A research group has made a groundbreaking discovery about how plants protect themselves from salt stress, which could lead to new ways to bolster food security.
According to the United Nations, soil salinization affects between 20% and 40% of the world’s arable land due to human activities and climate change, particularly rising sea levels. While humans need sodium to survive, most plants do not share this requirement. Excessive salt surrounding plant roots gradually inhibits their water absorption, stunting growth, poisoning the plants, and ultimately leading to their demise. Each year, soil salinization destroys around ten million hectares of farmland, threatening global food security.
Researchers from EPFL, the University of Lausanne (UNIL), and collaborators from Spain studied the ‘Salt Overly Sensitive 1’ (SOS1) gene, discovered in 2000, which provides protection to plant cells against salt. The team utilized an advanced tool called CryoNanoSIMS (Cryo Nanoscale Secondary Ion Mass Spectrometry) to capture unprecedented images. This unique cryogenic microscopy technology enables them to pinpoint where specific nutrients are either stored or utilized within a cell or tissue sample. Their findings indicate that, under significant salt stress, the SOS1 ion transporter does not expel sodium. Instead, it facilitates the storage of sodium in vacuoles within cells. By unraveling this mechanism and discovering why some plant species endure sodium better than others, scientists believe they can develop innovative strategies to enhance food security. Their research has been published in Nature.
First visual evidence
“Our study presents the first visual evidence at the cellular level of how plants shield themselves from excess sodium,” states Priya Ramakrishna, a postdoctoral researcher at EPFL’s Laboratory for Biological Geochemistry (LGB) and the study’s lead author. “Past theories were based on indirect observations. Now, we can see exactly how sodium is distributed under various levels of salt stress—something that was impossible to achieve at this resolution before.” The collaborative EPFL and UNIL team conducted detailed observations using the newly developed CryoNanoSIMS, which can create chemical images of biological materials with a resolution of 100 nanometers, focusing on plant root samples that were rapidly frozen in liquid nitrogen to maintain their structure.
This innovative method enabled them to map individual plant cells and determine where essential elements like potassium, magnesium, calcium, and sodium are stored in the tips of plant roots, known as the “root apical meristem,” which contains the stem cells contributing to root development. CryoNanoSIMS imaging revealed the state of roots under two different salt stress conditions.
A shift in strategy
Under mild salt stress, the cells are capable of preventing sodium from entering. However, the team noted a strategic shift during high salt stress: rather than expelling sodium as previously assumed, the SOS1 transporter aids in sequestering it into vacuoles that serve to store excess substances. “This defense strategy demands a lot of energy, hindering the plant’s growth and efficiency, and may eventually lead to its death if the salt stress continues,” Ramakrishna explains. The researchers confirmed their findings by repeating the experiments with mutant samples that lacked the SOS1 transporter gene, which displayed an inability to transfer sodium into the vacuoles, resulting in heightened sensitivity to salt. They also tested root samples from rice, the world’s most widely cultivated crop, and found similar results: sodium was routed to the vacuoles under high salt stress.
Linking location to function
For Ramakrishna, the capability for chemical imaging through the CryoNanoSIMS is a significant advancement. This tool can also be applied to study how plants defend themselves against other dangers like heavy metal contamination and pathogens. “Through interdisciplinary cooperation that intertwines biology and engineering, we can connect location to function and uncover mechanisms and processes that have never been observed before,” remarks Anders Meibom, a professor at EPFL’s School of Architecture, Civil and Environmental Engineering (ENAC) and the Faculty of Geosciences and Environment at UNIL, who was instrumental in the development of the CryoNanoSIMS.
Niko Geldner, a co-author of the study and head of research at UNIL’s Faculty of Biology and Medicine, shares his excitement for this partnership: “Plants rely heavily on extracting mineral nutrients from the soil, but we previously lacked the means to observe their transportation and accumulation with sufficient precision. The CryoNanoSIMS technology now makes this possible and has the potential to revolutionize our understanding of plant nutrition, extending beyond just salt issues.” Professor Christel Genoud, another paper co-author and Director of the Dubochet Center for Imaging, adds: “This technique is paving the way for entirely new possibilities in imaging biological tissues and establishing our institutions as leaders in this field.”
