Eukaryotic chaperonin coordinates root meristem activity by regulating SEC31B‐dependent COPII vesicle trafficking of PIN auxin efflux carrier in Arabidopsis

The eukaryotic chaperonin containing T-complex polypeptide-1 (CCT/TRiC) complex, composed of eight distinct subunits (CCT1-CCT8), is essential for cytosolic protein folding; however, its function in plants remains largely unexplored. Moreover, a direct link between CCT and coat protein complex II (COPII) vesicle trafficking-a key step in the early secretory pathway-has not been established in any eukaryotic system. Here, leveraging plant genetics, we investigated the functional relationship between CCT8 and COPII-mediated trafficking in the Arabidopsis root apex. The point mutant cct8-1 exhibited a short-root phenotype resulting from impaired cell division in the root meristem, which was accompanied by disrupted auxin homeostasis. This defect stemmed from a marked reduction in the abundance of multiple PIN-FORMED (PIN) auxin efflux carriers at the plasma membrane, without affecting their polar localization. Mechanistically, CCT8 directly interacted with SEC31B, a core component of the COPII coat. Accordingly, the sec31b-3 mutant phenocopied cct8-1 in root growth, auxin response, and PIN accumulation defects. The CCT8 mutation reduced SEC31B abundance at both the transcriptional and protein levels and compromised ER-to-Golgi transport, thereby diminishing PIN delivery to the plasma membrane. Importantly, overexpression of SEC31B partially rescued the root growth defects and restored PIN2 levels in cct8-1. Together, our findings uncover a previously unrecognized chaperonin-trafficking module in which CCT8 regulates SEC31B to modulate COPII-mediated delivery of PIN proteins, thus linking chaperonin function to auxin-dependent root development.