Salt as a nutrient for humans is a double-edged sword, being tasty in small amounts but generating an adverse response as the concentration rises. Distinct protein receptors have been shown to mediate these opposing reactions in animals. Excessive uptake of salt is not only unhealthy for humans but also detrimental for plants, because high levels of salt in the soil limit plant growth and crop yields. This is of concern, given that such conditions affect approximately 7% of land globally, including areas used for agriculture, and high salinity affects about 30% of irrigated crops1. Writing in Nature, Jiang et al.2 shed light on how plants recognize salt in their surroundings.
The salt sodium chloride (NaCl) is the main cause of salt stress in plants. It is toxic to cells because at high intracellular concentrations, Na+ ions compete with other ions for involvement in biological reactions. It also has a negative effect on cellular functions by perturbing the balance of ions and thus of water — generating what is called an osmotic perturbation. It was not known how plants perceive stress generated by high salt and whether they can distinguish between ionic and osmotic perturbations.
The exposure of plants to salt stress triggers an immediate temporally and spatially defined rise in the concentration of cytoplasmic calcium ions (Ca2+). It is thought that a calcium channel, of as yet unknown identity, provides a route for Ca2+ to enter cells during such calcium signalling. This Ca2+ signal leads to cellular adaption to salt stress in plant roots, and the subsequent formation of Ca2+ waves that spread over long distances and mediate adaptation responses throughout the entire plant3,4. Central to salt tolerance is the evolutionarily conserved SOS pathway. In this pathway, proteins such as SOS3, which can bind Ca2+ ions, decode the Ca2+ signal and activate5 a protein kinase enzyme called SOS2. This enzyme, in turn, activates a protein in the cell membrane called SOS1, which is a type of protein known as an antiporter that can transport Na+ ions out of the cell. SOS2 also promotes the sequestration of Na+ from the cytoplasm into an organelle called a vacuole6. However, the components and mechanisms governing the perception of extracellular Na+ and driving salt-induced Ca2+ signalling were unknown.
Jiang and colleagues performed a genetic screen using the model plant Arabidopsis thaliana to identify mutant plants that had an abnormally low Ca2+-signalling response to high Na+ exposure, but that could still generate Ca2+ signals when challenged with other types of stress. Taking this approach, they identified a plant that had a mutation in the gene encoding the protein IPUT1. IPUT1 acts at a central step required for the synthesis of a type of lipid called a sphingolipid. This is surprising because, in animals, Na+ ions are sensed by protein receptors rather than through the involvement of lipids.
IPUT1 catalyses the formation of the lipid glycosyl inositol phosphorylceramide (GIPC). GIPCs are major constituents of the outer layer of the lipid bilayer in the plasma membranes of plants, accounting for up to 40% of plasma-membrane lipids, and they can be considered equivalent in function to lipids called sphingomyelins that are found in animals7.
Other mutations previously identified8 in the gene for IPUT1 severely affect plant development; the mutation studied by the authors did not impair development, however, which enabled the role of this protein in the response to salt to be investigated. Emphasizing the importance of Ca2+ signalling for plant tolerance to high salt levels, the authors report that the abnormal Ca2+ signals and long-distance Ca2+ waves in these mutant plants were associated with the plants’ high sensitivity to salt stress. Remarkably, these mutants showed no alterations in their resilience to comparably severe osmotic stress that was induced experimentally in ways that did not require the manipulation of Na+ levels.
Jiang and colleagues report that salt-stress-triggered changes in membrane polarization (the difference in electrical charges between the interior and exterior of the cell) and activation of the SOS pathway were impaired in the mutant plants, compared with wild-type plants. The authors carried out biochemical tests revealing that GIPCs can bind Na+ ions and other ions that have a single positive charge, such as potassium (K+) and lithium (Li+). This observation is interesting because there is evidence for an inverse relationship between the concentrations of K+ and Na+ in plant cells during salt stress5. It would be worth investigating whether and, if so, how K+ binding GIPCs modulates the ability of GIPC to bind Na+, and vice versa. Taken together, the authors’ evidence supports their conclusion that direct binding of Na+ by GIPCs is an essential step in sodium sensing in plants that then triggers the calcium signals that lead to salt-tolerance responses.
The authors propose that plant GIPCs function in the same way as a type of lipid called a ganglioside that is found in animal cells. In neuronal cells, gangliosides directly or indirectly regulate important properties of receptors and ion channels in specific regions of the plasma membrane known as microdomains, which have a distinctive lipid composition9. The authors suggest that, like ganglioside function in animals, GIPCs in plants interact directly with Ca2+ channels. Na+ binding to GIPCs might modulate channel activity, leading to the generation of Ca2+ signals in the cell (Fig. 1a).
However, the evidence currently available also supports a different model, in which GIPCs stimulate Ca2+ signals through an indirect and more complex mechanism (Fig. 1b). There is growing evidence that microdomains in lipid membranes, and specifically GIPCs in these microdomains, aid the regulation of signalling in plants.
Salt stress also triggers the generation of molecules called reactive oxygen species (ROS)4,10, which can induce Ca2+ signalling in plants11. Moreover, salt stress affects the formation and dynamics of microdomains in the plasma membrane, consequently affecting the activity and lateral mobility (the speed and range of movements) of enzymes called NADPH oxidases that act in the production of ROS signals12. Such stress also affects the lateral mobility of enzymes called GTPases that regulate NADPH oxidases12. These changes in microdomain arrangement in response to salt stress depend on the GIPC composition of the plasma membrane12,13.
It is therefore tempting to speculate that the binding of Na+ ions or other positively charged ions to GIPCs modulates the dynamics and assembly of protein complexes in microdomains. Thus, Na+ binding to GIPCs might lead to the assembly of signalling complexes in a microdomain that enables a Ca2+ signal to be generated in response to salt-induced stress. In this way, Ca2+-ion-channel activation might be an indirect consequence of Na+ binding to GIPCs, and might involve the dynamic assembly and activation of other signalling proteins (such as NADPH oxidases) in these microdomains. It would be interesting to investigate whether SOS1 might be incorporated into such a microdomain.
There is evidence in plants that another type of membrane lipid called phosphatidylserine can also affect the formation of microdomains that mediate the regulation of GTPases, Ca2+ or ROS signalling13. It has been reported14 that phosphatidylserine can regulate GTPase-mediated signalling in plants and enable the formation of hormone-induced (rather than salt-stress mediated) clustering of GTPases in lipid membranes. Moreover, GIPCs can contribute to the generation of other signalling events in plants. For example, they act as receptors for specific toxins that cause plant disease, and plants with altered GIPC composition are more resistant to such toxins than are plants with a normal GIPC composition15. These observations, together with those reported by Jiang and colleagues, indicate that GIPCs fulfil versatile sensing and signalling functions in plants. This work also points to a crucial role for membrane-lipid composition in organizing functionally important signalling domains for many key processes in plants.
Nature 572, 318-320 (2019)