Battling micronutrient deficiency in barley
The World Health Organization estimates that two billion people suffer from micronutrient malnutrition including iron (Fe), zinc (Zn), and copper (Cu). Plant breeding based biofortification, i.e. the delivery of micronutrients via micronutrient-rich crops is considered the most cost-effective and sustainable approach to alleviate this so called 'hidden hunger'. In this study, we investigated phytosiderophore (PS) biosynthesis and exudation as a potential trait in crop breeding for producing micronutrient dense crops.  Two barley lines (Hordeum vulgare (B13 Balashere and BJ4648) with contrasting micronutrient grain yields were grown in a hydroponic experiment and PS release (analyzed with LC-ESI-MS/MS), root gene expression patterns (RNAseq), as well as biomass parameters were determined after 24 days of either Fe, Zn, or Cu withdrawal. Our two experimental cultivars accumulated the respective micronutrient in significantly lower quantities under Fe, Zn or Cu starvation despite showing no biomass effect. We found that the activation intensity of the PS pathway genes and PS exudation rates differed for different micronutrient deficiencies (Fe > Zn > Cu). In addition, we observed a stronger upregulation of PS pathway genes and greater PS exudation in the barley line with high micronutrient grain yield, suggesting that a strong response of the PS pathway might be a major trait involved in large micronutrient accumulation in cereal crops. The expression of desoxymugineic acid synthase (DMAS) showed a strong positive correlation with phytosiderophore exudation and our results further indicate that the composition of different phytosiderophores exuded was determined by the ratio of IDS3 and IDS2 expression. We also identified several other genes (such as several HSP20 genes) commonly triggered under Fe, Zn or Cu deficiency. These findings can contribute to developing more nutrient rich crops by revealing the main determinants of micronutrient uptake in cereal crops.