PDK1 regulates auxin transport and Arabidopsis vascular development through AGC1 kinase PAX

The 3-phosphoinositide-dependent protein kinase 1 (PDK1) is a conserved master regulator of AGC kinases in eukaryotic organisms. pdk1 loss of function causes a lethal phenotype in animals and yeasts, but only mild phenotypic defects in Arabidopsis thaliana (Arabidopsis). The Arabidopsis genome contains two PDK1-encoding genes, PDK1 and PDK2. Here, we used clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) to generate true loss-of-function pdk1 alleles, which, when combined with pdk2 alleles, showed severe developmental defects including fused cotyledons, a short primary root, dwarf stature and defects in male fertility. We obtained evidence that PDK1 is responsible for AGC1 kinase PROTEIN KINASE ASSOCIATED WITH BRX (PAX) activation by phosphorylation during vascular development, and that the PDK1 phospholipid-binding Pleckstrin Homology domain is not required for this process. Our data indicate that PDK1 regulates polar auxin transport by activating AGC1 clade kinases, resulting in PIN phosphorylation.