Direct Measurement of 59Fe-Labeled Fe2+ Influx in Roots of Pea Using a Chelator Buffer System to Control Free Fe2+ in Solution

Abstract

Fe2+ transport in plants has been difficult to quantify because of the inability to control Fe2+ activity in aerated solutions and non-specific binding of Fe to cell walls. In this study, a Fe(II)-3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-4[prime]4″-disulfonic acid buffer system was used to control free Fe2+ in uptake solutions. Additionally, desorption methodologies were developed to adequately remove nonspecifically bound Fe from the root apoplasm. This enabled us to quantify unidirectional Fe2+ influx via radiotracer (59Fe) uptake in roots of pea (Pisum sativum cv Sparkle) and its single gene mutant brz, an Fe hyperaccumulator. Fe influx into roots was dramatically inhibited by low temperature, indicating that the measured Fe accumulation in these roots was due to true influx across the plasma membrane rather than nonspecific binding to the root apoplasm. Both Fe2+ influx and Fe translocation to the shoots were stimulated by Fe deficiency in Sparkle. Additionally, brz, a mutant that constitutively exhibits high ferric reductase activity, exhibited higher Fe2+ influx rates than +Fe-grown Sparkle. These results suggest that either Fe deficiency triggers the induction of the Fe2+ transporter or that the enhanced ferric reductase activity somehow stimulates the activity of the existing Fe2+ transport protein.

A novel iron-regulated metal transporter from plants identified by functional expression in yeast.

“…Most other iron-efficient plants use strategy I and respond to iron deprivation by inducing the activity of membrane-bound Fe(III) chelate reductases that reduce Fe(III) to the more soluble Fe(II) form. The Fe(II) product is then taken up into the roots by an Fe(II)-specific transport system that is also induced by iron-limiting growth conditions (4). Furthermore, the roots of strategy I plants release more protons when iron-deficient, lowering the rhizosphere pH and thereby increasing the solubility of Fe(III).…”

The Role of Iron-Deficiency Stress Responses in Stimulating Heavy-Metal Transport in Plants1

“…Fe uptake in the dicots and the nongrass monocots is mediated by a plasma membrane-bound ferric reductase that transfers electrons from intracellular NADH (Buckhout et al, 1989) to Fe(III) chelates in the rhizosphere (Chaney et al, 1972). The ferrous ions (Fe 2ϩ ) released from the chelates by this process are subsequently transported into the cytoplasm via a separate transport protein (Kochian, 1991;Fox et al, 1996). When Fe deficient, dicot and nongrass monocots stimulate a number of processes to enhance Fe accumulation from the soil.…”

“…Finally, root Fe 2ϩ influx is regulated by the Fe status of the plant. Fox et al (1996) found that Fe-deficient pea (Pisum sativum L.) seedlings exhibit significantly higher rates of root Fe 2ϩ influx than Fe-sufficient seedlings. In addition to these responses, which are usually linked specifically to Fe accumulation, tissue concentrations of other mineral elements also appear to be influenced by plant Fe status.…”

Iron homeostasis related genes in rice

“…The ferric ion is then chelated by the released compounds, as well as by soil bacteriosiderophores (Römheld and Marschner, 1983;Bar-Ness et al,1992). Subsequently, chelated Fe 3+ is reduced by a low iron-inducible enzymatic activity present in the plasma membrane of root epidermal cells (Moog and Brüggermann, 1994) and Fe +2 is then absorbed by a specific transporter (Fox et al, 1996). A different strategy to overcome the problem of low iron availability in the soil evolved in grasses (such as rice).…”