Wanted: Micronutrients – Exploring the phytosiderophore pathway for micronutrient acquisition in plant-soil systems

Micronutrient (MN) deficiencies, particularly of iron (Fe), zinc (Zn), and copper (Cu), are major constraints to crop productivity in arid and semi-arid regions characterized by high-pH calcareous soils. Effective strategies for acquiring MN are essential to ensure high yields on nutrient-depleted soils and produce MN-rich crops. In the case of iron, grass species (Poaceae) increase Fe phytoavailability by releasing root exudates called phytosiderophores (PS), which have the capability to chelate and mobilize Fe from the soil, thereby facilitating its uptake by plants. So far, eight naturally occurring PS compounds have been identified among the Poaceae: mugineic acid (MA), 3"-hydroxymugineic acid (HMA), 3"-epi-hydroxymugineic acid (epi-HMA), hydroxyavenic acid (HAVA), deoxymugineic acid (DMA), 3"-hydroxydeoxymugineic acid (HDMA), 3"-epi-hydroxydeoxymugineic acid (epi-HDMA) and avenic acid (AVA). Given the commercial unavailability of all eight PS, research until now has largely focused on DMA and, occasionally, MA, primarily in relation to Fe acquisition, with much of the research conducted under artificial conditions like hydroponics. These limitations restrict our understanding of the PS-MN acquisition mechanisms involving other PS types, the diversity of PS released by different grasses, and their molecular responses. Additionally, it remains unclear how these findings translate to natural soil conditions or what happens to PS once released into the soil.

With access to the full set of chemically synthesized PS, we conducted experiments to explore PS biosynthesis and exudation across grass species (e.g., barley, rye, oat, sorghum) under Fe, Zn, or Cu deficiency in both hydroponic and soil systems. We assessed the efficiency of each PS in mobilizing these MNs through soil interaction experiments conducted in MN-deficient soils.

Our findings indicate that PS biosynthesis and exudation exhibit species-specific and genotype-dependent variations. Focusing on barley and supported by root gene expression data (RNAseq), we found a stronger PS pathway response in a MN efficient genotype compared to an inefficient line. Additionally, our soil-PS interaction experiments revealed that PS-aided metal mobilization is specific to soil type. Despite the structural similarities among the eight PS, we observed differences in their metal mobilization efficiencies, which were both time and PS-concentration dependent. Our findings offer valuable insights into the complex dynamics of the PS-mediated MN acquisition mechanism. These novel insights into plant MN nutrition can serve as a foundation for future studies as well as to develop breeding programs tailored to thrive in MN-deficient soils.