Figure 2.

Models of the abscisic acid (ABA) and the gibberellin (GA) hormone signalling network. (A) Model of ABA receptor activation showing the formation of receptor–ABA–PP2C (R–A–P) ternary complexes for the monomeric and the dimeric PYR/PYL/RCAR proteins considered in the modelling study [52]. The dissociation constants (K_d) for the reactions were measured experimentally and used to parameterise the model. Adapted from [39]. (B) The three functional modules of the GA signalling network (perception, response, and biosynthesis) are shown. Perception (yellow box): GA4 first binds to the GID receptor and the complex then interacts with DELLA proteins, leading to the ubiquitination (denoted with a green chain) and degradation of DELLA proteins. Response (green box): GID1, GA20OX, and GA3OX genes are transcriptionally activated by DELLA proteins but repress their own transcription. Biosynthesis (blue box): GA12 is converted to GA15 then to GA24, and finally to GA9 by the GA20ox enzyme. GA9 is then converted to GA4 by the GA3ox enzyme. Hence, DELLA-mediated upregulation of GA biosynthesis transiently elevates the levels of the hormone and the GID1 receptor, leading to DELLA degradation, thus creating a negative feedback loop. Reproduced, with permission, from [40].