Final Report Summary - ENDOFUN (The endodermis - unraveling the function of an ancient barrier) This ERC consolidator project was aiming to address endodermal functionality, specifically its role in plant nutrition, using mutants, markers and mechanistic insights obtained in the course of a previous ERC Starting Grant. Early on in the project we discovered that suberin formation in the endodermis displays an unexpected degree of physiological plasticity. We found that this cork-like, protective polymer can not only be produced precociousy and in larger amounts in responses to nutrient deficiencies, but pre-existing suberin can also be reduced in response to other type of deficiencies. This was an important, entirely unexpected finding (Barberon et al., Cell, 2016). Based on these and other preliminary findings, we then investigated in more detail how a previously overlooked cell type of the endodermis is developing in Arabidopsis. We found that the so-called "endodermal passage cells" are actually late emanations of a broader specification of endodermal cells into two differently localised endodermal sub-cell types, which we named "xylem pole" and "phloem pole" endodermis, due to their association with the underlying vascular strands. We found that xylem pole endodermis has a decreased responsiveness to the plant hormone cytokinin, which translates into additional cell divisions, less elongation and a great resistance to the stress hormone ABA, compared to phloem pole endodermis. The latter difference is thought to cause the delayed suberisation of xylem pole endodermis and the eventual formation of non-suberised passage cells. We furthermore found that passage cells, express, or maintain expression of a number of nutrient transporters that are not expressed in suberised endodermal cells (Andersen et al., Nature, 2018). Based on these findings, we have now obtained cell-type specific, genome-wide expression profiles of endodermal sub-cell types that will be an invaluable ressources for understanding the physiological function and relevance of endodermal passage cells for plant nutrition and biotic interactions (Andersen, Vermeer et al., unpublished). During the course of this project we moreover made great progress in our understanding of the role of the SCHENGEN mutants, obtained in a previous, forward genetic screen. We are now able to place most of the SCHENGEN mutants in a single, endodermal signaling pathway, whose function is to surveil the endodermal diffusion barrier and to boost endodermal differentiation, and overall lignification and suberisation in the case of persistent defects. This unique pathway relies on the strict subcellular localisation of its different signaling components and is crucial for understanding endodermal differentiation mutants (Alassimone, Fujita et al., Nature Plants, 2016 ; Doblas et al., Science, 2017)
