Metabolic changes of iron uptake in N2-fixing common bean nodules during iron deficiency

Abstract

Iron is an important nutrient in N(2)-fixing legume nodules. The demand for this micronutrient increases during the symbiosis establishment, where the metal is utilized for the synthesis of various iron-containing proteins in both the plant and the bacteroid. Unfortunately, in spite of its importance, iron is poorly available to plant uptake since its solubility is very low when in its oxidized form Fe(III). In the present study, the effect of iron deficiency on the activity of some proteins involved in Strategy I response, such as Fe-chelate reductase (FC-R), H(+)-ATPase, and phosphoenolpyruvate carboxylase (PEPC) and the protein level of iron regulated transporter (IRT1) and H(+)-ATPase proteins has been investigated in both roots and nodules of a tolerant (Flamingo) and a susceptible (Coco blanc) cultivar of common bean plants. The main results of this study show that the symbiotic tolerance of Flamingo can be ascribed to a greater increase in the FC-R and H(+)-ATPase activities in both roots and nodules, leading to a more efficient Fe supply to nodulating tissues. The strong increase in PEPC activity and organic acid content, in the Flamingo root nodules, suggests that under iron deficiency nodules can modify their metabolism in order to sustain those activities necessary to acquire Fe directly from the soil solution.

Immunolocalization of H+-ATPase and IRT1 enzymes in N2-fixing common bean nodules subjected to iron deficiency

“…Like in roots, transporters homologues to iron regulated transporter (IRT1) should be present in nodules. The amount of this protein was increased under Fe deficiency also in nodules enabling them to acquire Fe directly from the soil nutrient solution (Slatni et al, 2011). Additionally, Slatni et al (2011) showed that also H + -ATPase activity and protein level were increased under Fe deficiency in nodules of both cultivars, mainly in Flamingo.…”

“…The amount of this protein was increased under Fe deficiency also in nodules enabling them to acquire Fe directly from the soil nutrient solution (Slatni et al, 2011). Additionally, Slatni et al (2011) showed that also H + -ATPase activity and protein level were increased under Fe deficiency in nodules of both cultivars, mainly in Flamingo. From the data obtained in this previous work we might now hypothesise that, when common bean plants grow with low Fe availability, nodules acquire the capacity to take up Fe not only from the plant cytosol, but also directly from the nutrient solution.…”

“…The same studies suggested that the nodules of tolerant cultivar contributed to the rhizosphere acidification and to the reduction of apoplastic Fe 3+ . It is also noteworthy that the activity of FC-R under Fe deficiency was higher in nodules than in roots (about two fold), suggesting that under this condition nodules play an important role in Fe 3+ reduction and Fe uptake and consequently in Fe supply to the bacteroids (Slatni et al, 2011). Like in roots, transporters homologues to iron regulated transporter (IRT1) should be present in nodules.…”

“…Under symbiotic N 2 -fixing condition, in which the demand for Fe increases, the preferential allocation of Fe to nodules and the Fe-use efficiency for nodules growth and functions were found to be the base of tolerance of some common bean cultivars Slatni et al, 2008Slatni et al, , 2009). This adaptive mechanism includes the induction of three activities localized at the PM of roots cells and PBM and BM of symbiosome cells: (i) a proton pump acidifies the rhizosphere or PBS, thus driving more Fe into solution, (ii) a FC-R converts Fe 3+ -chelates to Fe 2+ and (iii) a Fe 2+ transporter moves Fe across the plasma or PBM into cells (Slatni et al, 2011).…”

“…Previous results focused to verify the ability of nodules to acquire Fe directly from the soil solution were obtained and showed, by detection both "in vivo" and "in vitro", the stimulation of FC-R activity in nodulated roots and detached nodules of Fe-deficient plants (Slatni et al, 2009(Slatni et al, , 2011. Two common bean cultivars were used in these studies, Coco blanc, sensitive and Flamingo, tolerant to Fe deficiency .…”