SMALLER TRICHOMES WITH VARIABLE BRANCHES (SVB) and its homolog SVBL act downstream of transcription factor NAC089 and function redundantly in Arabidopsis unfolded protein response.

The unfolded protein response (UPR) is a cellular mechanism that alleviates endoplasmic reticulum (ER) stress to maintain protein homeostasis. Although SMALLER TRICHOMES WITH VARIABLE BRANCHES (SVB) is characterized as an emerging UPR factor downstream of the IRE-bZIP60 pathway, whether its homologs participate in the plant UPR remains unknown. Here, we showed that an SVB homolog, SVB-like (SVBL), functions redundantly with SVB in ER stress tolerance. The svb-1 svbl-1 double mutant showed a hypersensitivity phenotype and had higher UPR gene expression under ER stress than single mutants and the wild type. SVB responded to ER stress by accumulating in the root epidermis and phloem cells, but SVBL did not. Ectopic expression of the UPR factor NAC089 upregulated both SVB and SVBL, which suggests that they work downstream of NAC089. Thus, SVB and SVBL play distinct roles that are modulated by the common upstream regulator NAC089 to cope with ER stress in Arabidopsis.