NICOTIANAMINE - MEDIATOR OF TRANSPORT OF IRON AND HEAVY-METALS IN THEPHLOEM

Recent work has demonstrated that minerals in plants are circulated be tween root and shoot. This occurs during the whole life time and rende rs possible response to changing environmental conditions. This minera l circulation occurs through intensive solute exchange between xylem a nd phloem in roots, stems, and leaves. The transport form of heavy met als such as iron, manganese, zinc and copper in the phloem, whether io nic or chelated, is unclear in most cases. The unusual amino acid nico tianamine (NA) is ubiquitous throughout the plant kingdom. It is a che lator of several divalent transition metals. Its physiological role wa s investigated with the tomato mutant chloronerva, the only known NA-f ree multicellular plant. The mutant also exhibits disturbances of its iron metabolism and that of other heavy metals. This leads, among othe rs, to a typical intercostal chlorosis and progressive iron accumulati on in the leaves. From the heavy metal chelating properties of NA and from the phenotype of the mutant chloronerva it is concluded that NA i s needed for normal distribution of heavy metals in young growing tiss ues fed via the phloem. This function could be fulfilled by mediating phloem loading or unloading of heavy metals as well as by preventing, their precipitation in the alkaline phloem sap. An attempt is made to explain the chloronerva phenotype in the light of the phloem transport hypothesis of chelated iron.