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The endodermis is the main apoplastic diffusion barrier in young, actively absorbing roots of higher plants and it is central to our understanding of how plant roots mediate selective uptake of nutrients. Classical anatomical literature on the endodermis defines three different stages of endodermal differentiation, of which only two are occuring in Arabidopsis. The formation of the “tight-junction-like" Casparian strips represents the first stage of differentiation, which is followed by a secondary stage, defined by the formation of suberin lamellae all around the endodermal surface. We have found an unexpected plasticity in the onset and strength of suberisation to a wide range of nutrient conditions, regulated by the stress hormones abscisic acid and ethylene. We demonstrate that this plasticity is an adaptive response, assisting in maintaining nutrient homeostasis. Interestingly, some endodermal cells never form suberin lamellae. These cells were suggestively termed „passage cells“, hinting towards a possible function of those cells in continued uptake of nutrient withinin a mature and otherwise non-permissive endodermis. Passage cells were shown to be always occurring above xylem poles. I will report on the occurence of passage cells in Arabidopsis and on our efforts to understand the mechanisms controlling their numbers and positioning.
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