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Summary: Brassinosteroids (BRs) are a group of plant‐specific steroid hormones, which induces the rapid nuclear localization of the positive transcriptional factors BRASSINAZOLE RESISTANT1/2 (BZR1/2). However, the mechanisms underlying the regulation of nucleocytoplasmic shuttling of BZR1 remain to be fully elucidated.In this study, we show that the scaffold protein Receptor for Activated C Kinase 1 (RACK1) from Arabidopsis is involved in BR signaling cascades through mediating the nuclear localization of BZR1, which is tightly retained in the cytosol by the conserved scaffold protein 14‐3‐3s.RACK1 can interact with BZR1 and competitively decrease the 14‐3‐3 interaction with BZR1 in cytosol, which efficiently enhances the nuclear localization of BZR1. 14‐3‐3 also retains RACK1 in cytosol through their interaction. Conversely, BR treatment enhances the nuclear localization of BZR1 by disrupting the 14‐3‐3 interaction with RACK1 and BZR1.Our study uncovers a new mechanism that integrates two kinds of conserved scaffold proteins (RACK1 and 14‐3‐3) coordinating BR signaling event. [ABSTRACT FROM AUTHOR]

Maintenance of root growth is essential for plant adaptation to soil drying. Here, we tested the hypothesis that auxin transport is involved in mediating ABA's modulation by activating proton secretion in the root tip to maintain root growth under moderate water stress., Rice and Arabidopsis plants were raised under a hydroponic system and subjected to moderate water stress (−0.47 MPa) with polyethylene glycol ( PEG). ABA accumulation, auxin transport and plasma membrane H+- ATPase activity at the root tip were monitored in addition to the primary root elongation and root hair density., We found that moderate water stress increases ABA accumulation and auxin transport in the root apex. Additionally, ABA modulation is involved in the regulation of auxin transport in the root tip. The transported auxin activates the plasma membrane H+- ATPase to release more protons along the root tip in its adaption to moderate water stress. The proton secretion in the root tip is essential in maintaining or promoting primary root elongation and root hair development under moderate water stress., These results suggest that ABA accumulation modulates auxin transport in the root tip, which enhances proton secretion for maintaining root growth under moderate water stress. [ABSTRACT FROM AUTHOR]

Summary Light and brassinosteroids (BR) are indispensable for plant growth and control cell division in the apical meristem. However, how external light signals cooperate with internal brassinosteroids to program root meristem development remains elusive. We reveal that the photoreceptor phytochrome B (phyB) guides the scaffold protein RACK1 to coordinate BR signaling for maintaining root meristematic activity. phyB and RACK1 promote early root meristem development. Mechanistically, RACK1 could reinforce the phyB–SPA1 association by interacting with both phyB and SPA1, which indirectly affects COP1‐dependent RACK1 degradation, resulting in the accumulation of RACK1 in roots. Subsequently, RACK1 interacts with BES1 to repress its DNA‐binding activity toward the target gene CYCD3;1, leading to the release of BES1‐mediated inhibition of CYCD3;1 transcription, and hence the promotion of root meristem development. Our study provides mechanistic insights into the regulation of root meristem development by combination of light and phytohormones signals through the photoreceptors and scaffold proteins. [ABSTRACT FROM AUTHOR]