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 developed adaptive strategies to cope with low iron and low 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 the root growth of Arabidopsis thaliana by activating brassinosteroid signaling, whereas low-phosphate-induced high iron accumulation inhibited 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 required the brassinosteroid receptor but the hormone ligand was negligible. The brassinosteroid receptor inhibitor BKI1, the transcription factors BES1/BZR1 and the ferroxidase LPR1, stood 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. Overall design: Arabidopsis (Arabidopsis thaliana) lines in the Columbia-0 (Col-0) background, wild type and bzr1-D mutant, were germinated on one-half-strength Murashige and Skoog medium supplemented with 0.2% (w/v) sucrose for 6 days. Seedlings were then transferred for 24h to medium with either adequate (0.625 mM) or deficient (1µM) Pi levels (as described in Singh et al, Plant Phys, 2014). 8 samples of mRNA from two biological replicates were collected (i.e. from roots of 7-day-old seedlings). Samples were sequenced on an Illumina HiSeq 2500, to obtain 50 bp single end reads.

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