Abstract
Iron uptake in dicots depends on their ability to induce a set of responses in root cells including rhizosphere acidification through H+ extrusion and apoplastic Fe(III) reduction by Fe(III)-chelate reductase. These responses must be sustained by metabolic rearrangements aimed at providing the required NAD(P)H, ATP and H+. Previous results in Fe-deficient cucumber roots showed that high H+ extrusion is accompanied by increased phosphoenolpyruvate carboxylase (PEPC) activity, involved in the cytosol pH-stat; moreover 31P-NMR analysis revealed increased vacuolar pH and decreased vacuolar [inorganic phosphate (Pi)]. The opposite was found in soybean: low rhizosphere acidification, decreased PEPC activity, vacuole acidification, and increased vacuolar [Pi]. These findings, highlighting a different impact of the Fe deficiency responses on cytosolic pH in the two species, lead to hypothesize different roles for H+ and Pi movements across the tonoplast in pH homeostasis. The role of vacuole in cytosolic pH-stat involves the vacuolar H+-ATPase (V-ATPase) and vacuolar H+-pyrophosphatase (V-PPase) activities, which generating the ΔpH and ΔΨ, mediate the transport of solutes, among which Pi, across the tonoplast. Fluxes of Pi itself in its two ionic forms, H2PO4- predominating in the vacuole and HPO42- in the cytosol, may be involved in pH homeostasis owing to its pH-dependent protonation/deprotonation reactions. Tonoplast enriched fractions were obtained from cucumber and soybean roots grown with or without Fe. Both V-ATPase and V-PPase activities were analyzed and the enrichment and localization of the corresponding proteins in root tissues were determined by Western blot and immunolocalization. V-ATPase did not change its activity and expression level in response to Fe starvation in both species. V-PPase showed a different behavior: in cucumber roots its activity and abundance were decreased, while in Fe-deficient soybean roots they were increased. The distinct role of the two H+ pumps in Pi fluxes between cytoplasm and vacuole in Fe-deficient cucumber and soybean root cells is discussed.
Highlights
Among Strategy I plants there is an inter- and intra-specific variability in susceptibility to lime-induced Fe deficiency, despite a similar demand for Fe. This variability is strongly related to the ability of plants to enhance the activities involved in the so-called reduction based Strategy I (Marschner et al, 1986; Schmidt, 2006; Kim and Guerinot, 2007; Ivanov et al, 2012), residing at the root cell plasma membrane (PM), which consists of three main steps: (1) acidification of the root apoplast and rhizosphere, mainly due to the enhanced H+ extrusion driven by the plasma membrane H+-ATPase (PMATPase); (2) reduction of the extracellular Fe3+ to Fe2+, which is the only form transported into the root by these species, by means of Fe(III)-chelate reductase (FC-R) which utilizes NAD(P)H as reducing substrate; (3) Fe2+ transport into the root symplast by IRT1, a member of the ZIP family
Cucumber and soybean plants grown in Fe-deprived nutrient solution for 8 and 11 days, respectively, showed highly chlorotic leaves and the typical Fe deficiency responses already described in previous works at the root level (Rabotti et al, 1995; Espen et al, 2000; Zocchi et al, 2007): in cucumber, a strong increase in both Fe reduction and H+ extrusion, by induction of FC-R and PM-ATPase activities respectively, enhanced activity of glycolytic enzymes and phosphoenolpyruvate carboxylase (PEPC); in soybean, increased Fe reduction activity but weak acidification of the nutrient solution, no induction of the PM-ATPase activity, only slight increase of glycolytic enzymes and decrease of PEPC activity
By comparing the response to Fe deficiency in two Strategy I species, cucumber and soybean, it emerged that the different aptitudes to induce the PM-ATPase are accompanied by other differences, related to the root cell vacuolar composition: in cucumber roots, in which PM-ATPase and PEPC activities are strongly induced, the vacuole undergoes an alkalization and a depletion of Pi, while in soybean, in which PM-ATPase and PEPC are weakly and/or not induced, the vacuolar pH is decreased and the Pi concentration increased (Espen et al, 2000; Zocchi et al, 2007)
Summary
Among Strategy I plants (dicots and non-graminaceous monocots) there is an inter- and intra-specific variability in susceptibility to lime-induced Fe deficiency, despite a similar demand for Fe. In Fe-deficient cucumber plants the high induction of the Strategy I responses is concomitant with the increase in carbohydrate catabolism, through the up-regulation of glycolysis and oxidative pentose phosphate pathway (Rabotti et al, 1995; Espen et al, 2000), providing ATP and reducing equivalents. In such conditions, phosphoenolpyruvate carboxylase (PEPC) activity results to be enhanced even more than the primary responses (De Nisi and Zocchi, 2000), leading to PEP consumption (with further increase in the glycolysis rate) and synthesis of organic acids, which are protogenic.
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