Modified nucleosides in tRNAs are critical for protein translation. N 1 -methylguanosine-37 and N 1 -methylinosine-37 in tRNAs, both located at the 3'-adjacent to the anticodon, are formed by Trm5 and here we describe Arabidopsis thaliana AtTrm5 (At3g56120) as a Trm5 ortholog. We show that AtTrm5 complements the yeast trm5 mutant, and in vitro methylates tRNA guanosine-37 to produce N 1 -methylguanosine (m 1 G). We also show in vitro that AtTRM5 methylates tRNA inosine-37 to produce N 1methylinosine (m 1 I) and in Attrm5 mutant plants, we show a reduction of both N 1methylguanosine and N 1 -methylinosine. We also show that AtTRM5 is localized to the nucleus in plant cells. Attrm5 mutant plants have overall slower growth as observed by slower leaf initiation rate, delayed flowering and reduced primary root length. In Attrm5 mutants, mRNAs of flowering time genes are less abundant and correlated with delayed flowering. Finally, proteomics data show that photosynthetic protein abundance is affected in mutant plants. Our findings highlight the bifunctionality of AtTRM5 and the importance of the post-transcriptional tRNA modifications m 1 G and m 1 I at tRNA position 37 in general plant growth and development.