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Understanding how plants control organ size is critical to plant productivity and is determined by cell proliferation within plant meristems coupled with cell elongation. In the plants, cell proliferation and division plane orientation are critical to tissue form and function and directional signaling is frequently proposed to have a key role in coordinating these developmental processes. In the Arabidopsis root, with its precise organization of cells and tissues, we investigate how polarized transmembrane kinases modulate the timing and orientation of cell division. We have identified, KINASE ON THE INSIDE (KOIN), which restricts cell proliferation with koin alleles having a larger root meristem with more numerous cells in the longitudinal axis. KOIN-GFP unexpectedly accumulates only in the interior cell types, including the endodermis where it localizes to the inner polar domain. KOIN expression specifically in the endodermis is sufficient rescue koin phenotypes and control overall meristem size. Additionally, closely related RLKs, INFLORESCENCE AND ROOT APICES KINASE (IRK) and PXY/TDR CORRELATED 2 (PXC2) function to restrict stele area and inhibit specific divisions in the endodermis where they localize to the outer polar domain. Compared to each single mutant, irk pxc2 roots have enhanced phenotypes indicating redundant activities of PXC2 and IRK and suggesting broader functions in the root. We propose that repression of cell division and root size occur through polarized receptor kinase-mediated signaling pathways that converge on the endodermis where KOIN and IRK/PXC2 function are required for coordinated root growth and development.

Complete list of authors : Jaimie M. Van Norman, Cecilia Rodriguez-Furlan, Imtiaz Rony, Roya Campos, Lala (Karina) Vargas, Jason Goff and Jessica Toth