While biological research is broad and vast, conducted by people all around the world, there are still often gaps and areas where no one has thought to look or just not bothered to take the time to do so. And it is in those areas that major new discoveries can often be found, whether we’re speaking of physical regions of the world or pieces of forgotten research waiting to be unearthed. Today, we’ll be considering the former, alongside the topics of climate change and engineering better drought resistance and salt tolerance in plants.
Salt and Bacteria
The ability for plants to better tolerate such conditions remains a major part of agricultural research due to oncoming environmental temperature shifts. Alongside that is the problem of increasing salinity in the soil, due to buildups over time from both human actions and general deposition from evaporating waters. Arid regions especially have to deal with both at the same time and the encroachment of soil destroying deserts. Osmotic impacts that try to equalize the high salt conditions outside of a plant by excreting water is one of the primary damaging factors of high salinity and overall prevents the uptake of both water and nutrients. Increased sodium levels inside the plant cells can also cause any number of other health impacts that prevent proper growth.
To combat this, outside of scientists overexpressing resistance genes and searching for new ones to transform into plants, another avenue is the use of plant growth promoting bacteria (PGPBs), which form a symbiotic relationship with plants and improve tolerance to multiple forms of stress conditions. We’ve previously discussed this area of research and it once again comes into play in a similar manner.
The Oncoming Desert
Scientific groups from the Desert Agriculture Initiative in Saudi Arabia have been focusing on whether a microbiome exists for the few plants that naturally grow in near-desert soils and also whether these benefits can be transitioned to other plants as well. Soils samples were collected nearby the town of Jizan from area around a common indigo shrub called Indigofera argentea and then a later experiment using further soil around four more desert plants. When the soil was filtered and tested, an unknown species of Enterobacter bacteria was isolated from it, along with several other diverse species from across the bacterial tree and with no meaningful relation to each other.
The bacteria were tested with the model organism Arabidopsis thaliana and also alfalfa, both showcasing greater growth and also resistance to salinity. The researchers wanted to know more about the underlying genetic factors involved in the benefits and so characterized not only the bacteria themselves, but also the genomic expression changes in the Arabidopsis plants after inoculation with the bacteria.
Manipulating Salt Transport
What was found directly involves the roots themselves. Normally, plants grow their roots in broad spatial areas to increase surface area and also access to more soil in case some part of their roots are blocked or become unusable. Soils with high salinity, however, inhibit this root growth and even prevent cellular division and spreading of root cells through water loss and osmotic pressure. When the bacteria being investigated were thrown into the mix, this all changed. Root growth in general began expanding at a rapid rate and in total general biomass, without any apparent favoring of vertical or horizontal growth.
An observed potential mechanism that the bacteria assist with requires an explanation of a main plant defense mechanism against salinity. In order to counter the effects of sodium ions, one method is for cells to retain potassium ions and then exclude sodium ions from particular regions of the plant, shunting them to the other parts of the plant body. Therefore, when high amounts of sodium ions begin to be taken up by the plant, it will usually lock off the shoots and upper areas of the plant and sequester those ions in the roots. This is obviously not good for root growth, per just mentioned problems, but it does protect the plant from even worse health impacts.
A cell membrane transport protein named HKT1 is used to take sodium ions traveling up the root xylem system and move them back to the root tissue itself. Overexpression of the HKT1 gene has been shown to reduce the accumulation of sodium ions in plants. What was seen in the plants cultured with the unknown bacterial species was high transcriptional upregulation of this gene in response to volatile organic compounds released by the bacteria.
Future Salinity Testing
Additional forms of similar manipulation of genetic expression in the plants by the bacterial chemical emissions appears to overall help with salinity tolerance and allow for plants to survive and even thrive in high salinity soils. The research group was able to qualitatively prove the benefits of the five bacterial species isolated and how they alter ion transportation and sodium and potassium distribution throughout a plant to further protect their symbiotic host from harm.
The next test the scientists will be conducting is seeing if the same results play out when used with barley and wheat, to see if agricultural crops can be grown in regions like the Middle East despite the saline environment caused from large deserts. With additional irrigation, large-scale farming may become more feasible in the area and anywhere in the world that deals with arid environments.
Photo CCs: Tribulus terrestris 3 from Wikimedia Commons