Catalyzes conversion of folates to polyglutamate derivatives allowing concentration of folate compounds in the cell and the intracellular retention of these cofactors, which are important substrates for most of the folate-dependent enzymes that are involved in one-carbon transfer reactions involved in purine, pyrimidine and amino acid synthesis. Essential for organellar and whole-plant folate homeostasis. (6S)-5,6,7,8-tetrahydrofolyl-(gamma-L-Glu)(n) + ATP + L-glutamate = (6S)-5,6,7,8-tetrahydrofolyl-(gamma-L-Glu)(n+1) + ADP + H(+) + phosphate A monovalent cation. Cofactor biosynthesis; tetrahydrofolylpolyglutamate biosynthesis. Vegetative phenotype does not differ visually from wild-type. No obvious defects in root development. Polyglutamylated folates still detectable, but loss of activity leads to a significant reduction (45%) in total foliar folate abundance compared to wild-type. The reduced total folate content is a result of reduced levels of 5-formyl-THF, 10-formyl and 5,10-methenyl-THF, 5-methyl-THF and THF (42, 42, 53 and 48%, respectively) compared to wild-type. The plastid and mitochondrial folate levels are also reduced by approximately 50 and 55%, respectively compared to wild-type. Folate polyglutamylation levels are significantly reduced but not abolished within the respective compartments. Combined loss of FPGS2 and FPGS1 result in embryo lethality. This double mutant has abnormal seeds that are readily distinguishable as albinos which do not proceed beyond the globular stage of embryogenesis. The absence of a developing embryo lead to collapse of seed walls, leaving shrivelled seed reamnants. Combined loss of FPGS2 and FPGS3 results in seedling lethality. Seedlings fail to proceed beyond the expanded cotyledon stage, exhibit an albino phenotype and are unable to thrive beyond germination. Belongs to the folylpolyglutamate synthase family.