A Mathematical Model for BRASSINOSTEROID INSENSITIVE1-Mediated Signaling in Root Growth and Hypocotyl Elongation  

Abstract

(S.v.M., J.M.) Brassinosteroid (BR) signaling is essential for plant growth and development. In Arabidopsis (Arabidopsis thaliana), BRs are perceived by the BRASSINOSTEROID INSENSITIVE1 (BRI1) receptor. Root growth and hypocotyl elongation are convenient downstream physiological outputs of BR signaling. A computational approach was employed to predict root growth solely on the basis of BRI1 receptor activity. The developed mathematical model predicts that during normal root growth, few receptors are occupied with ligand. The model faithfully predicts root growth, as observed in bri1 loss-of-function mutants. For roots, it incorporates one stimulatory and two inhibitory modules, while for hypocotyls, a single inhibitory module is sufficient. Root growth as observed when BRI1 is overexpressed can only be predicted assuming that a decrease occurred in the BRI1 half-maximum response values. Root growth appears highly sensitive to variation in BR concentration and much less to reduction in BRI1 receptor level, suggesting that regulation occurs primarily by ligand availability and biochemical activity.

A Mathematical Model for the Coreceptors SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE1 and SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE3 in BRASSINOSTEROID INSENSITIVE1-Mediated Signaling

“…All computational and modeling procedures were performed within the Matlab environment (Supplemental File S1; van Esse et al, 2012). Out of the different models tested, throughout this paper biochemical model 1 and physiological model 1 are used unless specified otherwise.…”

“…All three only transduce the signal when BL is bound to BRI1. To incorporate the coreceptors into the existing model describing BR-activated BRI1-mediated signaling (van Esse et al, 2012), several assumptions were made. The first is that BRI1 binds to SERK proteins in a 1:1 ratio.…”

“…For fluorescence microscopy, wild-type seedlings expressing BRI1-GFP under the control of its own promoter (Geldner et al, 2007) were crossed with the serk3-2 mutant. Root growth and hypocotyl elongation assays were performed as described previously (van Esse et al, 2012).…”

“…Recently, we developed a quantitative model to explain and predict how root and hypocotyl growth depend on the BRI1 receptor and BL ligand concentration. With the mass action kinetics on which the biochemical part of the model is based, phenotypes of bri1 mutants could be correctly predicted (van Esse et al, 2012).…”

“…The middle and right panels summarize the modeling approach. The biochemical model describes BL-dependent BRI1 activation (van Esse et al, 2012). Using genetically identified components, the model is now extended using mutated SERK1 and SERK3 coreceptors as an example.…”