Plant leaves can withstand much higher salt concentrations than roots. The underlying mechanism may help develop more salt-tolerant crops.
In the absence of water, heat or intensive irrigation, the content of table salt (sodium chloride) in the soil increases. However, most crops are sensitive to salt. They respond to the increasing salinity of the soil by greatly reducing their growth. This leads to a reduction in harvest.
Once taken up from the soil by the roots and carried with the water flow to the shoots and leaves, the salt can exert its toxic effect on the plant’s metabolism. How the plant can escape this dilemma, plant researchers from the Julius-Maximilians-University (JMU) Würzburg in Bavaria, Germany show in their latest publication in the journal New phytologist.
Biophysicist Professor Rainer Hedrich and his team have developed a methodology that can easily and quickly record how plants detoxify the salt supply in their leaves.
Leaf movement as an indicator of salt transport
To investigate the mechanisms of salt detoxification in leaves, Dr. Dorothea Graus as lead author of the publication, Professor Irene Marten and Dr. Kai Konrad tobacco plants as a model system. The intercellular spaces of tobacco leaves can be easily and quickly filled with test solutions using a syringe.
To document coping with acute salt stress, the insides of the tobacco leaves were flooded with a 30 percent sea salt solution and the response was recorded with a video camera. This salt stress caused a drop in pressure in the leaf cells, which became noticeable as the leaf continued to sink.
“We were prepared for this”says Rainer Hedrich. “But the fact that the blade fully recovered from the salt flood and returned to its original blade position after just 30 to 40 minutes was more than astonishing.” The injected salt dose remained in the leaf, but not in the intercellular spaces. Instead, it was absorbed into the cell plasma.
The salt, which reduced the pressure in the leaf, was thus introduced into the cell and then led to the largest cell compartment, the vacuole. Through this step, the water initially lost through osmosis returns to the cell, after which the cell pressure builds up again and the leaf stretches.
How does the salt get into the cell and how does it get into the vacuole?
Kai Konrad and Irene Marten explain that “sodium ions enter the cell through ion channels and are driven by the negative potential of the cell membrane. Chloride ions are taken up by chloride-proton cotransporters, which are fed by the proton motive force.
As a result of the incorporation of sodium chloride salt into the cell plasma, the membrane potential drops temporarily while the net proton concentration decreases. These signals, along with sodium ion sensors, initiate salt transport from the cytoplasm to the vacuole. The studies have shown that transport on the vacuole membrane strongly determines what happens in the cytoplasm and on the cell membrane.
Kai Konrad adds: “Using fluorescence-based detection of proton concentrations, we were able to show that the incorporation of sodium ions into the vacuole is accompanied by a change in the proton concentration in the cytosol and the vacuole.” This was indicative of the involvement of the NHX1 transporter located in the vacuole membrane, which exchanges sodium ions for protons from the vacuole during salt stress. “We were able to substantiate this assumption with plant lines whose vacuoles showed increased activity of the sodium ion-proton antiporter NHX1″explains Kai Konrad further.
Groundbreaking exception to the calcium dogma of salt tolerance
In roots, an increase in calcium ions in the cytoplasm causes sodium ion-repelling forces that repel invading salts into the soil. This salt protection mechanism, also called the SOS pathway, is also active in the tobacco root. However, the Würzburg research team was surprised to find that the leaves were able to detoxify the applied salt load without any calcium signal.
This means that the SOS dogma based on calcium ions is no longer valid with regard to salt stress control in leaves.
“Roots of most plants already suffer when confronted with a quarter of the salt dose we have imposed on the tobacco leaf”, explains Kai Konrad. So leaves apparently have better salt stress control and thus salt tolerance than roots. With persistent soil salinization, however, the salt reservoir in the vacuole of cultivated plants fills up and therefore pushes the salt tolerance in the leaf to the limit.
Better understanding the salt toxicity mechanisms in leaves could help develop new strategies for producing salt-tolerant crops. To this end, the Würzburg research team aims to specifically change the ion ratios of sodium, chloride, protons and calcium in the cell using light-directed ion transport proteins, so-called optogenetic tools, and thus further decipher the salt transport mechanisms and signal pathways involved.
Video: Why does salt change the taste of everything?
Dorothea Graus et al, Tobacco leaf tissue rapidly detoxifies direct salt load without calcium activation and SOS signaling, New phytologist (2022). DOI: 10.1111/nph.18501
Provided by Julius-Maximilians-University (JMU) Würzburg
Quote: Extreme salt stress causes leaf movement (2022, October 6), retrieved October 6, 2022 from https://phys.org/news/2022-10-extreme-salt-stress-triggers-leaf.html
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