Cambium LBDs promote radial growth by regulating PLL-mediated pectin metabolism

Plant growth originates from the interlinked action of cell division and cell growth. During radial growth of secondary tissues, bifacial cambial stem cells grow and divide to produce xylem and phloem precursors, which subsequently undergo expansion characteristic of their respective differentiation processes. In Arabidopsis roots, cytokinins and four downstream LATERAL ORGAN BOUNDARIES DOMAIN (LBD) transcription factors are key players in promoting radial growth, though the underlying mechanisms remain unknown. Here our results indicate that these LBD genes primarily regulate cell growth rather than proliferation. Through a large-scale CRISPR-Cas9-aided reverse genetic screen, we identified a set of PECTATE LYASE-LIKE (PLL) genes that function downstream of cytokinin and the LBDs in the regulation of radial growth. We show that at least one of these PLLs, PLL18, possesses pectate lyase activity. In accordance with this activity, PLLs and LBDs promote radial growth by modifying the pectin composition and mechanical properties of the primary cell wall. Our findings thus connect the central role of cytokinins in radial growth with cell wall remodelling and pave a way for further research on hormone-mediated plant growth regulation and cell wall metabolism.