Mechanism of the synergistic impact of Piriformospora indica and Azotobacter chroococcum on Zn and Fe biofortification

This study investigates the mechanisms underlying the enhanced zinc (Zn) and iron (Fe) uptake in a low Zn- and Fe-accumulating Indian wheat genotype HD2967 upon co-inoculation of Piriformospora indica, a phyto-promotional fungal endophyte, and Azotobacter chroococcum, a rhizobacteria, in a hydroponic system. Modifications in root architecture, low molecular-weight organic acid (LMWOA) profile in root exudates, and expression of TaZIP genes in roots and shoots of treated and control plants were analysed. The LMWOA profiles of treated and control plants were investigated using high-performance liquid chromatography, which showed the presence of four LMWOAs: citric, malic, acetic, and succinic acids. A 3.6-fold increase was observed in the total organic acid content of colonised over control plants with citric acid being predominant – representing 90% of the total organic acids. Microbial colonisation caused profuse branching in roots of treated plants with increases in root volume, total surface area, total length, and numbers of tips, forks, and crossings by 1.39-, 1.37-, 1.67-, 1.4-, 1.32-, and 1.81-fold, respectively. Four targeted TaZIP genes (TaZIP1, TaZIP3, TaZIP5, and TaZIP7) were overexpressed in roots and shoots of treated plants as compared to control plants. The results clearly suggest that endophytic colonisation modified the LMWOA profile in root exudates, root morphology, and expression of TaZIP genes in roots and shoots, of colonised hosts would have synergistically facilitated enhanced accumulation of Fe and Zn in colonised plants.