Plants fabricate Fe-nanocomplexes at root surface to counter and phytostabilize excess ionic Fe

Abstract

While evaluating the impact of iron nanoparticles (NPs) on terrestrial plants we realized potential of root system of intact plants to form orange-brown complexes constituted of NPs around their roots and at bottom/side of tubes when exposed to FeCl3. These orange-brown complexes/plaques seen around roots were similar to that reported in wetland plants under iron toxicity. Transmission electron microscopy coupled with energy dispersive X-ray analysis revealed that orange-brown complexes/plaques, formed by root system of all 16 plant species from 11 distinct families tested, were constituted of NPs containing Fe. Selected area electron diffraction and powder X-ray diffraction spectra showed their amorphous nature. Thermogravimetric and fourier transform infra-red analysis showed that these Fe-NPs/nanocomplexes were composed of iron-oxyhydroxide. These plant species generated orange-brown Fe-NPs/nanocomplexes even under strict sterile conditions establishing inbuilt and independent potential of their root system to generate Fe-NPs. Root system of intact plants showed ferric chelate reductase activity responsible for reduction of Fe(3+) to Fe(2+). Reduction of potassium ferricyanide by root system of intact plants confirmed that root surface possess strong reducing strength, which could have played critical role in reduction of Fe(3+) and formation of Fe-NPs/nanocomplexes. Atomic absorption spectrophotometric analysis revealed that majority of iron was retained in Fe-nanocomplexes/plaques, while only 2-3 % was transferred to shoots, indicating formation of nanocomplexes is a phytostabilization mechanism evolved by plants to restrict uptake of iron above threshold levels. We believe that formation of Fe-NPs/nanocomplexes is an ideal homeostasis mechanism evolved by plants to modulate uptake of desired levels of ionic Fe.

Reducing Strength Prevailing at Root Surface of Plants Promotes Reduction of Ag+ and Generation of Ag0/Ag2O Nanoparticles Exogenously in Aqueous Phase

“…Earlier, it had been unequivocally proven that root system of plant species possess strong reducing capacity [32]–[36].…”

“…Therefore, we believe that some of the Ag 0 NPs formed by reduction of Ag + at the root surface were oxidized to Ag 2 O NPs in aqueous phase under ambient conditions. Recently, we also reported that the root surface of plants possesses potential to form iron oxy-hydroxide NPs [36].…”

“…However, these biological methods also have certain limitations, such as (i) use of microorganism requires special facilities for their maintenance and safety measures [32]; (ii) it is difficult to precisely identify the biomolecule(s) amongst the cocktail of molecules in extracts of various biological materials/components, that are responsible for generation of silver NPs; (iii) it would be difficult to extract NPs synthesized intracellular in cells of plants or microorganisms and moreover, due to the presence of wide variety of biomolecules (having reducing strength) within the cells, NPs formed intracellular would be of broad size/shape range (31–32). In search for an alternate andideal green method, we felt it wise to exploit root system of intact plants for generating silver nanoparticles, as plant biologists clearly established that root system of plants possess immense reducing strength [32]–[36] and generation of metal nanoparticles from ions primarily involves reduction [10], [11], [31], [32], [36]. Therefore, the present investigations were carried with the aim to evaluate the potential of plants to generate silver nanoparticles exogenously at the root surface.…”

Differential Response of Floating and Submerged Leaves of Longleaf Pondweed to Silver Ions

“…Earlier, Pardha-Saradhi et al (2014a,b,c) reported that plants reduce toxic ionic forms of heavy metals into non/less-toxic NPs as a defense mechanism. During our investigations, we found that both floating and submerged leaves of longleaf pondweed could turn clear colorless AgNO 3 solutions to colloidal brown ( Figures 1A,B and Supplementary Figure 1).…”

“…A large number of plants are hyper-accumulators of heavy metals, so many researchers are now trying to understand hyper-accumulating strategies in these plants (Kamal et al, 2004; Dhir et al, 2009; Sharma and Dietz, 2009). Pardha-Saradhi et al (2014a,b,c) have shown that terrestrial plants have the potential to biotransform precious heavy metal ions (e.g., Au 3+ and Ag + ) and essential metal ions (e.g., Fe 3+ ) into less/non-toxic nanoparticles (NPs)/nanocomplexes. Shabnam et al (2017) have recently demonstrated that Ag-NPs are significantly less toxic than ionic Ag.…”