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

Key message 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 .