The expression of heterologous Fe (III) phytosiderophore transporter HvYS1 in rice increases Fe uptake, translocation and seed loading and excludes heavy metals by selective Fe transport

Abstract

SummaryMany metal transporters in plants are promiscuous, accommodating multiple divalent cations including some which are toxic to humans. Previous attempts to increase the iron (Fe) and zinc (Zn) content of rice endosperm by overexpressing different metal transporters have therefore led unintentionally to the accumulation of copper (Cu), manganese (Mn) and cadmium (Cd). Unlike other metal transporters, barley Yellow Stripe 1 (HvYS1) is specific for Fe. We investigated the mechanistic basis of this preference by constitutively expressing HvYS1 in rice under the control of the maize ubiquitin1 promoter and comparing the mobilization and loading of different metals. Plants expressing HvYS1 showed modest increases in Fe uptake, root‐to‐shoot translocation, seed accumulation and endosperm loading, but without any change in the uptake and root‐to‐shoot translocation of Zn, Mn or Cu, confirming the selective transport of Fe. The concentrations of Zn and Mn in the endosperm did not differ significantly between the wild‐type and HvYS1 lines, but the transgenic endosperm contained significantly lower concentrations of Cu. Furthermore, the transgenic lines showed a significantly reduced Cd uptake, root‐to‐shoot translocation and accumulation in the seeds. The underlying mechanism of metal uptake and translocation reflects the down‐regulation of promiscuous endogenous metal transporters revealing an internal feedback mechanism that limits seed loading with Fe. This promotes the preferential mobilization and loading of Fe, therefore displacing Cu and Cd in the seed.

Iron homeostasis in plants – a brief overview

“… 110 This has so far not been a problem, for example overexpression of barley Yellow Stripe 1 ( HvYS1 ) led specifically to increases in iron. 111 …”

Physiological background of the remarkably high Cd2+ tolerance of the Aspergillus fumigatus Af293 strain

“…Therefore, the heterologous expression of the pcaA gene will be able to increase the cadmium tolerance of fungal strains with bioremediation and biomining potential, and this approach may represent an attractive future strain improvement strategy when the application of living fungal biomass is considered in various environmental biotechnological processes. Moreover, our results also suggest that strategies based on the inhibition of cadmium uptake by plants can be efficiently combined with strategies aimed at increasing the cadmium efflux of plant cells to prevent the accumulation of this toxic metal in crops.…”

“…However, in plants, Cd 2+ enters root cells predominantly via Fe 2+ transporters, for example through OsIRT1 and OsNRAMP5 in rice . The upregulation of iron transport mechanisms specific to iron but not to cadmium—for example (phyto)siderophore‐mediated iron transport or certain metal transporters—can reduce cadmium uptake by reducing the activity of iron transporters also used by Cd 2+ . Therefore, the observed correlation between the ability for TAF‐C production and for growth in the presence of CdCl 2 in A. fumigatus (Fig.…”

“…Therefore, coping with both iron shortages and iron overloads can be a real challenge for soil microbes, including fungi. Under these ecological conditions, the overproduction of TAF‐C and the overexpression of pcaA would also lead to a high cadmium tolerance merely by accident, but this phenotype would then be independent of the Cd 2+ burden of the habitat. Interestingly, in manure, animal litter, sewage sludge and compost, which are typical habitats for A. fumigatus , the fungus typically seems to face high‐iron, low‐cadmium environments , which also makes the evolution of highly specific Cd 2+ ‐specific pumps rather unlikely on these substrates.…”

Analysis of Yellow Striped Mutants of Zea mays Reveals Novel Loci Contributing to Iron Deficiency Chlorosis

“…In an early study using constitutive expression of barley YS1 in rice, plants showed superior growth in alkaline soil conditions but did not contain significantly more iron in grains (Gomez-Galera et al, 2012). Later, barley YS1 expressed in rice was shown to promote the preferential mobilization and loading of Fe in seeds while displacing Cd and Cu (Banakar et al, 2017). At present, two key uptake genes, Ys1 and Ys3 ( TOM1 ), for the grass specific mechanism are understood, as are the genes involved in PS synthesis, but it is unclear whether additional grass specific components exist.…”