Halophytes: what makes them special? Revealing ionic mechanisms of salinity tolerance

The increasing problem of global land salinisation and associated multibillion dollars losses in crop production require a better understanding of key physiological mechanisms conferring salinity tolerance in crops. The effective way of gaining such knowledge comes from studying halophytes. Halophytes have always attracted the attention of plant physiologists, due to their remarkable ability to tolerate and even benefit from salt concentrations that kill most other plant species. At the very least, halophytes may provide genes that allow transgenic conference of salinity tolerance to crops. In addition, some halophytes have already been tested as vegetable, forage and oilseed crops in agronomic field trials, whilst others show good potential to be developed as crops. Surprisingly, our knowledge of fundamental ionic and molecular mechanisms conferring salinity tolerance in halophytes is rather limited, and at best is restricted to several model species. This talk summarises the current knowledge of physiological mechanisms regulating ion uptake and sequestration in halophytes and provides insights into the identity of membrane-transport systems mediating ion transport in halophyte root and leaf tissues. The focus of this study was on two species: quinoa (Chenopodium quinoa Willd.) and Atriplex (Atriplex lentiformis L.). A range of physiological techniques (leaf gas exchange and photosynthetic characteristics; sap osmolality; tissue elemental composition) were used to reveal mechanisms of osmotic adjustment and tissue-specific ion compartmentation in both species at the whole-plant level. It was found that 95% of osmotic adjustment in old leaves and between 80 and 85% of osmotic adjustment in young leaves was achieved by means of accumulation of inorganic ions (Na⁺, K⁺ and Cl^-) when plants were grown at elevated (up to 500 mM NaCl) salinity levels, whilst the role of organic osmolytes was very limited. Both species also possessed an efficient mechanism to control Na⁺ and K⁺ loading into the xylem, as well as for efficient Na+ sequestration in leaves. Whole-plant experiments were complemented by a range of microelectrode studies (noninvasive ion flux measurements; membrane potential; patch clamp) aimed to reveal the identity of specific ion transporters mediating the above process. We compare the kinetics of net K⁺ and Na⁺ fluxes between different root zones (e.g. elongation vs mature zone) and report a differential sensitivity of quinoa and Atriplex root tissues to NaCl and oxidative (hydroxyl-generating Cu/ascorbate mixture) stress. We show that regulation of both depolarization-activated outward-rectifying K⁺-selective (GORK) channels and non-selective cation (NSCC) channels are instrumental to halophytes adaptation to saline conditions, and reveal an important role for a H⁺- ATPase pump in this regulation. We also demonstrate a feasibility of using the MIFE technique to map ion flux profiles from intact plant leaves and report, for the first time, in situ data on patterns of net K⁺, Na⁺ and H⁺ flux kinetics from halophyte bladders in response to a range of salinity treatments.