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Roots transport water and nutrients from the soil but at the same time they must prevent pathogen invasion and loading of toxic substances. The ground tissue system regulates the transport of water and different compounds from the soil to the vascular tissue and restricts the movement of pathogens and toxic substances to the rest of the plant. This tissue is composed of two major cellular systems: endodermis and cortex. Most of land plants have a single layered endodermis but a more complex multilayered cortex. Our first objective was to identify the genetic and developmental process underlying ground tissue formation in the Brassicaceae family with special emphasis to species adapted to extreme environments since previous data suggest that specific organization of the ground tissue is involved in such adaptation. A preliminary phenotypic analysis of the Brassicaceae family roots has revealed several species with different ground tissue phenotypes. From ground tissue single layered endodermis and cortex in Arabidopsis lyrata roots to double cortex in Olimarabidopsis pumila or multi-layered cortex in the crops Brassica rapa and Brassica napus; the Brassicaceae family provides an immense source of phenotypic variation in ground tissue organization. We have classified and document this distribution of ground tissue organization in the Brassicaceae phylogeny. After analysing roots of 25 species across several lineages of the Brassicaceae family we have correlate ground tissue features and specific ecological trends. Moreover, this preliminary distribution suggest that the more likely ancestral ground tissue organization in this family was a single endodermis and a multi-layered cortex and a second layer of endodermis (peri-endodermis) has been acquired several times during the evolution of this family.
With the aim to get a complete picture of how this tissue complexity is genetically regulated and how has been acquired during plant evolution, we have define at cellular level the developmental programme followed by two different species Cardamine hirsuta and Eutrema halophila to produce their specific root tissue systems: a double layered cortex and an additional layer of endodermal-periendodermal cells in Eutrema. We are now performing an in deep analysis of this developmental process in E. halophila. We are characterising the pattern of cell divisions comparing the expression of the orthologous genes of three key ground tissue regulatory transcription factors (SCHIZORIZA (SCZ), SCARECROW (SCR) and SHORTROOT (SHR) with specific expression in endodermis/cortex cell types in A. thaliana. Based on the model of the ground tissue formation in Arabidopsis, our hypothesis is that different patterns of expression of some of the key ground tissue transcription factors SCHIZORIZA (SCZ), SCARECROW (SCR) and SHORTROOT(SHR) underlie the acquisition of complex ground tissue organization in these species. To test this hypothesis we are using a comparative approach, we are comparing the expression patterns and protein localization of the orthologous genes of these three key ground tissue transcription factors in these three species.
Aditionally, to understand how ground tissue diversity has been acquired during the evolution of Brassicaceaea we have used Arabidopsis as a system to test the evolutionary and adaptative consequence of the acquisition of different ground tissue organizations. For that reason, we have generated Arabidopsis transgenic plants that express all the three ground tissue regulators SCZ, SCR and SHR of E.halophila fused to reporter GUS. We have found that in Arabidopsis roots, EuSHR gene is expressed in the stele, EuSCR 1 and EuSCR2 are both expressed in the endodermis and EuSCZ is expressed in the ground tissue and vascular tissue. All these patterns of expression of the Eutrema genes are similar to the Arabidopsis genes, suggesting that the regulatory elements that direct their expression has been conserved between these two species.
Finally, due to the extremophile behavior of Eutrema such as extreme tolerance to a stresses like low humidity, freezing and high salinity, we have analysed the development of the casparian stripe in E. halophila in response to saline stress and we have seen an early development of the casparian stripe in roots of E.halophila that has been treated with NaCl. We are now using the E. halophila lines expressing EuCASP1protein fused to GFP to analyse in more detail this induced casparian formation but also the E. halophila transgenic lines (expression and overexpressor lines) described above to check if any of the genes that we are studying are also regulated by this environmental condition. Altogether, these analyses will help us to evaluate the adaptive value of this trait. Root science promises to improve agriculture by enhancing resistance of crops to environmental challenging conditions and this project could help to deliver this promise. Further characterization of the role of ground tissue in plant adaptation will provide us with the genetic tools to design better crops with an enhanced tolerance to adverse conditions.
Addressing the global challenges of climate change and food security is linked to the use of our plant biodiversity. A better understanding of the developmental and evolutionary basis of root tissue formation is essential to facilitate the exploitation of plant root diversity in Brassicaceae and the results obtained in this project will contribute significantly to reach this understanding. In summary, this project will help us to face two of the global issues of our society, food security and impact of changing environment in plant diversity. To address both challenges we need a better understanding of plant development and plant adaptation and this Marie Curie funded project would help to improve our knowledge of both processes.