Agriculture, Osmoregulation, and GMOs

A blog about how halophytic plants might save the future of agriculture in the San Joaquin Valley

David Gallagher

Why does agriculture need to be saved in the San Joaquin Valley?

The San Joaquin Valley contains seven of the top ten counties in California in terms of total value of crops produced. In 2007, an estimated $25 billion of crop commodities were produced in the San Joaquin Valley. Some important crops are grapes, oranges, cotton, almonds, walnuts, and cherries. It has been called “The food basket of the world”. However, the productivity and sustainability of the San Joaquin Valley is threatened by the build up of salt in the groundwater that has occurred over the last 60 years. Hydro-salinity models have suggested that irrigation in the San Joaquin Valley is not sustainable and salinity will continue to rise because of the closed groundwater systems (systems that do not have sufficient leeching of the groundwater to remove excess salt). Salinity has increased primarily because of evaporation and transpiration (the movement of water within a plant from the roots to the leaves and into the air as water vapor) of pure water from the soil solution, thereby concentrating salt in the groundwater. Irrigation with fresh water does not solve the problem because the increase in the water table results in the capillary rise of the already high-salinity groundwater into the rooting zones, thereby increasing salt concentrations in the shallow groundwater zone. One of the big problems for San Joaquin Valley agriculture is that high salt concentrations are toxic to most agricultural crops.

Change in total salt storage and dissolved salts (in megatons) since 1940 in a 1,400 square km area in western Fresno County on the western side of the San Joaquin Valley

Salt deposits on land formerly used for agriculture in the San Joaquin Valley, CA
http://ucce.ucdavis.edu/files/repository/calag/img6301p42.jpg

What is too salty?

The presence of ions in the groundwater is expressed in terms of the electrical conductivity of a soil-saturated extract (EC_e) and is expressed in decisiemens (siemens is a unit of electrical conductance) per meter (dS/m). You might recall that sodium chloride (salt) dissociates in water into Na⁺ and Cl^-  ions. The more ions dissolved in water, the better the water conducts electricity. Of course, other ions are present, but in the San Joaquin Valley salt ions are generally responsible for the electrical conductivity of the soil. Values for EC_e range form less than 1 dS/m to as high as 16 dS/m for the San Joaquin Valley depending on the depth of the measured groundwater. The deepest parts (20-40 meters) of the groundwater system have the smallest EC_e  values and the shallower parts (6 meters or less) of the groundwater system have the largest EC_e values. In general, when the EC_e values exceed 2.0 dS/m, many plants start to experience stress. Some typical threshold values (the highest value of electrical conductivity the plant can grow in) for EC_e for various crops are almonds at 1.5 dS/m, grapes at 1.5 dS/m, and walnuts at 1.0 dS/m.

Why is salt toxic to plants?

The nutrient status of a plant is disrupted because salt ions can easily move through the vascular tissue of the plant by way of the transpirational stream (the movement of water throughout the plant). Under normal conditions the cytosol of higher vascular plants (all agricultural plants) contain approximately 10mM Na^+. Under saline stress, cytosolic salt ions increase to more than 100mM and become cytotoxic. The high concentrations of salt ions result in protein denaturation and membrane destabilization and ultimately the disruption of biochemical pathways essential to photosynthesis and sugar production. High salt concentrations also result in osmotic stress. 

How do plants respond to salty environments?

In addition to the toxic effects of salt, salt also decreases the water potential of the soil around roots, resulting in osmotic stress. Plants can only continue to absorb water if the water potential at the leaf surface is more negative than the water potential in the soil. This is analogous to sucking water up through a straw from a glass - your intake of air is creating a negative pressure at the top of the straw relative to the pressure at the bottom of the straw and the resulting pressure differential results in the movement of water up the straw. Plant physiologists call these pressures water potentials. Plants use the difference in water potential between the roots and the leaves to maintain turgor pressure (think about it in terms of water pushing against the inside of the cell wall trying to get out) and it is what maintains a plant’s rigidity, and ultimately allows for growth. The reason that salt lowers the water potential is that as the salt concentration in the soil increases, the amount of water in the soil decreases. When the amount of water in the roots becomes greater than in the soil (roots are relatively permeable to water), the water wants to move into the soil. This movement of water is called osmosis.

