When it comes to biological stresses on plants that reduce their ability to grow and stunts their development, high salt levels is one of the worst possibilities. A high salinity in the soil can interfere with a plant’s roots in being able to take up water through osmosis. At the same time, some sodium ions are also taken up with the water and accumulate in plant cellular chloroplasts, eventually causing damage to cellular organelles and the chloroplasts themselves, resulting in the loss of green photosynthetic cells in a process known as chlorosis. This in turn impacts photosynthesis efficiency and later health and yields of the plants.
The Volatility of Being a Plant
But when a plant is under stress, there are ways to detect and follow those issues, even to the point of being able to determine the types of stress directly from the plant’s response to them. Their signaling response includes the production of what are known as volatile organic compounds (VOCs) that are able to both send signals to the rest of the cells and also defend directly against the stress, such as how some VOCs are antioxidants and are produced when free radical oxidation stress is being applied.
For salinity stress, a strong diversity of VOC responses are activated, including classes like green leaf volatiles (GLVs), monoterpenes, sesquiterpenes, short-chained oxygenated VOCs, and many more. We’ve discussed before how rice farms, due to the growing conditions of rice within water sources themselves, are highly sensitive to changes in salinity in the water. While efforts are ongoing to develop plants that can themselves resist high salt concentrations, there are other factors outside of them that can influence that resistance.
Another relevant component are the plant growth-promoting bacteria (PGPB) that live symbiotically with their plant host. Specifically it is a subset of these bacteria that are able to contain the enzyme 1-aminocyclopropane-1-carboxylate deaminase (ACC deaminase) that are currently being investigated. A collaboration between South Korea, Estonia, and Kentucky has been looking into these particular bacteria and whether they can alleviate salt stress in addition to the capabilities of the plant itself.
Plants and Bacteria in Perfect Harmony
So far, the only thing that is known is that ACC deaminase is able to lower the amount of the stress hormone ethylene that is produced, though the mechanism of how it does so is not well understood, but it does directly improve stress resistance. To test this outcome, the researchers took two rice cultivars, one salt-sensitive and one salt-resistant and set up measurements of their production of VOCs.
The rice plants were then potted into experimental groups with differing levels of sodium chloride mixed in and then separate groups as well for whether the symbiotic bacteria were included or not. The scientists then tested whether the photosynthetic capabilities of the foliage improved during salt stress thanks to the inclusion of the bacteria, how the amount and type of VOCs emitted under foliage stress changed with or without the bacteria and at differing levels of NaCl, and lastly how all of the above differed between the two rice cultivars that were being used.
The group went with the hypothesis that salt stress directly stimulates the production of VOC stress hormones and that those sorts of emissions would be the strongest in the salt sensitive cultivar. At the same time, they expected the bacteria to help with resisting the stress and thus lower the amount of emissions by doing that.
For both cultivars, the expected physiological results occurred, though in more detail than past studies on salt stress in particular have noted. They observed the reduction in the leaf photochemical rate, lowering the efficiency of photosynthesis, along with osmotic potential going down and reducing the amount of water that could be taken up. This in turn made the stomata on the underside of leaves close in order to reduce water loss through evaporation, but this closure also negatively impacts photosynthesis. Such an outcome was observed in both cultivars, though to differing extents.
For the other experimental groups, inoculation with the bacterial strain Brevibacterium linens RS16 showcased their ability to maintain photosynthesis, likely by regulating the usage of water by the plant to more efficiently process it, along with blocking the uptake of sodium ions. They appear to do so by binding the ions to polysaccharides on their surface and then storing the ions in a storage vacuole inside of themselves.
Giving Clues Into Salt Protection
Overall, the salt sensitive cultivar clearly did worse than the resistant one, but the symbiotic bacteria were able to improve their state in both cases and negate the negative outcomes of high salinity. It is apparent that the bacteria are able to inhibit ethylene and ACC accumulation in the leaves by using ACC deaminase, but how this regulation occurs and in what manner it helps the plant is unclear at this point. A focused and more expansive molecular dive is needed to see the individual facets of the process.
But a greater understanding of this relationship between plants and these growth promoting bacteria in the soil microbiome is of key importance when dealing with stresses such as salt. An investigation into how they help each other may also give us clues on how we can improve our plants themselves to also help them survive in high salinity soils lacking such a hospitable bacterial environment. And that would be a benefit to everyone.
Photo CCs: Riso maturo from Wikimedia Commons