Effects of high levels of zinc and manganese ions on Strategy I responses to iron deficiency in citrus

The aim of this work is to study the effect of elevated concentrations of either Zn2+ or Mn2+ in the culture media on the components of the system of iron acquisition by roots from Fe-deficient and Fe-supplied citrus seedlings. Seeds of Citrus aurantium (L.) were germinated in vitro. Nutrient solutions without iron, zinc or manganese were prepared for assays and then combinations of Fe-EDDHA, ZnSO4 center dot 7H(2)O or MnSO4 center dot H2O were added to obtain the following media: -Fe-Zn, +Fe-Zn, -Fe+Zn, +Fe+Zn, -Fe-Mn, +Fe-Mn, -Fe+Mn and +Fe+Mn. Data indicated that when either 100 mu M Zn2+ or 250 mu M Mn2+ were added to culture media containing 20 mu M Fe, iron concentration in roots and shoots was reduced with respect to the seedlings cultured in the same medium lacking Zn2+ or Mn2+. Reciprocally, the presence of iron in the medium lowered Zn and Mn levels in the plant. Addition of either 100 mu M Zn2+ or 250 mu M Mn2+ to the medium enhanced the transcription levels of FRO2, IRT1 and NRAMP3 genes in roots, this effect being more marked in Fe-starved plants. FC-R activity followed the same behaviour as the gene FRO2. Both Zn2+ and Mn2+ stimulated the activity of some genes (FRO2, IRT1 and NRAMP3) which was also induced by Fe-deficiency, although these effects appear to be independent of the inhibition of Fe uptake by these cations.