Plant Class I formins represent a unique family of cytoskeletal organizers that are at the same time integral membrane proteins. We have previously reported that the GFP fusion of the Arabidopsis formin AtFH1, which is considered to be the main housekeeping Class I formin expressed in majority of tissues according to the available gene expression data, also transiently localised to the tonoplast in the root transition zone at or around the time of major membrane rearrangements that generate the large central vacuole typical for differentiated plant cells (Oulehlová et al., 2019 – https://doi.org/10.1093/pcp/pcz102). It is currently unknown whether AtFH1 has an active role in tonoplast rearrangements or whether it transiently passes through the tonoplast while being sorted for vacuolar degradation. A newly prepared fusion of AtFH1 with a different, pH-insensitive fluorescent protein, as well as pharmacological treatment with concanamycin A, showed, in addition to known AtFH1-GFP localisation, also presence of AtFH1 within vacuolar lumen in the root elongation zone and mature tissues, supporting the degradation hypothesis. However, we also found quantitative, sometimes significant changes in vacuolar organization in mutants lacking AtFH1, using both established vacuolar morphology metrics and a newly developed method for quantitative evaluation of tonoplast topology, supporting a tonoplast-associated function of AtFH1. Therefore, depending on the exact developmental stage and conditions, both of the hypotheses may be correct.