Functional redundancy and synthetic haploinsufficiency among Arabidopsis ATP-citrate lyase genes.

ATP-citrate lyase (ACL) catalyzes the formation of cytosolic acetyl-CoA, which is the precursor for the generation of a variety of metabolites that have significant roles in plant growth and development. In Arabidopsis thaliana, ACL is encoded by three ACLA and two ACLB genes. The genetic functions of these genes were characterized by using combinations of null alleles of each subunit-gene. Individual acla mutants and double mutants that retained only the functional ACLA1 or ACLA3 gene exhibited normal growth, indicating substantial functional redundancy among the ACLA paralogs. These mutants showed minimal effects on cytosolic acetyl-CoA-derived metabolites, although anthocyanin content consistently increased. In contrast, aclb1; aclb2 double mutants were not recoverable, rather embryo development was arrested at the torpedo stage, demonstrating that ACL activity is essential for embryogenesis. Moreover, ACLB paralogs were not functionally equivalent during vegetative growth. Plants carrying a single functional ACLB2 allele in the absence of ACLB1 functionality displayed a severe dwarf phenotype, whereas plants carrying a single ACLB1 allele in the absence of ACLB2 appeared wild type, revealing synthetic haploinsufficiency of ACLB2. Reduced ACLB2 dosage also decreased cuticular wax accumulation in a gene dosage-dependent manner. Collectively, these findings demonstrate that the Arabidopsis ACL gene family combines paralog redundancy with subunit-specific synthetic haploinsufficiency essential for normal growth and development.