H2020 PLISUS Project: Plant lipid signaling under drought and salt stress

How anti-stress lipids help make plants more resilient Describing how a group of proteins helps warn plants of stressors like drought and salinity could help sow the seeds for more sustainable agriculture and food security for a rapidly growing global population. Harsh environmental conditions limit where crops can be grown and can lead to yield losses. As stressors, such as drought and salt, trigger changes in plant physiology and biochemistry, understanding how plants sense and adapt to them has become a pressing research question.

A number of organic compounds, collectively known as lipids, that form part of a plant cell membrane are known to be key. “Two lipids, diacylglycerol and phosphatidic acid, known as essential ‘second messengers,’ trigger physiological changes at the cellular level of a plant,” explains Miguel Botella, coordinator of the EU-funded PLISUS project. “Given this important role, they must be tightly regulated, so we wanted to explore some of the key mechanisms that make this possible.” The aim of the project was to explore the role of contact sites between a cell’s working units (organelles), especially those located on the so-called plasma membrane of the endoplasmic reticulum, known to be essential for communication and the regulation of cellular processes.

This research was supported by the Marie Skododowska-Curie Actions programme. Exploring the Endoplasmic Reticulum-Plasma Membrane In plants, cell membranes form numerous contact sites between most organelles in the cell, including the plasma membrane that surrounds cells, providing protection from the external environment. Plant cells have evolved several signaling processes to warn of stressors and trigger protective processes. Lipid signaling moves lipids from the plasma membrane to the endoplasmic reticulum membrane. A family of proteins called synaptotagmins (SYTs) are essential for this process, as they "tether" membranes together. Previous studies had shown that SYTs contain a region called the SMP domain that has been shown to bind a class of lipids called phospholipids, suggesting the mechanism by which SYT proteins actually regulate lipid signaling. "SYT proteins could be moving these phospholipids from the plasma membrane to the endoplasmic reticulum, where they are modified before being sent back to the plasma membrane to counteract stress," Botella adds.

To investigate this, the team used confocal microscopy to study a model plant, thale cress (Arabidopsis thaliana), using wild plants as a control group, along with SYT mutants that had a non-functioning SYT protein inserted. To analyze how endocytosis, the process cells use to regulate substances entering and leaving cells, is affected by stress, the plants were studied under control conditions and after cold treatments. They found that endocytosis was impaired in the mutants, as cold stress altered the diacylglycerol lipid content in the plasma membrane. To learn more about this process, the project used transcriptomic analysis to search for other genes that might be controlling the endocytosis process. While the project found proteins of interest, more research is required before conclusions can be drawn. "Previous work has described how SYTs maintain the stability of diacylglycerol lipid in the plasma membrane when under stress.

PLISUS links this homeostasis to the endocytic process,” says Botella. “We were surprised that the mutated SYT complexes cannot perform endocytosis at all, suggesting that the diacylglycerol content in the plasma membrane is an essential regulator of this process.” More resilient crops Insights from PLISUS contribute to efforts to improve crop resilience through selective breeding programs or genetic manipulation. “Since water scarcity and high salinity restrict the locations where crops and horticultural plants can be grown, our findings could have very specific practical applications,” explains José Aznar-Moreno, a researcher on the project.

Since the exact underlying molecular mechanism and physiological consequences of the link between the SYT complex and the endocytic pathway remain unknown, this is now the focus of researchers.