Drought and salt stress tolerance: osmotic adjustment and endosomal trafficking as targets
Résumé
To cope with drought stress, plants can reduce water loss through reduced leaf surface or leaf conductance or take up water more efficiently by altering root hydraulic conductance, in order to maintain tissue water potential. At the cell level, plants can maintain cell turgor and reduce evaporative water loss by accumulating compatible solutes (osmotic adjustment). To cope with salt stress, at the cellular and tissue levels, several mechanisms are developed by plants: (i) Osmotic adjustment by means of the accumulation of ions/solutes/organic compounds maintains cell turgor and consequently water uptake by the root system. (ii) The root system may be involved in Na+ exclusion. (iii) Na+ and Cl− compartmentalization (also termed sequestration), for instance into the vacuoles, avoids toxic concentrations within the cytoplasm of cells.
Since water channels, also termed aquaporins, play a crucial role in water transport, they may be related to controlling osmotic adjustment in plants and would be good targets for improving our understanding of plant responses to drought and salinity stress. However, several contrasting results did not clearly establish a causal link. Therefore, more research is necessary to elucidate the roles of aquaporins. Sequestration of Na+ into vacuoles to prevent cellular toxicity by tonoplast transporters has been well described. However, recent studies pointed to the involvement of endosomal compartments and the trafficking between these compartments in the sequestration of Na+. Loss of functions of two transporters localized at endosomal compartments associated with Golgi apparatus and the trans-Golgi network (TGN) rendered the Arabidopsis atnhx5/atnhx6 double mutant plants more salt sensitive. These findings suggest that endosomal trafficking may be crucial to control the salt tolerance and potentially drought tolerance in rice plants.
The rationale of our research is to use a strategy of over-expression of proteins suspected to be involved in (1) osmotic adjustment and (2) endosomal trafficking for a better understanding of drought and salt tolerance in rice plants:
1-Six aquaporin isoforms expressed either in the root system or in the shoot or both may have an important function in the osmotic adjustment under drought and salinity.
2-Four proteins identified as components of key compartments in route towards to the vacuole. This role in the trafficking towards the vacuole may be essential for osmotic adjustment and for the optimal sequestration of Na+ in this compartment.
Rice transgenic lines will be challenged to drought and salt stress, and several parameters have been recorded (shoot development, Na+ accumulation, ROS production, seed yield, …).
This research receives a financial support from the program "Investissements d'avenir" of the Agence Nationale de la Recherche (ANR-10-LABX-001-01 Labex Agro) coordinated by Agropolis Fondation.