The total folate content of various tissues from pea seedlings was determined using a microbiological assay. In the seed the bulk of folate was located in cotyledons, but the embryo, representing only 2% of the total seed fresh weight, had a folate concentration about three times higher. In the presence of sulphanilamide, an inhibitor of folate biosynthesis, the initial folate content supported root elongation during the first 1–2 days. However, further growth of the seedling was not possible and required a de novo synthesis of folate. Following organogenesis, the folate content of young roots and shoots was similar to that in the initial embryo. In contrast, the folate content of young leaves increased rapidly to reach a value two to three times higher than in other tissues after 7 days of growth and remained roughly constant thereafter. The intracellular distribution of folate was estimated in 12-day-old leaves. Mitochondria, the site of folate synthesis, contained approximately 30% of the total cellular folate. The folate level in chloroplasts was about 100 times lower than in mitochondria, representing, on a protein basis, less than 4% of the total pool. We associate the bulk of folate with the cytosol plus nucleus (65–70% of the pool), although mitochondria (less than 5% of the cytoplasmic volume) had the highest concentration. Why there should be more folate in leaves, on a fresh weight basis, than in other tissues is not clear. The folate-dependent enzymes involved in photorespiration (glycine decarboxylase and serine hydroxymethyltransferase) are located in mitochondria where they accumulate during greening. In contrast, the folate within mitochondria purified from either roots, etiolated leaves or green leaves was, on a protein basis, similar, suggesting that other factors related to light and photosynthesis might be involved.
© 2001 Society of Chemical Industry