Abstract
Legumes, which develop a symbiosis with nitrogen-fixing bacteria, have an increased demand for iron. Iron is required for the synthesis of iron-containing proteins in the host, including the highly abundant leghemoglobin, and in bacteroids for nitrogenase and cytochromes of the electron transport chain. Deficiencies in iron can affect initiation and development of the nodule. Within root cells, iron is chelated with organic acids such as citrate and nicotianamine and distributed to other parts of the plant. Transport to the nitrogen-fixing bacteroids in infected cells of nodules is more complicated. Formation of the symbiosis results in bacteroids internalized within root cortical cells of the legume where they are surrounded by a plant-derived membrane termed the symbiosome membrane (SM). This membrane forms an interface that regulates nutrient supply to the bacteroid. Consequently, iron must cross this membrane before being supplied to the bacteroid. Iron is transported across the SM as both ferric and ferrous iron. However, uptake of Fe(II) by both the symbiosome and bacteroid is faster than Fe(III) uptake. Members of more than one protein family may be responsible for Fe(II) transport across the SM. The only Fe(II) transporter in nodules characterized to date is GmDMT1 (Glycine max divalent metal transporter 1), which is located on the SM in soybean. Like the root plasma membrane, the SM has ferric iron reductase activity. The protein responsible has not been identified but is predicted to reduce ferric iron accumulated in the symbiosome space prior to uptake by the bacteroid. With the recent publication of a number of legume genomes including Medicago truncatula and G. max, a large number of additional candidate transport proteins have been identified. Members of the NRAMP (natural resistance-associated macrophage protein), YSL (yellow stripe-like), VIT (vacuolar iron transporter), and ZIP (Zrt-, Irt-like protein) transport families show enhanced expression in nodules and are expected to play a role in the transport of iron and other metals across symbiotic membranes.
Highlights
All plants require the micronutrient iron for optimum growth
As iron is likely to be imported into infected cells from the apoplast in M. truncatula (Rodriguez-Haas et al, 2013), it is likely that similar transporter families are involved in uptake in indeterminant nodules
LjSEN1 is expressed in infected cells of the nodule and the protein it encodes is proposed to be a ferrous iron transporter based on its distant homology to AtVIT1 (Kim et al, 2006) and CCC1 (Li et al, 2001)
Summary
All plants require the micronutrient iron for optimum growth. legumes, which develop symbiotic relationships with nitrogen-fixing bacteria, have an increased demand for the micronutrient (Tang et al, 1990a). As iron is likely to be imported into infected cells from the apoplast in M. truncatula (Rodriguez-Haas et al, 2013), it is likely that similar transporter families are involved in uptake in indeterminant nodules. Whether uptake of ferrous iron is favored by the symbiosome within the natural nodule environment is yet to be determined and will most likely depend on the concentrations of Fe(II) and Fe(III) in the infected cell cytosol.
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