Most plants can overcome a moderate increase in salt concentration and the resulting change in water potential because they can make osmotic adjustments (osmoregulate) in response to their environment. Osmoregulation is the ability of plant cells (or most cells for that matter) to adjust the osmotic concentrations of osmolytes (molecules that affect the movement of water) within their cells to maintain the needed amount of water in the cytosol and the external environment. The movement of Na⁺ and Cl^- ions during osmoregulation takes place mainly in the vacuoles (an organelle of a plant cell that forms a closed compartment within the cell). When ions are compartmentalized in the vacuole, other solutes (osmolytes) must accumulate in the cytosol to maintain the proper water potential or water will diffuse out of the cell (remember we want the water to come into the cell to maintain turgor pressure). Many plants use organic compounds as osmolytes, such as proline (an amino acid), sorbitol, glycine betaine, and 3-dimethylsulfoniopropionate (DMSP). The reason plants use these compounds is that the cell can maintain large concentrations without experiencing detrimental effects on metabolism. Some of these compounds, such as proline actually protect the cell from toxic byproducts during periods of stress, such as periods of water shortage or osmotic stress.

A cell with an external water potential of -0.8MPa because of salt-induced osmotic stress. The cell can osmoregulate by increasing the cellular concentration of solutes in the vacuole and cytosol. The water potential is brought down to -0.9MPa thereby allowing water to flow from the external environment to inside the cell.

What is a halophytic plant?

Plants that are adapted to grow in saline environments are called halophytes. Halophytes have a greater capacity for vacuolar sequestration of sodium and chloride ions in leaf cells. They also posses the ability to reduce cellular uptake of Na⁺ across the cellular membrane. These two features allow halophytes to tolerate a higher Na⁺ in the roots and in the transpirational stream. A large part of salt tolerance in halophytes stem (pun intended!) from the synthesis of additional vacuolar transporters. Plant cells need a way to move ions from the cytosol into the vacuole. This movement is accomplished by trans- membrane proteins (they span the width of the plasma membrane and form a sort of bridge from the inside to the outside of the cell) that are specialized in moving specific ions in specific directions. This specificity can thought as bridge that only allows one type of car to cross. These transporter proteins are critical for sequestering Na⁺ and Cl^- ions. You can see from the diagram below that the cell uses a combination of primary and secondary active transporters. The primary transporters uses ATP (the energy currency of a cell) to establish a proton (H⁺ ions) gradient and then the secondary transporters use the proton gradient (the protons are now moving down their concentration gradient) to move Na⁺ ions against their concentration gradient by coupling the two movements together.

Primary and active transport for a typical plant cell.

Other halophytes, such as Atriplex spp. (saltbush) have evolved specialized hairs on the surfaces of their leaves that excrete salt. Saltbush is found worldwide in saline environments, from estuaries to inland salt sinks and can even tolerate irrigation with seawater. The specialized hairs are called vesiculated trichomes. Each trichome has a stalk and a balloon-like tip called the bladder cell. The leaves sequester the salt ions in the bladder cells and when ruptured, release salt back into the environment. The plant actively transports Na⁺  ions and Cl^-  ions into the vacuoles of the bladder cells using the mechanism previously described. The leaves of Atriplex have a silvery reflectance due to the presence of the trichomes.

A. Apical bud of Atriplex spp. showing abundance of trichomes. B. Cross-section of leaf showing a vesicular trichome (white arrow). C. Single trichome with stalk and bladder cell. (bc=bladder cell, sc=stalk cell, ec=epidermal cell)

Atriplex leucophylla in Morro Bay, CA showing the silvery reflectance of the trichomes
http://calphotos.berkeley.edu/cgi-bin/img_query?rel-taxon=begins+with&where-taxon=Atriplex+leucophylla

GMO’s to the rescue?

