T-cytoplasm induced male sterility (cms-T) is a maternally inherited inability to produce viable pollen in maize. The causal factor, the URF13 protein, is encoded by the mitochondrial genome and accumulates in the mitochondrial inner membrane. Fertility restoration of cms-T is mediated by the complementary action of two nuclear genes, rf1 and rf2. The rf2 gene was cloned via transposon tagging. Sequence analysis revealed that it has high sequence similarity to mammalian mitochondrial aldehyde dehydrogenases (mtALDHs). Sequence, mRNA, and protein analyses of the spontaneous mutant allele rf2-R213 demonstrated that the ALDH activity is necessary for rf2's function as a cms-T restorer. Detailed genetic characterizations of rf2 mutants revealed that loss of rf2 function not only causes male sterility in T-cytoplasm plants, but also causes anther arrest in the lower florets of maize plants that carry the normal (N) cytoplasm. To identify the biochemical pathway through which the rf2 gene functions in both N and T cytoplasms, the fermentation pathway was tested as a candidate. Mutations in three pdc genes, which encode the rate-limiting pyruvate decarboxylase (PDC) enzyme, were isolated via a reverse genetic approach. Detailed characterizations revealed that pdc3 mutants dramatically reduced anaerobic stress resistance; however, pdc3 mutants did not affect male fertility in either T- or N-cytoplasm plants.;Analyses of rf2 Mu-insertion mutant alleles revealed that some of these alleles could lose their capacity to condition a mutant phenotype without excisional loss of the associated Mu transposons, which is a phenomenon known as Mu suppression. These analyses have revealed that Mu suppression can occur not only at alleles caused by Mu insertions in 5' UTRs via the recruitment of alternative transcription initiation sites as reported previously, but also at alleles caused by Mu insertions in 3' UTRs. Suppression of this new class of Mu -suppressible alleles occurs via the recruitment of alternative polyadenylation sites within the TIRs of the inserted Mu transposon. In addition, these studies establish that insertions of two additional classes of Mu transposons can generate suppressible alleles.