Tyrosine sulfation is a post-translational modification that has been found in several peptide hormones in plants. This modification is mediated by tyrosylprotein sulfotransferase (TPST). In loss-of-function mutant of TPST (tpst-1), root stem cells are not maintained and root meristematic activity is considerably decreased, suggesting that tyrosine-sulfated peptide(s) plays critical roles in root meristem development. By using in silico screening and practical bioassay, we identified a novel family of sulfated peptides that restore root stem cell and transit amplifying cell proliferation in tpst-1. We named these peptides root meristem growth factors (RGF). RGFs are expressed mainly in the stem cell area and the innermost layer of central columella cells. We also found that RGF1 defines expression levels and patterns of the root stem cell transcription factor PLETHORA mainly at the post-transcriptional level. Interestingly, RGFs function independently of the auxin pathway. Our results reveal that secreted peptide signals play a crucial role in root meristem development.

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