Combating Mineral Malnutrition through Iron and Zinc Biofortification of Cereals

Spatially resolved analysis of variation in barley (Hordeum vulgare) grain micronutrient accumulation

Genetic biofortification requires knowledge on natural variation and the underlying mechanisms of micronutrient accumulation. We therefore studied diversity in grain micronutrient concentrations and spatial distribution in barley (Hordeum vulgare), a genetically tractable model cereal and an important crop with widespread cultivation. We assembled a diverse collection of barley cultivars and landraces and analysed grain micronutrient profiles in genebank material and after three independent cultivations. Lines with contrasting grain zinc (Zn) accumulation were selected for in-depth analysis of micronutrient distribution within the grain by micro-proton-induced X-ray emission (μ-PIXE). Also, we addressed association with grain cadmium (Cd) accumulation. The analysis of > 120 lines revealed substantial variation, especially in grain Zn concentrations. A large fraction of this variation is due to genetic differences. Grain dissection and μ-PIXE analysis of contrasting lines showed that differences in grain Zn accumulation apply to all parts of the grain including the endosperm. Cd concentrations exceeded the Codex Alimentarius threshold in most of the representative barley lines after cultivation in a Cd-contaminated agricultural soil. Two important conclusions for biofortification are: first, high-Zn grains contain more Zn also in the consumed parts of the grain; and second, higher micronutrient concentrations are strongly associated with higher Cd accumulation.

Improving Rice Zinc Biofortification Success Rates Through Genetic and Crop Management Approaches in a Changing Environment

Though rice is the predominant source of energy and micronutrients for more than half of the world population, it does not provide enough zinc (Zn) to match human nutritional requirements. Moreover, climate change, particularly rising atmospheric carbon dioxide concentration, reduces the grain Zn concentration. Therefore, rice biofortification has been recognized as a key target to increase the grain Zn concentration to address global Zn malnutrition. Major bottlenecks for Zn biofortification in rice are identified as low Zn uptake, transport and loading into the grain; however, environmental and genetic contributions to grain Zn accumulation in rice have not been fully explored. In this review, we critically analyze the key genetic, physiological and environmental factors that determine Zn uptake, transport and utilization in rice. We also explore the genetic diversity of rice germplasm to develop new genetic tools for Zn biofortification. Lastly, we discuss the strategic use of Zn fertilizer for developing biofortified rice.

Phosphate and zinc transport and signalling in plants: toward a better understanding of their homeostasis interaction

Inorganic phosphate (Pi) and zinc (Zn) are two essential nutrients for plant growth. In soils, these two minerals are either present in low amounts or are poorly available to plants. Consequently, worldwide agriculture has become dependent on external sources of Pi and Zn fertilizers to increase crop yields. However, this strategy is neither economically nor ecologically sustainable in the long term, particularly for Pi, which is a non-renewable resource. To date, research has emphasized the analysis of mineral nutrition considering each nutrient individually, and showed that Pi and Zn homeostasis is highly regulated in a complex process. Interestingly, numerous observations point to an unexpected interconnection between the homeostasis of the two nutrients. Nevertheless, despite their fundamental importance, the molecular bases and biological significance of these interactions remain largely unknown. Such interconnections can account for shortcomings of current agronomic models that typically focus on improving the assimilation of individual elements. Here, current knowledge on the regulation of the transport and signalling of Pi and Zn individually is reviewed, and then insights are provided on the recent progress made towards a better understanding of the Zn-Pi homeostasis interaction in plants.

Zn and Fe biofortification: the right chemical environment for human bioavailability

A considerable fraction of global disease burden and child mortality is attributed to Fe and Zn deficiencies. Biofortification, i.e. the development of plants with more bioavailable Zn and Fe, is widely seen as the most sustainable solution, provided suitable crops can be generated. In a cereal-dominated diet availability of Fe and Zn for absorption by the human gut is generally low and influenced by a highly complex chemistry. This complexity has mostly been attributed to the inhibitory effect of Fe and Zn binding by phytate, the principal phosphorus storage compound in cereal and legume seeds. However, phytate is only part of the answer to the multifaceted bioavailability question, albeit an important one. Recent analyses addressing elemental distribution and micronutrient speciation in seeds strongly suggest the existence of different Fe and Zn pools. Exploration of natural variation in maize showed partial separation of phytate levels and Fe bioavailability. Observations made with transgenic plants engineered for biofortification lend further support to this view. From a series of studies the metal chelator nicotianamine is emerging as a key molecule. Importantly, nicotianamine levels have been found to not only increase the loading of Fe and Zn into grains. Bioavailability assays indicate a strong activity of nicotianamine also as an enhancer of intestinal Fe and Zn absorption.

Phosphate/zinc interaction analysis in two lettuce varieties reveals contrasting effects on biomass, photosynthesis, and dynamics of Pi transport

Inorganic phosphate (Pi) and Zinc (Zn) are essential nutrients for normal plant growth. Interaction between these elements has been observed in many crop plants. Despite its agronomic importance, the biological significance and genetic basis of this interaction remain largely unknown. Here we examined the Pi/Zn interaction in two lettuce (Lactuca sativa) varieties, namely, "Paris Island Cos" and "Kordaat." The effects of variation in Pi and Zn supply were assessed on biomass and photosynthesis for each variety. Paris Island Cos displayed better growth and photosynthesis compared to Kordaat under all the conditions tested. Correlation analysis was performed to determine the interconnectivity between Pi and Zn intracellular contents in both varieties. Paris Island Cos showed a strong negative correlation between the accumulation levels of Pi and Zn in shoots and roots. However, no relation was observed for Kordaat. The increase of Zn concentration in the medium causes a decrease in dynamics of Pi transport in Paris Island Cos, but not in Kordaat plants. Taken together, results revealed a contrasting behavior between the two lettuce varieties in terms of the coregulation of Pi and Zn homeostasis and provided evidence in favor of a genetic basis for the interconnection of these two elements.