RESUMO
Acerola (Malpighia glabra) is an exotic fruit cultivated primarily for its abundant ascorbic acid (AsA) content. The molecular mechanisms that regulate the metabolism of AsA in acerola have yet to be defined. Monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR) are key enzymes of the ascorbate-glutathione cycle that maintain reduced pools of ascorbic acid and serve as important antioxidants. cDNAs encoding MDHAR and DHAR were isolated from acerola using RT-PCR and RACE. Phylogenetic trees associated acerola MDHAR and DHAR with other plant cytosolic MDHARs and DHARs. Expressions of the two genes correlated with their enzymatic activities and were differentially regulated during fruit ripening. Interestingly, MDHAR expression was only detected in overripe fruits, whereas the transcript level of DHAR was highest at the intermediate stage of fruit ripening. Under dark conditions, there was a sharp and significant decline in the total and reduced ascorbate contents, accompanied by a decrease in the level of transcripts and enzyme activities of the two genes in acerola leaves. MDHAR and DHAR transcripts and enzyme activities were significantly up-regulated in the leaves of acerola under cold and salt stress conditions, indicating that expression of both genes are transcriptionally regulated under these stresses.
Assuntos
Ácido Ascórbico/metabolismo , Malpighiaceae/enzimologia , Malpighiaceae/genética , NADH NADPH Oxirredutases/genética , Oxirredutases/genética , Proteínas de Plantas/genética , Sequência de Aminoácidos , DNA Complementar/genética , Frutas/genética , Frutas/metabolismo , Frutas/efeitos da radiação , Regulação da Expressão Gênica de Plantas , Malpighiaceae/metabolismo , Malpighiaceae/efeitos da radiação , Dados de Sequência Molecular , NADH NADPH Oxirredutases/química , NADH NADPH Oxirredutases/metabolismo , Oxirredutases/química , Oxirredutases/metabolismo , Filogenia , Proteínas de Plantas/química , Proteínas de Plantas/metabolismo , RNA Mensageiro/genética , Salinidade , Alinhamento de Sequência , Estresse Fisiológico , TemperaturaRESUMO
Phosphomannomutase (PMM; EC 5.4.2.8) catalyzes the interconversion of mannose-6-phosphate to mannose-1-phosphate in the Smirnoff-Wheeler pathway for the biosynthesis of l-ascorbic acid (AsA). We have cloned the PMM cDNA from acerola (Malpighia glabra), a plant containing an enormous amount of AsA. The AsA contents correlate with the PMM gene expression of the ripening fruits and leaves. The PMM activities in the leaves of acerola, tomato and Arabidopsis correlate with their respective AsA contents. Transgenic tobacco plants overexpressing the acerola PMM gene showed about a 2-fold increase in AsA contents compared with the wild type, with a corresponding correlation with the PMM transcript levels and activities.
