Root meristem growth factor RGF, a sulfated peptide hormone in plants

In recent decades, small secreted peptides have been recognized as a new class of intercellular signaling phytohormones in plants. Tyrosine sulfation plays crucial roles in peptide hormone bioactivities in plants. The Arabidopsis tyrosylprotein sulfotransferase mutant tpst-1 causes severe abnormalities in the root tip due to deficiency in the biosynthesis of all functional tyrosine-sulfated peptides. Root meristem growth factor RGF, a sulfated peptide hormone specifically expressed in the root tip, was found to complement tpst-1 root defects. This review summarizes the history of the identification of RGF, the characteristics of RGF, the identification of RGF receptors, and the target of RGF. In brief, RGF is a 13 amino acid sulfated peptide. The RGF peptide mutant rgf1,2,3 exhibited a reduced size of the root apical meristem, indicating that RGF maintains cell proliferation activity in the root apical meristem. RGF receptors were identified by comprehensive binding analysis with a custom-made receptor expression library. The RGF receptor mutant rgfr1,2,3 showed a phenotype of reduced root length due to a reduction in the root apical meristem and was insensitive to RGF. The signaling cascade through RGF-RGF receptor pairs regulates the gradient formation of PLETHORA (PLT), which is known as the master regulator of root formation. In the peptide mutant rgf1,2,3 and receptor mutant rgfr1,2,3, the gradient of PLT proteins disappeared, indicating that RGF defines the PLT protein gradient to ensure robust root growth and root development. Recent studies of the downstream signaling of RGF-RGF receptor pairs are also described in this review.