Interdependent Nutrient Availability and Steroid Hormone Signals Facilitate Root Growth Plasticity

Plants acquire essential elements from inherently heterogeneous soils, in which phosphate and iron availabilities vary. Consequently, plants have developed adaptive strategies to cope with low iron or phosphate levels, including alternation between root growth enhancement and attenuation. How this adaptive response is achieved remains unclear. Here, we found that low iron accelerates root growth in Arabidopsis thaliana by activating brassinosteroid signaling, whereas low-phosphate-induced high iron accumulation inhibits it. Altered hormone signaling intensity also modulated iron accumulation in the root elongation and differentiation zones, constituting a feedback response between brassinosteroid and iron. Surprisingly, the early effect of low iron levels on root growth depended on the brassinosteroid receptor but was apparently hormone ligand-independent. The brassinosteroid receptor inhibitor BKI1, the transcription factors BES1/BZR1, and the ferroxidase LPR1 operate at the base of this feedback loop. Hence, shared brassinosteroid and iron regulatory components link nutrient status to root morphology, thereby driving the adaptive response.