Fe deficiency causes transcriptional shift in roots leading to disruption of drought tolerance in soybean

Iron (Fe) deficiency in alkaline soils, exacerbated by drought, collectively affects soybean health. This study aimed to evaluate the physiological and transcriptional changes in Fiskeby IV, a drought-tolerant genotype that loses its tolerance when exposed to simultaneous Fe deficiency and drought. In this growth incubator study, Fe deficiency and drought stress resulted in substantial reductions in plant biomass, photosynthetic efficiency, and nutrient uptake in Fiskeby IV. Despite these disruptions, the photochemical efficiency of photosystem II remained stable, suggesting the activation of protective mechanisms to maintain essential photosynthetic functions. RNA-seq analysis highlighted a complex response, showing the upregulation of ethylene-responsive genes (Ethylene-response sensor 2, Ethylene-responsive TF018, Ethylene-responsive TF5) as well as the genes related to rhizosphere acidification (ATPase 1) and redox homeostasis (Glutaredoxin-3). It suggests that ethylene signaling and rhizosphere acidification may be responsive in coordinating Fe homeostasis and drought adaptation in soybean. On the flip side, combined stresses caused the downregulation of several genes related to nutrient uptake (nicotianamine transporter YSL1, ammonium transporter 2, sulfate transporter 3.4, and major facilitator family protein). In a targeted study, supplementation with 1-aminocyclopropane-1-carboxylic acid (ACC), an ethylene precursor, led to substantial improvements in morpho-physiological traits and Fe status under combined stress conditions. This ACC treatment enhanced root flavonoid content and rhizosphere siderophore levels accompanied by restoration of 16S and ITS microbial community under Fe deficiency and drought. It underscores the potential of targeting ethylene signaling that may facilitate Fe mobilization and microbial interactions to enhance soybean tolerance to concurrent Fe deficiency and drought. This is the first report on the transcriptional response and requirement of Fe status underlying drought tolerance, potentially guiding future strategies for improving combined stress resilience in soybean.