance signals mediating the shoot response to NO3- perception in roots.