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Understanding Halophytes for an Agriculture Worth its Salt

Periodic Reporting for period 2 - HALO (Understanding Halophytes for an Agriculture Worth its Salt)

Período documentado: 2018-12-01 hasta 2019-11-30

Global demand for food and farmland is rapidly growing due to increasing world population and urbanisation. As a result current estimates indicate that food production will have to increase by up to 70% by 2050 to keep pace with projected demands. Suitable resources for future agricultural expansion are however limited due to competing land and water uses for human consumption and non-food crops production. Thus, looking forward, we currently face one of the greatest challenges of the 21st century: to meet the world’s future food security and sustainability needs, food production must grow substantially despite a substantial decline in the availability of productive resources (soil and water). It is now clear that this conundrum cannot simply be solved with currently available soil and water resources and that the brackish/saline ones, nowadays unproductive, have to be included in the equation. Despite this, there is a growing recognition in the scientific and research and development communities of the limitations of current food production technologies. This perhaps is a reflection of the fact that crop selection process has been developed without considering the constraints occurring in more marginal and extreme environments. As a result, this selection for higher yields under optimal conditions during the green revolution of late 20th century has dramatically reduced the tolerance of elite crops to abiotic stresses. Fortunately, 450 million years of land plant evolution has generated biological complexity, which has allowed the so-called “extremophiles” to adapt to extreme environments, ranging from high salinity environments to extreme temperature changes and drought conditions in desert environments. Amongst these extremophiles, halophytes are an exciting group of plants that shows an elevate tolerance to salinity, thriving in salt concentrations damaging for most other angiosperms.

The main objective of the HALO project is to elucidate the complementary morphological, physiological and anatomical characteristics that enable dicotyledonous halophyte to be successful on saline soils, including their unique ability to sequester cytotoxic Na and Cl ions in specialised external structures called salt bladders. This will reveal the fine print of one of the most interesting mechanisms evolved by plants over the course of evolution not only to deal with NaCl toxicity but also use it to thrive in these otherwise hostile NaCl-rich environments, opening up novel and previously unexplored breeding targets to improve salt tolerance in crops.
Overall, from a scientific point of view, this project has two main objectives:
1. Dissect the physiological and molecular modification occurring in halophytes under saline conditions underlying their ability to thrive in saline environments;
2. Identify (electrophysiological and molecular characterisation) the transporters involved in ion transport in stalk cells within the epidermal bladder cells.

For objective 1 the response of four accessions of the halophyte quinoa with contrasting salt tolerance and epidermal bladder cell (EBC) density were evaluated to determined why accessions with a high density in EBC (thus elevated sequestration of cytotoxic ions) fail to achieve high salt tolerance compared with two accessions that have low EBC density. To elucidate possible mechanism(s) underlying this enhanced tolerance in accessions with low EBC density, different physiological and electrophysiological parameters were evaluated after long and short-term treatment with NaCl. The combined data indicate that additional tolerance mechanisms operate in roots rather than shoots in salt tolerant accessions. This in turn results in an improved responses to NaCl and ROS stress, which improves root functionality following salt stress and ultimately results in improved leaf stomatal regulation and enhanced salt tolerance.

For objective 2, ion fluxes and membrane potentials were measured in stalk cells in petioles of leaves of control and salt grown quinoa plants using non-invasive microelectrode ion flux measuring MIFE technique. Subsequent pharmacological experiments using a range of channel blockers and metabolic inhibitors revealed the most likely transporters involved in Na, Cl and K loading via the stalk cells in EBC.
Salinity costs the global farming economy around $27 billion per year and all attempts to reduce this economic burden by targeting “traditional” traits in the breeding programs have yet to be successful. The results of this project will arm breeders with the ability to target previously unexplored traits, opening a new avenue for crop breeding for salinity stress tolerance and increasing the chances for success in creating truly salt tolerant varieties. In particular, the expected results will provide the first comprehensive electrophysiological characterisation of key membrane transport systems mediating ion transport via the stalk cells into the salt bladders. This will give useful insights into mechanisms behind bladder cell-based desalination and open a highly new and exciting possibility to introduce salt tolerance into cereal crops by targeting ion loading in other types of trichomes. Indeed given that bladders are modified trichomes, one could overcome NaCl toxicity of crops by arming ordinary trichomes with the ability to sequester toxic ions through the expression of molecular salt bladder elements in them.

Halophytes’ ability to thrive in saline soils has increasing potential in a world where most known agricultural crop are salt-sensitive glycophytes and where changes in climate and land use will increase resources salinisation. Indeed, halophytes could represent the “silver bullet” approach for maintaining food supply over the coming years as they could play an important role as crops in their own right and also as models for generating salt tolerance in traditional crops. Overall, this will enable farmers to use currently unproductive lands and saline waters to increase agricultural outputs while limiting the global diversions of “good” resources to agriculture, which is critical for the state of the environment and for the civil society at large.
Project diagram