Folate distribution during higher plant development

The total folate content of various tissues from pea seedlings was determin ed using a microbiological assay. m the seed the bulk of folate was located in cotyledons, but the embryo, representing only 2% of the total seed fres h weight, had a folate concentration about three times higher. In the prese nce of sulphanilamide, an inhibitor of folate biosynthesis, the initial fol ate content supported root elongation during the first 1-2 days. However, f urther growth of the seedling was not possible and required a de novo synth esis of folate. Following organogenesis, the folate content of young roots and shoots was similar to that in the initial embryo. In contrast, the fola te 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 roug hly constant thereafter. The intracellular distribution of folate was estim ated in 12-day-old leaves. Mitochondria, the site of folate synthesis, cont ained approximately 30% of the total cellular folate. The folate level in c hloroplasts was about 100 times lower than in mitochondria, representing, o n 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 mitoch ondria (less than 5% of the cytoplasmic volume) had the highest concentrati on. Why there should be more folate in leaves, on a fresh weight basis, tha n in other tissues is not clear. The folate-dependent enzymes involved in p hotorespiration (glycine decarboxylase and serine hydroxymethyltransferase) are located in mitochondria where they accumulate during greening. In cont rast, the folate within mitochondria purified from either roots, etiolated leaves or green leaves was, on a protein basis, similar, suggesting that ot her factors related to Light and photosynthesis might be involved. (C) 2001 Society of Chemical Industry.