Computational modeling and 1 quantitative physiology reveal 2 central parameters for 3 brassinosteroid-regulated early cell 4 physiological processes linked to 5 elongation growth of the Arabidopsis 6 root 7

Brassinosteroids (BR) are key hormonal regulators of plant development. However, 19 whereas the individual components of BR perception and signaling are well characterized 20 experimentally, the question of how they can act and whether they are sufficient to carry out the 21 critical function of cellular elongation remains open. Here, we combined computational modeling 22 with quantitative cell physiology to understand the dynamics of the plasma membrane 23 (PM)-localized BR response pathway during the initiation of cellular responses in the epidermis of 24 the Arabidopsis root tip that are be linked to cell elongation. The model, consisting of ordinary 25 differential equations, comprises the BR induced hyperpolarization of the PM, the acidification of 26 the apoplast and subsequent cell wall swelling. We demonstrate that the competence of the root 27 epidermal cells for the BR response predominantly depends on the amount and activity of 28 H + -ATPases in the PM. The model further predicts that an influx of cations is required to 29 compensate for the shift of positive charges caused by the apoplastic acidification. A potassium 30 channel was subsequently identified and experimentally characterized, fulfilling this function. 31 Thus, we established the landscape of components and parameters for physiological processes 32 potentially linked to cell elongation, a central process in plant development. 33