The Storage, Compartmentalisation and Transport of Salt in Plants that Grow in Areas of High Salinity
In areas where irrigation is used up to 40% of arable land has high salinity. Unfortunately many crop plants are glycophytes and are therefore not tolerant to high salt levels. Much research has been carried out using the glycophyte Arabidopsis thaliana and salt tolerant (halophyte) plants with the aim of discovering the mechanisms of salt control in plants, and to identify genes that may be involved in salt tolerance.
It is hoped that a deeper understanding of mechanisms employed by salt tolerant plants to deal with ion toxicity will enable breeders to develop crops that can grow in areas of high salinity and have increased yields. Two of the mechanisms employed by halophyte plants to cope with high levels of salt are storage through compartmentalisation and restriction of long distance transport of salts from the root to the aerial parts of plants.
Salt Storage - Compartmentalisation
One of the ways in which plants are able to control excess salts is to store them in their vacuoles. This results in a reduction of sodium levels found in the cytoplasm Compartmentalisation of salt in this way has two major positive impacts:
1. Sodium is separated from cytosolic enzymes - excess sodium leads to an inhibition in enzyme function
2. compartmentalisation of sodium in the vacuoles helps to maintain the osmotic potential.
The ability to compartmentalise salt into the vacuoles is one of the major determinants of the levels of salt that are plant is able to tolerate, and hence has a large impact upon the soil that a plant is able to grow in. The way in which salt is up took into vacuoles is well known and involves tonoplast pyrophosphatase and ATPases.
Control of Long Distance Salt Transport
A further way in which plants are able to control the impact of salt and ion toxicity is through long distance transport mechanisms. By restricting the uptake of salt from the root to the aerial parts of the plant the impact of salt on developing tissue, and areas of respiration can be reduced. Another way in which a plant is able to deal with increased salt levels is to store salt in older leaves; this helps to keep the concentration of salt low in new still developing leaves.
How Plants Deal With Osmotic Stress
Much work is being carried out into plant salt tolerance. It is thought that a better understanding of how plants are able to deal with stresses that growing in saline environments will help breeders develop plants that will be able to generate higher yields in lands that have high salinity. As ~20% of all arable lands is known to have high levels of salt, the development of salt tolerant crops may greatly help in eliminating in world hunger.
Dealing with Increased Osmotic Potential
There are many mechanisms that plants have developed for dealing with increased salt levels and these include compartmentalisation of salt into vacuoles and the limitation of long distance transport of salt from the root to a plants aerial parts.
One of the main things that a plant needs to grow is water, in areas of high salinity the osmotic potential is dramatically altered, leading to osmotic stress and possible injury from salt stress. The next section takes a look into the mechanisms that a plant can employ to overcome osmotic stress related problems.
One of the ways in which plants are able to cope with high levels of osmotic potential is to store sodium in vacuoles; this process results in a lowering of osmotic potential and a lowering of stress.
However, when plants are growing in arable land that has a high salinity then the accumulation of sodium may become a limiting factor, this is especially the case in glycophyte crop plants that do not have the mechanisms to store large quantities of salts in their vacuoles. In these circumstances there will be an increase in the osmotic potential resulting in a plant not being to take on-board the required amount of water, leading to a decrease in growth and a lack of vigour.
Altering Osmotic Potential Through Osmolytes
In addition to compartmentalisation, a way in which a plant may overcome osmotic stress is to increase the production and accumulation of osmolytes such as proline, polyols and glyceine betaine; these osmolytes can help to reduce the damage done to plant cells through osmotic related stress. The production of osmolytes leads to a plant becoming more tolerant of salt.
Osmolytes work to reduce osmotic stress and increase a plants ability to cope with salt by reducing osmotic potential, reducing reactive oxygen species, and perhaps most importantly, protecting proteins from misfolding.
Gaxiola et al. (2001). Drought- and salt-tolerant plants result from over-expression of the AVP1 H+-pump. Proceedings of the National Academy of Science 98: 11444 to 11449.
Lazof and Bernstein (1999). The NaCl induced inhibition of shoot growth: the case for distributed nutrition with special consideration of calcium. Advances in Botanic research 29: 113-189.
Yeo (1983). Salinity resistance: Physiologies and prices. Physiology Plant. 58: 214 to 222.
Nuccio et al. (1999). Metabolic engineering of plants for osmotic stress resistance. Current Opinions in Plant Biology 2: 128 to 134.