The Anatomy of Phi Thickening Development in Brassica oleracea Roots

In response to jasmonic acid, inner cortical cells in Brassica oleracea roots sequentially form two different secondary cell wall arrays. Initially, these cells develop phi thickenings, thick bands looped around the radial walls that are coordinated between adjacent cells to form a reinforcing scaffold. These thickenings develop later than the formation of the Casparian strip and protoxylem. Subsequently, inner cortical cells develop a reticulate network of delicate thickenings on their inner face adjacent to the endodermis. While phi thickenings initially develop in a single layer in the inner cortex, we demonstrate that multiple rings can develop in two ways, either through the occasional development of isolated thickenings in individual cells in the middle cortex, or through cell divisions in the endodermis that generate a new inner cortical layer which forms extensive phi thickenings. We also show that the phi thickening network in developing lateral roots remains separate from the network in the primary root, even as the Casparian strip re-forms a contiguous network. These anatomical observations have implications not only for understanding the developmental pathways leading to phi thickening formation but also for investigating the rare example of two different secondary wall structures being deposited in the same cell type.