The detyrosination/retyrosination cycle is the most common post‐translational modification of α‐tubulin. Removal of the conserved C‐terminal tyrosine of α‐tubulin by a still elusive tubulin tyrosine carboxypeptidase, and religation of this tyrosine by a tubulin tyrosine ligase (TTL), are probably common to all eukaryotes. Interestingly, for plants, the only candidates qualifying as potential TTL homologs are the tubulin tyrosine ligase‐like 12 proteins. To get insight into the biological functions of these potential TTL homologs, we cloned the rice TTL‐like 12 protein (OsTTLL12) and generated overexpression OsTTLL12‐RFP lines in both rice and tobacco BY‐2 cells. We found, unexpectedly, that overexpression of this OsTTLL12‐RFP increased the relative abundance of detyrosinated α‐tubulin in both coleoptile and seminal root, correlated with more stable microtubules. This was independent of the respective orientation of cortical microtubule, and followed by correspondingly changing growth of coleoptiles and seminal roots. A perturbed organization of phragmoplast microtubules and disoriented cell walls were further characteristics of this phenotype. ... mehrThus, the elevated tubulin detyrosination in consequence of OsTTLL12 overexpression affects structural and dynamic features of microtubules, followed by changes in the axiality of cell plate deposition and, consequently, plant growth.