| Animals cannot synthesize the B-vitamin folate and depend mainly on plants to supply them with this essential cofactor. The majority of plant folates, like those from other organisms, are conjugated to a short polyglutamate tail. However, the physiological significance of folate polyglutamylation in plants remains unclear, and therefore was investigated in the following study.;This work focused on the family of enzymes responsible for polyglutamate tail removal, gamma-glutamyl hydrolase (GGH). Characterization of the tomato GGH gene family revealed three genes encoding two homodimeric enzymes with distinct catalytic properties and a third catalytically inactive isoform. It was further demonstrated that each GGH member can dimerize with one another, resulting in heterodimers with catalytic properties midway between those of the corresponding homodimers.;The consequences of enhanced and impaired folate polyglutamylation were probed by modulating GGH expression in both Arabidopsis and tomato fruit. Overexpression of GGH caused extensive deglutamylation and decreased total folate in Arabidopsis and tomato. Conversely, RNAi-mediated knockdown of GGH activity by 99% in Arabidopsis increased total folate content by 34%, which, at least in part, accumulated as polyglutamylated derivatives in the vacuole.;Therefore, the results of this study are consistent with a model whereby polyglutamylated folates enter the vacuole, are hydrolyzed by GGH and exit as monoglutamates. This model further suggests that GGH inactivation could be employed as a strategy to enhance folate content in plants by allowing polyglutamates to accumulate in vacuoles. |
