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Plant roots integrate environmental signals with development using exquisite spatiotemporal control. This is apparent in the deposition of suberin, an apoplastic diffusion barrier, which regulates flow of water, solutes and gases, and is environmentally plastic. Suberin is considered a hallmark of endodermal differentiation but is absent in the tomato endodermis. Instead, suberin is present in the exodermis, a cell type that is absent in the model organism Arabidopsis thaliana. Here we demonstrate that the suberin regulatory network has the same parts driving suberin production in the tomato exodermis and the Arabidopsis endodermis. Despite this co-option of network components, the network has undergone rewiring to drive distinct spatial expression and with distinct contributions of specific genes. Functional genetic analyses of the tomato MYB92 transcription factor and ASFT enzyme demonstrate the importance of exodermal suberin for a plant water-deficit response and that the exodermal barrier serves an equivalent function to that of the endodermis and can act in its place.

Plants are exposed to cellular damage by mechanical stresses, herbivore feeding, or invading microbes. Primary wound responses are communicated to neighboring and distal tissues by mobile signals. In leaves, crushing of large cell populations activates a long-distance signal, causing jasmonate production in distal organs. This is mediated by a cation channel-mediated depolarization wave and is associated with cytosolic Ca2+ transient currents. Here, we report that much more restricted, single-cell wounding in roots by laser ablation elicits non-systemic, regional surface potential changes, calcium waves, and reactive oxygen species (ROS) production. Surprisingly, laser ablation does not induce a robust jasmonate response, but regionally activates ethylene production and ethylene-response markers. This ethylene activation depends on calcium channel activities distinct from those in leaves, as well as a specific set of NADPH oxidases. Intriguingly, nematode attack elicits very similar responses, including membrane depolarization and regional upregulation of ethylene markers. Moreover, ethylene signaling antagonizes nematode feeding, delaying initial syncytial-phase establishment. Regional signals caused by single-cell wounding thus appear to constitute a relevant root immune response against small invaders.

The endodermis is a key cell layer in plant roots that contributes to the controlled uptake of water and mineral nutrients into plants. In order to provide such functionality the endodermal cell wall has specific chemical modifications consisting of lignin bands (Casparian strips) that encircle each cell, and deposition of a waxy-like substance (suberin) between the wall and the plasma membrane. These two extracellular deposits provide control of diffusion enabling the endodermis to direct the movement of water and solutes into and out of the vascular system in roots. Loss of integrity of the Casparian strip-based apoplastic barrier is sensed by the leakage of a small peptide from the stele into the cortex. Here, we report that such sensing of barrier integrity leads to the rebalancing of water and mineral nutrient uptake, compensating for breakage of Casparian strips. This rebalancing involves both a reduction in root hydraulic conductivity driven by deactivation of aquaporins, and downstream limitation of ion leakage through deposition of suberin. These responses in the root are also coupled to a reduction in water demand in the shoot mediated by ABA-dependent stomatal closure.

Casparian strips are ring-like structures consisting of lignin, sealing the apoplastic space between endodermal cells. They are thought to have important functions in controlling radial transport of nutrients and toxic elements in roots. However, Arabidopsis mutants with a defective Casparian strip structure have been found to maintain nutrient homeostasis in ranges supportive of growth under standard laboratory conditions. In this study, we investigated the function of Casparian strips under excess boron (B) conditions using sgn3 and sgn4 mutants with defective Casparian strip development but which do not exhibit excessive deposition of suberin, another endodermal diffusion barrier. The growth of sgn3 and sgn4 mutants did not differ significantly from that of wild-type (WT) plants under different B conditions in plate cultures; however, they were highly sensitive to B excess in hydroponic culture, where transpiration drives the translocation of boric acid toward the shoot. In hydroponic culture with sufficient to excess boric acid, B accumulation in shoots of the sgn3 and sgn4 mutants was higher than that in the WT. A time-course tracer study using 10B-enriched boric acid at a sufficient or slightly excessive concentration showed higher translocation of B into shoots of the sgn3 and sgn4 mutants. Furthermore, a genetically encoded biosensor for boric acid expressed under a stele-specific promoter (proCIF2:NIP5;1 5′UTR : Eluc-PEST) visualized faster boric acid flux into the mutant steles. Collectively, our results demonstrate the importance of Casparian strips in preventing apoplastic diffusion of boric acid into the stele under excess supply. [ABSTRACT FROM AUTHOR]

Author(s): Tonni Grube Andersen (corresponding author) [1]; Sadaf Naseer [1]; Robertas Ursache [1]; Brecht Wybouw [2, 3]; Wouter Smet [2, 3, 4]; Bert De Rybel [2, 3, 4]; Joop E. [...]

Polarized epithelia are fundamental to multicellular life. In animal epithelia, conserved junctional complexes establish membrane diffusion barriers, cellular adherence and sealing of the extracellular space (1). Plant cellular barriers are of independent evolutionary origin. The root endodermis strongly resembles a polarized epithelium and functions in nutrient uptake and stress resistance (2). Its defining features are the Casparian strips, belts of specialized cell wall material that generate an extracellular diffusion barrier (2). The mechanisms localizing Casparian strips are unknown. Here we identify and characterize a family of transmembrane proteins of previously unknown function. These 'CASPs' (Casparian strip membrane domain proteins) specifically mark a membrane domain that predicts the formation of Casparian strips. CASP1 displays numerous features required for a constituent of a plant junctional complex: it forms complexes with other CASPs; it becomes immobile upon localization; and it sediments like a large polymer. CASP double mutants display disorganized Casparian strips, demonstrating a role for CASPs in structuring and localizing this cell wall modification. To our knowledge, CASPs are the first molecular factors that are shown to establish a plasma membrane and extracellular diffusion barrier in plants, and represent a novel way of epithelial barrier formation in eukaryotes., In 1865 Robert Caspary described a novel cell type surrounding the vascular cylinder of roots (3). Caspary named this layer 'Schutzscheide', protective sheath, but during the twentieth century it was [...]

Dynamically polarized membrane proteins define different cell boundaries and have an important role in intercellular communication--a vital feature of multicellular development. Efflux carriers for the signalling molecule auxin from the [...]

Author(s): Sandra Richter [1]; Niko Geldner [1, 5]; Jarmo Schrader [1]; Hanno Wolters [1]; York-Dieter Stierhof [2]; Gabino Rios [3, 5]; Csaba Koncz [3]; David G. Robinson [4]; Gerd Jürgens [...]