RESUMO
Branched-chain amino acids (BCAAs) are synthesized in plants from branched-chain keto acids, but their metabolism is not completely understood. The interface of BCAA metabolism lies with branched-chain aminotransferases (BCAT) that catalyze both the last anabolic step and the first catabolic step. In this study, six BCAT genes from the cultivated tomato (Solanum lycopersicum) were identified and characterized. SlBCAT1, -2, -3, and -4 are expressed in multiple plant tissues, while SlBCAT5 and -6 were undetectable. SlBCAT1 and -2 are located in the mitochondria, SlBCAT3 and -4 are located in chloroplasts, while SlBCAT5 and -6 are located in the cytosol and vacuole, respectively. SlBCAT1, -2, -3, and -4 were able to restore growth of Escherichia coli BCAA auxotrophic cells, but SlBCAT1 and -2 were less effective than SlBCAT3 and -4 in growth restoration. All enzymes were active in the forward (BCAA synthesis) and reverse (branched-chain keto acid synthesis) reactions. SlBCAT3 and -4 exhibited a preference for the forward reaction, while SlBCAT1 and -2 were more active in the reverse reaction. While overexpression of SlBCAT1 or -3 in tomato fruit did not significantly alter amino acid levels, an expression quantitative trait locus on chromosome 3, associated with substantially higher expression of Solanum pennellii BCAT4, did significantly increase BCAA levels. Conversely, antisense-mediated reduction of SlBCAT1 resulted in higher levels of BCAAs. Together, these results support a model in which the mitochondrial SlBCAT1 and -2 function in BCAA catabolism while the chloroplastic SlBCAT3 and -4 function in BCAA synthesis.
Assuntos
Família Multigênica , Solanum lycopersicum/enzimologia , Solanum lycopersicum/genética , Transaminases/genética , Aminoácidos de Cadeia Ramificada/biossíntese , Aminoácidos de Cadeia Ramificada/química , Vias Biossintéticas , Cromatografia Líquida de Alta Pressão , Clonagem Molecular , DNA Complementar/genética , Escherichia coli/crescimento & desenvolvimento , Perfilação da Expressão Gênica , Regulação Enzimológica da Expressão Gênica , Regulação da Expressão Gênica de Plantas , Teste de Complementação Genética , Endogamia , Cinética , Especificidade de Órgãos/genética , Mapeamento Físico do Cromossomo , Plantas Geneticamente Modificadas , Transporte Proteico , Locos de Características Quantitativas/genética , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Reprodutibilidade dos Testes , Homologia de Sequência de Aminoácidos , Frações Subcelulares/enzimologia , Transaminases/metabolismoRESUMO
Intercrossing different varieties of plants frequently produces hybrid offspring with superior vigor and increased yields, in a poorly understood phenomenon known as heterosis. One classical unproven model for heterosis is overdominance, which posits in its simplest form that improved vigor can result from a single heterozygous gene. Here we report that heterozygosity for tomato loss-of-function alleles of SINGLE FLOWER TRUSS (SFT), which is the genetic originator of the flowering hormone florigen, increases yield by up to 60%. Yield overdominance from SFT heterozygosity is robust, occurring in distinct genetic backgrounds and environments. We show that several traits integrate pleiotropically to drive heterosis in a multiplicative manner, and these effects derive from a suppression of growth termination mediated by SELF PRUNING (SP), an antagonist of SFT. Our findings provide the first example of a single overdominant gene for yield and suggest that single heterozygous mutations may improve productivity in other agricultural organisms.
Assuntos
Genes de Plantas/genética , Proteínas de Plantas/genética , Solanum lycopersicum/genética , Alelos , Flores/fisiologia , Biblioteca Gênica , Heterozigoto , Homozigoto , Vigor Híbrido , Solanum lycopersicum/metabolismo , Modelos Genéticos , Dados de Sequência Molecular , Mutação , Fenótipo , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Fatores de TempoRESUMO
Heterosis, or hybrid vigor, is a major genetic force that contributes to world food production. The genetic basis of heterosis is not clear, and the importance of loci with overdominant (ODO) effects is debated. One problem has been the use of whole-genome segregating populations, where interactions often mask the effects of individual loci. To assess the contribution of ODO to heterosis in the absence of epistasis, we carried out quantitative genetic and phenotypic analyses on a population of tomato (Solanum lycopersicum) introgression lines (ILs), which carry single marker-defined chromosome segments from the distantly related wild species Solanum pennellii. The ILs revealed 841 quantitative trait loci (QTL) for 35 diverse traits measured in the field on homozygous and heterozygous plants. ILs showing greater reproductive fitness were characterized by the prevalence of ODO QTL, which were virtually absent for the nonreproductive traits. ODO can result from true ODO due to allelic interactions of a single gene or from pseudoODO that involves linked loci with dominant alleles in repulsion. The fact that we detected dominant and recessive QTL for all phenotypic categories but ODO only for the reproductive traits indicates that pseudoODO due to random linkage is unlikely to explain heterosis in the ILs. Thus, we favor the true ODO model involving a single functional Mendelian locus. We propose that the alliance of ODO QTL with higher reproductive fitness was selected for in evolution and was domesticated by man to improve yields of crop plants.