5-Formyltetrahydrofolate is an inhibitory but well tolerated metabolite in Arabidopsis leaves.
Goyer A, Collakova E, Díaz de la Garza R, Quinlivan EP, Williamson J, Gregory JF 3rd, Shachar-Hill Y, Hanson AD. J Biol Chem. 2005 Jul 15; 280(28):26137-42. Epub 2005 May 11.