As a model organism, the ''

Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter ...

'' response to

salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

is studied to aid understanding of other more economically important crops. High concentration of salt in the soil has negative effects on plants. For example, it reduces the yield that crop plants can produce in 7% of the land. On the other side, some plants show adaptations to changes in soil salinity, in that the plant's exposure to salt initiates certain mechanisms for cell

osmotic Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of ...

regulation and causes changes in this plant's water obtaining and loss behaviors. One of such plants is the model plant ''Arabidopsis thaliana'', a member of the family

Brassicaceae Brassicaceae () or (the older) Cruciferae () is a medium-sized and economically important family of flowering plants commonly known as the mustards, the crucifers, or the cabbage family. Most are herbaceous plants, while some are shrubs. The l ...

. ''Arabidopsis thaliana'' is native to

Eurasia Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelag ...

and was introduced to some parts of North America. It grows in rocky, sandy and disturbed terrains. It has been found in many studies that ''Arabidopsis thaliana'' showed enhanced Na⁺ and H⁺ extrusion from their cells after exposure to high salinity. Part of ''Arabidopsis''’ range might have included high salinity soil and the plant started adapting to that. Upon high salt exposure, ''Arabidopsis'' experiences a negative osmotic pressure gradient between the salty solution and its

xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived fr ...

, and it absorbs Na⁺ through Na⁺ permeable transporters. The plant then reduces the impact of high Na+ abundance by improving Na+ efflux from its cells through SOS pathway Two different paths in the SOS pathway can activate SOS1, a molecule that causes sodium efflux. One path is the SOS2- SOS3, the other is the PLD path. This is shown in figure 1. SOS2-SOS3 path: #After exposure to high sodium level,

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

level increases in the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

. SOS3 can detect elevated calcium by making a

calcium-binding protein Calcium-binding proteins are proteins that participate in calcium cell signalling pathways by binding to Ca2+, the calcium ion that plays an important role in many cellular processes. Calcium-binding proteins have specific domains that bind to cal ...

, a protein that detects high calcium level in the cytosol and binds to it. #SOS3 proteins interact with protein

kinase In biochemistry, a kinase () is an enzyme that catalysis, catalyzes the transfer of phosphate groups from High-energy phosphate, high-energy, phosphate-donating molecules to specific Substrate (biochemistry), substrates. This process is known as ...

s, then get phosphorylated, which builds up the complex SOS2-SOS3 attached to calcium, then activates SOS2 #Activation of SOS2 pushes it to the

plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...

, then activates SOS1. Finally, this causes the extrusion of any extra Sodium to the outside through Na⁺/H⁺

antiporter An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma m ...

which is near SOS1. #The SOS2-SOS3 complex is necessary for generating the full response of SOS1. But in mutants missing SOS2-SOS3, Sodium can directly regulate SOS1.

(PLD) path

High salinity increases the activity of the enzyme PLD1, which causes the accumulation of

phosphatidic acid Phosphatidic acids are anionic phospholipids important to cell signaling and direct activation of lipid-gated ion channels. Hydrolysis of phosphatidic acid gives rise to one molecule each of glycerol and phosphoric acid and two molecules of fatty a ...

. PA activates MPK6, a protein kinase regulating translation efficiency in high salinity conditions. Then MPK6 phosphorylates SOS1 and again causes sodium efflux. One of the experiments providing the previous pathway utilized ''Arabidopsis'' seedlings grown inside

X-gal dish X-gal (also abbreviated BCIG for 5-bromo-4-chloro-3-indolyl-β--galactopyranoside) is an organic compound consisting of galactose linked to a substituted indole. The compound was synthesized by Jerome Horwitz and collaborators in 1964. The forma ...

es. Researchers used 6–8 days old plants. The MIFE technique was employed to assess the magnitudes of fluxes of Na⁺, K⁺, and H⁺. The experiment involved cutting 8–10 mm long root segments and placing them in a Perspex holder. Then they put the holder inside a 4 mL chamber containing the required solution. They gave around 50 minutes for that setting to reach equilibrium, then took the measurements. Through such a technique, they measured net ion fluxes. Looking at plants responses to salinity might help us distinguish the plants that show the best responses, that is plants that show the least negative impacts on their fitness upon salinity exposure. This might open up the possibility of planting them into soils that other plants cannot survive in.

References

{{Reflist Responses to salinity Plant ecology Plant physiology