Summary: Studies of Iron (Fe) uptake mechanisms by plant roots have focussed on Fe(III)‐siderophores or Fe(II) transport systems. Iron deficency also enhances root secretion of flavins and phenolics. However, the nature of these compounds, their transport outside the roots and their role in Fe nutrition are largely unknown. We used HPLC/ESI‐MS (TOF) and HPLC/ESI‐MS/MS (ion trap) to characterize fluorescent phenolic‐type compounds accumulated in roots or exported to the culture medium of Arabidopsis plants in response to Fe deficiency. Wild‐type and mutant plants altered either in phenylpropanoid biosynthesis or in the ABCG37 (PDR9) ABC transporter were grown under standard or Fe‐deficient nutrition conditions and compared. Fe deficiency upregulates the expression of genes encoding enzymes of the phenylpropanoid pathway and leads to the synthesis and secretion of phenolic compounds belonging to the coumarin family. The ABCG37 gene is also upregulated in response to Fe deficiency and coumarin export is impaired in pdr9 mutant plants. Therefore it can be concluded that: Fe deficiency induces the secretion of coumarin compounds by Arabidopsis roots; the ABCG37 ABC transporter is required for this secretion to take place; and these compounds improved plant Fe nutrition.

The identification of Fe transport forms in plant xylem sap is crucial to the understanding of long-distance Fe transport processes in plants. Previous studies have proposed that Fe may be transported as an Fe–citrate complex in plant xylem sap, but such a complex has never been detected. In this study we report the first direct and unequivocal identification of a natural Fe complex in plant xylem sap. A tri-Fe(III), tri-citrate complex (Fe₃Cit₃) was found in the xylem sap of Fe-deficient tomato (Solanum lycopersicum Mill. cv. ‘Tres Cantos’) resupplied with Fe, by using an integrated mass spectrometry approach based on exact molecular mass, isotopic signature and Fe determination and retention time. This complex has been modeled as having an oxo-bridged tri-Fe core. A second complex, a di-Fe(III), di-citrate complex was also detected in Fe–citrate standards along with Fe₃Cit₃, with the allocation of Fe between the two complexes depending on the Fe to citrate ratio. These results provide evidence for Fe–citrate complex xylem transport in plants. The consequences for the role of Fe to citrate ratio in long-distance transport of Fe in xylem are also discussed.

This article reviews the current state of knowledge of the roles of iron in chloroplast structure and function and in chloroplast development. The uptake and transport of iron to leaves and the relationship of chlorophyll to leaf iron content are also reviewed briefly. In addition, we present some original data on the protein composition of thylakoid membranes and on chlorophyll biosynthesis as affected by iron deficiency and resupply.