Conversion of folates to polyglutamate derivatives, including tetrahydrofolate (PubMed:11752472, PubMed:12535338). Required during embryogenesis; from maternal tissues until the globular stage, and from the embryo after the globular stage (PubMed:12535338). 7,8-dihydropteroate + ATP + L-glutamate = 7,8-dihydrofolate + ADP + H(+) + phosphate Cofactor biosynthesis; tetrahydrofolate biosynthesis; 7,8-dihydrofolate from folate: step 1/1. Expressed in seedlings, leaves, inflorescence stems, flower buds and roots. Observed, in embryos from the early stage to the mature stage, with higher levels in embryo proper, and weaker signals in suspensor cells and other tissues surrounding the embryo. Defective embryos with development arrested after the globular stage, at the transition to the heart stage. Belongs to the folylpolyglutamate synthase family.