Polarly localized receptor-like kinases PXC2 and IRK act redundantly during Arabidopsis root development in the radial axis

SUMMARYIn plants, coordination of cell division and differentiation is critical for tissue patterning and organ development. Directional cell signaling and cell polarity have been proposed to participate in coordination of these developmental processes. For instance, a leucine-rich repeat receptor-like kinase (LRR-RLK) named INFLORESCENCE AND ROOT APICES KINASE (IRK) functions to restrict stele area and inhibit longitudinal anticlinal divisions (LADs) in the endodermis where it is polarly localized. The LRR-RLK most closely related to IRK is PXY/TDR CORRELATED 2 (PXC2) and we find that PXC2 shows similar polarized accumulation as IRK in root cell types. To further understand how these proteins operate in directional cell-cell signaling and root development we explored PXC2 function. pxc2 roots have an increase in stele area, indicating that PXC2 also functions to restrict stele size. Additionally, compared to either single mutant, irk pxc2 roots have an enhanced phenotype with further increases in endodermal LADs and stele area indicating redundant activities of these receptors. The double mutant also exhibits abnormal root growth, suggesting broader functions of PXC2 and IRK in the root. However, PXC2 is not functionally equivalent to IRK, as endodermal misexpression of PXC2 did not fully rescue irk. We propose that PXC2 is at least partially redundant to IRK with a more predominant role for IRK in repression of endodermal LADs. Our results are consistent with the hypothesis that repression of specific endodermal cell divisions and stele area through a PXC2/IRK-mediated directional signaling pathway is required for coordinated root growth and development.SIGNIFICANCE STATEMENTIn the root, coordination of growth and developmental processes is critical for organ function and directional cell-to-cell signaling and cell polarity are implicated in these processes. Our studies indicate that laterally polar transmembrane receptor kinases, PXC2 and IRK, have redundant functions in restriction of specific endodermal cell divisions and stele size, and are important for gravitropic root growth.