The Arabidopsis thaliana CC-type glutaredoxin (GRX) ROXY1 and the bZIP TGA transcription factor (TF) PERIANTHIA (PAN) interact in the nucleus and together regulate petal development. The CC-type GRXs exist exclusively in land plants, and in contrast to the ubiquitously occurring CPYC and CGFS GRX classes, only the CCtype GRXs expanded strongly during land plant evolution. Phylogenetic analyses show that TGA TFs evolved before the CC-type GRXs in charophycean algae.

SUMMARY To unravel the function of a protein of interest, it is crucial to asses to what extent it associates via direct interactions or by overlapping expression with other proteins. ROXY1, a land plant‐specific glutaredoxin, exerts a function in Arabidopsis flower development and interacts with TGA transcription factors in the nucleus. We detected a novel ROXY1 function in the root meristem. Root cells that lack chlorophyll reducing plant‐specific background problems that can hamper colocalization 3D microscopy. Thus far, a super‐resolution three‐dimensional stochastic optical reconstruction microscopy (3D‐dSTORM) approach has mainly been applied in animal studies. We established 3D‐dSTORM using the roxy1 mutant complemented with green fluorescence protein‐ROXY1 and investigated its colocalization with three distinct RNAPII isoforms. To quantify the colocalization results, 3D‐dSTORM was coupled with the coordinate‐based colocalization method. Interestingly, ROXY1 proteins colocalize with different RNA polymerase II (RNAPII) isoforms that are active at distinct transcription cycle steps. Our colocalization data provide new insights on nuclear glutaredoxin activities suggesting that ROXY1 is not only required in early transcription initiation events via interaction with transcription factors but likely also participates throughout further transcription processes until late termination steps. Furthermore, we showed the applicability of the combined approaches to detect and quantify responses to altered growth conditions, exemplified by analysis of H2O2 treatment, causing a dissociation of ROXY1 and RNAPII isoforms. We envisage that the powerful dual‐color 3D‐dSTORM/coordinate‐based colocalization combination offers plant cell biologists the opportunity to colocalize and quantify root meristem proteins at an increased, unprecedented resolution level <50 nm, which will enable the detection of novel subcellular protein associations and functions.