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Title: “The endodermis – walking the thin line between protection and exchange”

Host: Jaimie Van Norman

Abstract: Roots must manage the opposing aims of providing for uptake of nutrients and water from the soil on the one hand and providing protection from potentially harmful microorganism and toxic elements on the other. The endodermis, the main barrier cell layer of roots, plays a pivotal role in this continuous tug-of-war. The localised network of cell wall impregnations between endodermal cells – the Casparian strips – appear optimised for effectively blocking apoplastic diffusion into the stele while maximising endodermal plasma membrane surface for uptake. Yet, in later stages, endodermal cells close off their surface by surrounding themselves with layers of hydrophobic suberin lamellae, forfeiting uptake capacity for strongly increased protection. Recently, we have been able to demonstrate that roots are able to change speed and degree of suberisation, depending on the specific nutrient deficiency that they experience. This plasticity is not only due to regulated biosynthesis, but also due to increased degradation of pre-existing suberin under certain conditions, revealing an unexpected level of control of roots over their protective barrier. Intriguingly, even in fully suberized root zones, some cells remain entirely unsuberised. These long-described, suberisation-resistant cells were intuitively termed “passage cells” and noted to always occur above xylem poles. We were able to demonstrate that passage cell formation is preceded by differentiation of endodermal cells into phloem/xylem pole endodermis. Symmetry breaking is caused by local differences of cytokinin and auxin signalling, due in part to the movement of the cytokinin repressor AHP6 from the stele into endodermal cells. Moreover, we show that passage cells are indeed privileged sites of transporter expression and appear to exert an effect on surrounding cortical and epidermal cells. I will report on our latest ideas on the functional relevance of this highly conserved cell type in roots.