Functional specification of Arabidopsis isopropylmalate isomerases in glucosinolate and leucine biosynthesis

The Arabidopsis genome encodes one potential isopropylmalate isomerase (IPMI) large subunit and three potential IPMI small subunits, which in bacteria and archaea form heterodimers to catalyze the isomerization of 2-isopropylmalate to 3-isopropylmalate in leucine biosynthesis. We demonstrate here that AtLeuC physically interacts with AtLeuD proteins to form functional IPMIs. The IPMIs are localized to chloroplast stroma. Tissue-specific expression analysis revealed that the patterns of AtLeuD1 and AtLeuD2 expression are similar, but distinct from that of AtLeuD3. This result indicates functional redundancy of AtLeuD1 and AtLeuD2, and functional specification of AtLeuD3. Reverse genetics and metabolite profiling showed that AtLeuD1 and AtLeuD2 function redundantly in aliphatic glucosinolate biosynthesis, but AtLeuD3 is not likely to be involved in this pathway. The lethal phenotype of the atleud3 mutant suggests functional specification of AtLeuD3 in leucine biosynthesis. A defect in female gametophyte development was found to contribute to the mutant lethality, suggesting the important role of AtLeuD3 in female gametophyte development.