Identifying key root and rhizosphere traits for efficient zinc uptake in barley

It is estimated that about half of the cultivated soils are deficient in zinc (Zn), contributing substantially to human Zn deficiency. As cereals constitute a major component of the human diet, improving their Zn content by breeding is a crucial agricultural goal to mitigate human Zn deficiency. However, breeding nutrient-rich cereals requires the identification of the plant traits that most strongly contribute to efficient Zn acquisition under deficient soil.

Plants can enhance Zn acquisition through multiple, potentially interacting mechanisms. They can enhance the uptake by adapting their root morphology or by increasing the expression of Zn cell-membrane transporters. Beyond the physical root system, roots secrete a chemically diverse blend of high- and low-molecular weight compounds that mobilise Zn from the soil. Cereals employ a strategy based on phytosiderophores (PS), metal-chelating agents released by roots into the soil. Root-associated microorganisms can also impact the plant's micronutrient status either by directly mobilising micronutrients or enhancing general plant health. While the mechanistic importance of individual traits has been demonstrated, their relative contributions have rarely been evaluated within a single integrative framework.

Using barley (Hordeum vulgare L.) as a model crop, sixteen genotypes representing the northern European germplasm were grown in a Zn-deficient soil. A diverse array of root and rhizosphere phenotypes was screened. We quantified and characterised the root exudate metabolome, placing a special focus on phytosiderophores. Root morphological traits such as root length and surface area were characterised. The expression levels of genes involved in Zn uptake were also assessed. Amplicon sequencing of the 16S rRNA gene and the ITS2 region was conducted to explore the root-associated microbiome.

Zn uptake efficiency varied substantially among barley genotypes. Barley genotypes that efficiently acquired Zn, exuded higher amounts of phytosiderophores and exhibited a distinct exudate metabolome profile, suggesting that exudates may play a key role in plant Zn nutrition. Specific root length also emerged as a possible key phenotype. While root-associated microorganisms were influenced by the plant’s Zn status and genotype, we found only subtle microbial differences between Zn-efficient and less efficient genotypes, providing little indication of their role in Zn uptake. This study establishes an integrative framework for root and rhizosphere phenotyping with the aim of identifying key traits for producing nutrient-rich crops.