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Molecular mechanisms controlling endodermis and exodermis differentiation in tomato roots

Periodic Reporting for period 2 - ROOT BARRIERS (Molecular mechanisms controlling endodermis and exodermis differentiation in tomato roots)

Okres sprawozdawczy: 2018-02-15 do 2019-02-14

It is predicted that the world human population will reach about 11.2 billion by 2100 and with the climate change ahead water and food security have become one of the major challenges that humanity will have to face in the near future. On this context, agriculture as main source of human and animal food is of special importance. Improving and creating more resilient crops that would be able to grow on water limiting conditions is of special urgency. The study at genetic level of plants species that have naturally evolved and adapted to drought environments provides a source of knowledge to accomplish this necessity. In plants, the root system is responsible to uptake water and nutrients from the soil. When the environment conditions are adverse, such as drought conditions, the roots have to respond to these conditions. There are specific cell-types into the plant root, the endodermis and exodermis, that when differentiate forms barriers that can help to face these adverse conditions.

Nevertheless, to date, the molecular players which control exodermis differentiation remain unknown as well as whether the regulatory programs that determine endodermis and exodermis development are the same or distinct. The ROOT BARRIERS project has addressed these questions with the study of endodermis and exodermis differentiation in tomato, a plant species that develops both layers. We have studied the exodermis and endodermis differentiation using the domesticated Solanum lycopersicum ´M82´ and wild Solanum pennellii species as they have different root morphology and cellular development and have found they differs in their differentiation features. Moreover, our results suggest the endodermis and exodermis differentiation in tomato do not share same molecular regulators. Some studies have revealed that endodermis and exodermis differentiation occurs precociously in response to salt stress. The ROOT BARRIERS project has aimed to study how salt stress affects endodermis and exodermis differentiation in tomato roots also using S. lycopersicum ´M82´and S. pennellii. In addition, the role of these two cell types is of special interest as S.pennellii is salt tolerant specie and suggests that the environment is able to directly regulate the development of a specific cell type. We have generated stable transgenics plants to isolate the messenger RNA specifically from the endodermis and exodermis in control and salt stress conditions. This way we will be able to identify the regulators implicated in the differentiation of these two cell types and how salt stress is influencing the control of their differentiation at the molecular level.
Work performed from the beginning of the project

- Expression pattern of SlCASPs, SlSGN3, SlCIF2, SlMYB36 genes in the tomato root.
- Localization of the SlCASP proteins in the tomato root.
- Developmental framework for the endodermis and exodermis differentiation in Solanum lycopersicum and Solanum pennelli in control and salt stress conditions.
- Identification of an exodermis specific promoter.
- Generation of INTACT and TRAP lines to profile the mRNA from the endodermis and exodermis in Solanum lycopersicum and Solanum pennelli.
- Testing the apoplastic pathway in the exodermis and endodermis using specific tracers.
- CRISPR mutants for candidate genes potentially implicated in the endodermis and exodermis differentiation.
- Endodermis and Exodermis transcript profiling in control and salt stress conditions.

Results achieved

- There are different developmental frameworks for the endodermis and exodermis differentiation in tomato.
- None of the Arabidopsis molecular players, which control endodermis differentiation, homologs in tomato show a specific expression pattern in the endodermis or exodermis, as they are at least being expressed in both. Some of them are also expressed in the cortex and epidermis cells.
- We have identified an exodermis specific promoter. We will be able to specifically profile the mRNA form the exodermis cells using the INTACT and TRAP protocols.
- Salt stress is affecting differentially root development, the endodermis and exodermis differentiation in Solanum lycopersicum and Solanum pennelli.
- The exodermis in tomato shows a plastic response in the formation of the apoplastic barriers in salt stress conditions.

Exploitation and dissemination of the results

The results of the Root Barriers project were presented in the following conferences:

- Poster presentation. 8th International Symposium of Root development. Umea (Sweden, 2017)
- Poster presentation. FASEB meeting. Vermont (USA) (2017)
- Oral presentation. Plant Development and drought stress. Asilomar (USA) (2017)
- Poster presentation. XIV Reunión de Biología Molecular de Plantas. Salamanca. (Spain) (2018).
- Poster presentation. ISRR 10th International Symposium. Israel (2018).
To date the molecular mechanism controlling the exodermis differentiation are not known since the model organism Arabidopsis thaliana do not form an exodermis. The endodermis and exodermis are forming barriers that control the entrance of water and solutes from the soil. With the ROOT BARRIERS project, using tomato as a model system, we will describe for the first time whether the molecular players controlling the endodermis and exodermis differentiation are same or distinct. Moreover, we are studying two tomato species that differs on their adaptability to drought. The comparison of the response of the endodermis and exodermis to saline conditions from these two species will allow us to elucidate if a differential response on these cell types is mediating their different tolerance to drought stress. Elucidating the genes that are controlling the formation of these barriers will help us to modulate the root response in stress conditions such as drought and salinity. The long term socioeconomic impact for this project will be based on the generation of more resilient crops that would be able to grow on drought conditions as it is predicted an increase of desert areas with the climate change ahead.
Wild and M82 tomato cultivars in the green house. Specific promoter expression in exodermis.