Post-translational and transcriptional regulation of phenylpropanoid biosynthesis pathway by Kelch repeat F-box protein SAGL1

A Kelch repeat F-box containing protein, SMALL AND GLOSSY LEAVES1 (SAGL1) regulates phenylpropanoid biosynthesis as a post-translational regulator for PAL1 (phenylalanine ammonia-lyase) and an indirect transcriptional regulator for ANTHOCYANIDIN SYNTHASE. Phenylpropanoid biosynthesis in plants produces diverse aromatic metabolites with important biological functions. Phenylalanine ammonia-lyase (PAL) catalyzes the first step in phenylpropanoid biosynthesis by converting l-phenylalanine to trans-cinnamic acid. Here, we report that SMALL AND GLOSSY LEAVES1 (SAGL1), a Kelch repeat F-box protein, interacts with PAL1 protein for proteasome-mediated degradation to regulate phenylpropanoid biosynthesis in Arabidopsis. Mutations in SAGL1 caused high accumulation of anthocyanins and lignin derived from the phenylpropanoid biosynthesis pathway. We found that PAL enzyme activity increased in SAGL1-defective mutants, sagl1, but decreased in SAGL1-overexpressing Arabidopsis (SAGL1OE) without changes in the transcript levels of PAL genes, suggesting protein-level regulation by SAGL1. Indeed, the levels of PAL1-GFP fusion protein were reduced when both SAGL1 and PAL1-GFP were transiently co-expressed in leaves of Nicotiana benthamiana. In addition, bimolecular fluorescence complementation analysis suggested an interaction between SAGL1 and PAL1. We also found that the transcript levels of ANTHOCYANIDIN SYNTHASE (ANS) increased in the sagl1 mutants but decreased in SAGL1OE. Our results suggest that SAGL1 regulates phenylpropanoid biosynthesis post-translationally at PAL1 and transcriptionally at ANS.