Beyond the barrier: communication in the root through the endodermis

Abstract

The root endodermis is characterized by the Casparian strip and by the suberin lamellae, two hydrophobic barriers that restrict the free diffusion of molecules between the inner cell layers of the root and the outer environment. The presence of these barriers and the position of the endodermis between the inner and outer parts of the root require that communication between these two domains acts through the endodermis. Recent work on hormone signaling, propagation of calcium waves, and plant-fungal symbiosis has provided evidence in support of the hypothesis that the endodermis acts as a signaling center. The endodermis is also a unique mechanical barrier to organogenesis, which must be overcome through chemical and mechanical cross talk between cell layers to allow for development of new lateral organs while maintaining its barrier functions. In this review, we discuss recent findings regarding these two important aspects of the endodermis.

Figure 1.

Endodermal barriers affect radial movement of water and solutes through the root. A, At the root tip, to move from the soil to the outer tissues of the root and then into the stele, water and solute molecules can use either the apoplastic (black lines), symplastic (dotted lines), or transcellular (dashed lines) pathways. B, The deposition of the Casparian strip in the endodermis prevents the free apoplastic diffusion of molecules between the outer part and the inner part of the root forcing molecules to pass through the symplast of endodermal cells. C, The deposition of suberin lamellae prevents the uptake of molecules from the apoplast directly into the endodermis forcing molecules to enter the symplast from more outer tissue layers. Suberin deposition is also likely to prevent the backflow of water and ions out of the stele. Passage cells are unsuberized and may facilitate the uptake of water and nutrients in older parts of the root. Cor, Cortex; End, endodermis; Epi, epidermis; Peri, pericycle; Vasc, vasculature. Figure redrawn and modified from Geldner et al. (2013).

Figure 2.

Mechanical and auxin signals mediate cell-to-cell communication during lateral root development. A, Cell swelling in the pericycle sends a mechanical signal to the endodermis. The endodermis responds in an auxin-dependent manner to allow the pericycle cell to undergo the first asymmetric division marking lateral root initiation. B, The endodermis responds to turgor-driven growth of the developing lateral root primordium by undergoing auxin-dependent cell morphological changes. These changes allow the primordium to pass through the endodermis without creating significant apoplastic gaps between the primordium and neighboring endodermal cells. C, Auxin produced by the developing primordium is taken up by overlying cells in the cortex and endodermis through the transporter LAX3. Auxin signaling in these cells then activates expression of cell wall-remodeling enzymes that allow the cells to separate from one another as the primordium pushes past them. Events are color coded according to the tissue layer they occur in. Cor, Cortex; End, endodermis; Epi, epidermis; LR, lateral root; Peri, pericycle.