Can the adaptations that halophytes utilize to grow and thrive in saline environments be used to improve the salt tolerance of agricultural crops?  Genetically modified organisms or in our case, plants are widely used in agriculture today.  Since the first genetic modified tomato plant became available for commercial sales in 1994, genetic engineering has proven a revolutionary technique to generate desirable traits in plants. GMO’s differ from conventional bred crop varieties in that a specific gene or genes can be transferred from any organism (not only a plant) to the host plant without the need for crossbreeding and the genes introduced are often genes that could never be crossed successfully. These genetically modified plants are referred to as transgenic plants. Transgenic plants include “golden rice”, a rice with large amounts of B-carotene that was created to reduce Vitamin A deficiency in children in many parts of the world. Also, there is a transgenic corn that is resistant to the herbicide Roundup and is widely planted in the United States. The benefit is that the corn fields can be treated with Roundup, killing only weeds and thereby increasing crop yields.

There are encouraging results in terms of improved salinity tolerance (by inserting genes that are known to impart improved resistance to salinity) in various crop plants (under controlled laboratory conditions) through the use of genetic engineering. However, there is further need of improvement for successful release of salt tolerant plants that will be successful in the field under real-world growing conditions. As you can see from the diagram below, there are many genes that have been identified as imparting salinity tolerance to different plants. In the real world, it probably will not involve just one gene that can be inserted into a crop of choice, but ultimately many genes.

Known genes that impart salinity tolerance to different plants

Recently, researchers in Australia were able to cross Triticum monococcum (einkorn wheat), one of the earliest cultivated species of salt tolerant wheat with Triticum durum (durum wheat), a commercially important and widely cultivated wheat that is used to make pasta. Traditional cross breeding methods were used and the process took over 15 years to complete. The new line outperformed the commercial variety by approximately 25% in salty soils in real world conditions of commercial agriculture. Even though traditional methods of crossbreeding were used, it was the knowledge from transgenic plants that allowed an understanding at the molecular level of what was going on. It was determined that the new variety of wheat had a gene for a particular trans-membrane sodium transporter protein. This protein was found in root cells that surround the xylem (vascular system) and it actively pumps sodium from water in the xylem before the water  travels to other parts of the plant.

The future of salt tolerant crops might just involve a combination of the traditional techniques of crossbreeding and the modern techniques of genetic modification. It is possible that the next time you drive by a grove of almond trees in the San Joaquin Valley the leaves might be shimmering with a silvery reflectance.

References

Apse, M.P., and E. Blumwald. 2007. Na⁺  transport in plants. FEBS Letters 581:2247-2254.

California Agricultural Resource Directory 2008-2009. Agriculture Statistical Review.

Hasegawa, P.M., R.A. Bressan, J.K. Zhu, and H.J. Bohnert. 2000. Plant cellular and molecular responses to high salinity. Annual Review of Plant Physiology and Plant Molecular Biology 51:463-499.

Hoffman, G.J. 2010. Salt tolerance of crops in the southern Sacramento-San Joaquin Delta. California Environmental Protection Agency State Water Resources Board Division of Water Rights, Final Report.

Munns R., R.A. James, B. Xu, A. Athman, S.J. Conn, C. Jordans, C.S. Byrt, R.A. Hare, S.D. Tyerman, M. Tester, D. Platt, and M. Gilliham. 2012. Wheat grain yield on saline soils is improved by an ancestral Na⁺  transporter gene. Nature Biotechnology 30:360-364.

Schoups, G., J.W. Hopmans, C.A. Young, J.A. Vrugt, W.W. Wallender, K.K. Tanji, and S. Panday. 2005. Sustainability of irrigated agriculture in the San Joaquin Valley, California. Proceedings of the National Academy of Sciences 102:352-356.

Smaoui, A., Z. Barhoumi, M. Rabhi, and C. Abdelly. 2011. Localization of potential ion transport pathways in vesicular trichome cells of Atriplex halimus L. Protoplasma 248:363-372.

Taiz, L., and E. Zeiger. 2010. Plant Physiology 5^th Edition. Sinauer Associates, Sunderland, MA.

Turan, S., K. Cornish, and S. Kumar. 2012. Salinity tolerance in plants: Breeding and genetic engineering. Australian Journal of Crop Science 6:1337-1348.