Improving the concentration and bioavailability of zinc (Zn) in cereal grains is an important way to solve the problem of Zn deficiency in human body. The bioavailability of Zn is related to both its distribution and speciation in grains. In the current study, we examined the differences of Zn concentration, distribution, and speciation within grains among wheat cultivars with similar high grain yield but contrasting grain Zn concentration using synchrotron micro X-ray fluorescence (μ-XRF) and X-ray absorption near-edge structure (XANES). Results showed that compared to the low-Zn cultivar, the Zn concentration was 103, 50, 76, 33, and 64% higher in the crease region, aleurone layer, scutellum, embryonic axis, and endosperm of the high-Zn cultivar, respectively. Zinc mainly colocalized with phosphorus (P) in the aleurone layer and the scutellum, but less colocalization of Zn with P and a much lower concentration ratio of P/Zn were found in the high-Zn cultivar. Sulfur (S) is present in the form of scattered spots in the endosperm in accord with Zn, but the colocalization of Zn with S was predominant in the modified aleurone layer and the nucellar projection of the high-Zn cultivar. XANES results showed the lower proportion of Zn-phytate in the high-Zn cultivar. Findings indicated that it is possible to select the high-yield wheat cultivar with both high grain Zn concentration and high bioavailability, which provide a new perspective for genetic Zn biofortification.
An effective solution to global human zinc (Zn) deficiency is Zn biofortification of staple food crops, which has been hindered by the low available Zn in calcareous soils worldwide. Many culturable soil microbes have been reported to increase Zn availability in the laboratory, while the status of these microbes in fields and whether there are unculturable Zn-mobilizing microbes remain unexplored. Here, we use the culture-independent metagenomic sequencing to investigate the rhizosphere microbiome of three high-Zn (HZn) and three low-Zn (LZn) wheat cultivars in a field experiment with calcareous soils. The average grain Zn concentration of HZn was higher than the Zn biofortification target 40 mg kg–1, while that of LZn was lower than 40 mg kg–1. Metagenomic sequencing and analysis showed large microbiome difference between wheat rhizosphere and bulk soil but small difference between HZn and LZn. Most of the rhizosphere-enriched microbes in HZn and LZn were in common, including many of the previously reported soil Zn-mobilizing microbes. Notably, 30 of the 32 rhizosphere-enriched species exhibiting different abundances between HZn and LZn possess the functional genes involved in soil Zn mobilization, especially the synthesis and exudation of organic acids and siderophores. Most of the abundant potential Zn-mobilizing species were positively correlated with grain Zn concentration and formed a module with strong interspecies relations in the co-occurrence network of abundant rhizosphere-enriched microbes. The potential Zn-mobilizing species, especially Massilia and Pseudomonas, may contribute to the cultivars’ variation in grain Zn concentration, and they deserve further investigation in future studies on Zn biofortification.
Increasing iron (Fe) and zinc (Zn) concentrations in
crop grains
with high yield is an effective measure to ensure food supply and
alleviate mineral malnutrition in humans. Micronutrient concentrations
in grains depend on not only their availability in soils but also
their uptake in roots and translocation to shoots and grains. In this
three-year field study, we investigated genotypic variation in Fe
and Zn uptake and translocation within six wheat cultivars and examined
in detail Fe and Zn distributions in various tissues of two cultivars
with similar high yield but different grain Fe and Zn concentrations
using synchrotron micro-X-ray fluorescence. Results revealed that
root Fe and Zn concentrations were 11 and 44% greater in high-nutrient
(HN) than in low-nutrient (LN) concentration cultivar. Although both
cultivars accumulated similar amounts of Fe in shoots, HN cultivar
had greater accumulation of Fe in grain and greater accumulation of
Zn in both shoots and grain. Grain Zn concentration was positively
correlated with shoot Zn accumulation, and grain Fe concentration
was positively correlated with the ability to translocate Fe from
leaves/stem to grains. In the first nodes of shoots, HN cultivar had
482% greater Fe and 36% greater Zn concentrations in the enlarged
vascular bundle (EVB) than LN cultivar. In top nodes, HN cultivar
had 225 and 116% greater Fe and Zn concentrations in the transit vascular
bundle and 77 and 71% greater in the EVB when compared to LN cultivar.
HN cultivar also had a greater ability to allocate Fe and Zn to the
grain than LN cultivar. In conclusion, HN cultivar had greater capacity
of Fe and Zn acquirement by roots and translocation and partitioning
from shoots into grains. Screening wheat cultivars for larger Fe and
Zn concentrations in shoot nodes could be a novel strategy for breeding
crops with greater grain Fe and Zn concentrations.
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