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. Has significantly reduced methionine levels compared to wild-type. Combined loss of FPGS3 and FPGS2 results in seedling lethality. Seedlings fail to proceed beyond the expanded cotyledon stage, exhibit an albino phenotype and are unable to thrive beyond germination. Fpgs3 and fpgs1 double mutant exhibits dwarfed leaves, late flowering (approximately 13 days after wild-type), reduced fecundity and delayed senescence. Pollination with fpgs3 and fpgs1 double mutant pollen yields at most one or two seeds per silique compared to the yield of full siliques when wild-type stigmas are pollinated with wild-type pollen. There is a 40% reduction in the total of 5-CH(3)-THF pool in fpgs3 and fpgs1 double mutant leaf tissue. Fpgs3 and fpgs1 double mutants have 70% lower methionine content than wild-type. Belongs to the folylpolyglutamate synthase family.