Abstract
The non-proteinogenic amino acid nicotianamine (NA) is ubiquitous among plants. In meristematic tissues it reaches concentrations of about 400μmol (g fresh weight)−1. NA forms complexes, among others, with the metal micronutrients (MN) copper, zinc, iron and manganese (logKMeNA 18.6-8.8). Calculations of the dissociation curves of the metal-NA complexes based on the complex formation constants and on the acid dissociation constants of NA revealed their stability at the neutral or weak alkaline pH of cytoplasm and sieve tube sap. For the Mn-NA complex, dissociation begins at about pH 6.5, for all others dissociation occurs at more acid pHs. Thus, metal-NA complexes could theoretically persist also in the apoplasm and in xylem sap. The octanol water partition coefficient of NA is about 1 and those of its metal complexes are in the range of 0.3–0.4. The reason for this shift is perhaps the negative charge of the complexes. The higher lipophilicity of the free NA indicates that the NA supply to sites of requirement is faster than the removal of the complexes as long as membranes are an integral part of the transport paths. Changing phloem transport rates of MN-NA complexes by manipulation of the cotyledon apoplasm of Ricinus commuais L. suggest a competition of MN for NA at the site(s) of phloem loading. Thus, NA could control MN transport via phloem including recirculation.
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