Triphosphate tunnel metalloenzymes (TTMs) are found in all biological kingdoms. Members of the TTM family have divergent biological functions and act on a range of triphosphorylated substrates (thiamine triphosphate, RNA and inorganic polyphosphate). TTMs are well-studied in microorganisms and animals, with limited attention in plants. Here, I explored their functions in Arabidopsis thaliana, focusing on TTM1 and TTM2. Tissue and cellular microscopy studies show that both TTM1 and TTM2 are expressed in actively dividing (meristem) tissue and are tail-anchored outer mitochondrial membrane proteins. Genetic investigations reveal that TTM2 plays a crucial role in the transition from vegetative to reproductive phase, while its closest paralog, AtTTM1, does not. Notably, both the P-loop kinase and TTM tunnel modules of TTM2 are essential for this developmental switch, as demonstrated through structural mutations. My research enhances our understanding of TTM1 and TTM2’s biochemical and physiological functions in Arabidopsis and highlights their functional distinction.