An AINTEGUMENTA phosphoswitch controls bilateral stem cell activity during secondary growth.

Plant stem cells have the remarkable ability to give rise to distinct tissues and organs throughout development. Two concentric cylinders of actively dividing stem cells are the main drivers of radial thickening during secondary growth, each producing distinct vascular cell types toward the inside and the outside. While the molecular mechanisms underlying their initiation have been well studied, it remains unclear how these stem cell layers determine which cell type is generated. Here, we demonstrate that the cross-talk between the ERECTA (ER) receptor pathway and auxin signaling controls the amount and choice of output tissue in these two stem cell regions. Mechanistically, we show that the ER pathway phosphorylates the transcription factor AINTEGUMENTA (ANT), thereby modulating bilateral stem cell activity and favoring the differentiation of only one type of cell. Our results thus show that the phosphorylation status of ANT regulates root girth and biomass accumulation by influencing bilateral stem cell production. This highlights the critical role of posttranslational modifications in plant growth and development.