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Since the first isolation of ascorbic acid (AsA) in 1928, few papers have been published regarding the biosynthesis of AsA in plants, especially in fruits. It took as long as 1998, before Wheeler, Jones and Smirnoff, based on a study with Arabidopsis leaves, proposed what can be considered the main pathway of biosynthesis of AsA, in which L-galactose (L-GAL) is a key precursor. This paper reports the effectiveness of some precursors (cold or radiolabeled) in the biosynthesis of AsA in different plants: green sweet pepper, white-pulp guava, red-pulp guava, papaya and strawberry at two ripening stages (mature green and ripe for papaya and mature green and half red for strawberry) and broccoli. The 'Smirnoff-Wheeler' pathway was functioning and active in all sources studied, as demonstrated by the increase in AsA contents and incorporation of labeled precursors into AsA. In papaya, the AsA content in the ripe fruit was higher than in the mature green, indicating the synthesis of AsA during ripening. On the other hand, the AsA content in the mature green strawberry was similar to that of the half red fruits. Our data demonstrate that L-GAL and L-Galactono-1,4-lactone (L-GL) are effective precursors for the biosynthesis of AsA in fruits and also provided additional evidence for the participation of D-mannose (D-MAN) and D-glucose-1P in the biosynthesis of AsA in plants. Apesar da importância do ácido ascórbico (AA) para os organismos animais e vegetal, sua biossíntese somente foi elucidada em 1998, quando Wheeler, Jones e Smirnoff demonstraram, em folhas de Arabidopsis, que L-galactose (L-GAL) é um precursor chave. Neste trabalho, investigou-se a atuação de supostos precursores na síntese do AA em diferentes fontes vegetais: pimentão-verde, goiabas de polpa branca e vermelha, mamão e morango em dois estádios do amadurecimento: verde e maduro para o mamão e verde e rosa para o morango e em brócolis, verificando-se a atuação da via "Smirnoff-Wheeler" mediante a constatação do aumento dos conteúdos de AA e incorporação de precursores radiativamente marcados. O conteúdo de AA no mamão maduro apresentou-se maior do que no verde, indicando que há predomínio da síntese durante o amadurecimento desse fruto. O mesmo não ocorreu com o morango, onde não houve diferenças significativas entre os conteúdos de AA nos frutos verde e intermediário. Os resultados confirmaram que a L-GAL e a L-galactono-1,4-lactona (L-GL) são precursores bastante eficientes do AA, e também que há síntese de AA a partir de D-manose (D-MAN), L-GAL e D-glicose-1P nos vegetais estudados

Ascorbic acid biosynthesis: a precursor study on plants