Arabidopsis <scp>BCAT1</scp> and <scp>BCAT2</scp> play distinct roles between branched‐chain wax biosynthesis and energy production

Branched-chain aminotransferase1 (BCAT1) and BCAT2 initiate branched-chain amino acid catabolism, which provides precursors for multiple metabolic pathways. Yet, whether their degradation products converge on the same or diverge into distinct metabolic pathways remains an open question. In Arabidopsis thaliana, disruption of AtBCAT1 (but not AtBCAT2) leads to a deficiency in iso-branched waxes, demonstrating its specific role in their biosynthesis. Expressing AtBCAT2 from the AtBCAT1 promoter fails to rescue this defect, proving that AtBCAT1 uniquely channels Valine-derived carbon into wax synthesis. Phylogenetic analysis places BCAT1 and BCAT2 in separate clades that arose from an ancient tandem duplication. Rapeseed BnBCAT2, like Arabidopsis AtBCAT2, cannot complement bcat1, whereas BCAT2 orthologues from rice and tobacco partially rescue bcat1, underscoring Brassicaceae-specific sub-functionalization of BCAT1 that likely originated in a common ancestor. In addition, under darkness, AtBCAT1 delays dark-induced senescence, whereas AtBCAT2 accelerates it. This reveals that under energy-deficient conditions, AtBCAT2 supports energy production via the TCA cycle, while AtBCAT1 is preferentially diverted to alternative biosynthetic pathways. Collectively, AtBCAT1 and AtBCAT2 play distinct roles in branched wax synthesis and ATP production; that is, AtBCAT1 is dedicated to supplying iso-branched-chain wax precursors, whereas AtBCAT2 preferentially feeds the resulting carbon skeletons into the TCA cycle under energy-deficient conditions.