SEB Bulletin July 2005
Salt of the Earth- How do plants cope?
Remember the feeling of eating too much popcorn? You're usually sitting in the cinema when the lips become dry and tingly because the salt is making them dehydrated. So you reach for the giant drink they foisted on you whilst buying the popcorn, aahh that's better! But we're lucky that we can do something to remedy our salty overdose. Think of a wheat plant; roots firmly embedded in the dry earth and in desperate need of water to dilute some of the salt in its system. Unlike us, plants are entirely at the mercy of their environment. So unfortunately for the immobile plant, when exposed to too much salt, it can be a fatal experience.
But all soil contains salts, many of which are essential nutrients for plants. Salinity occurs when soluble salts (usually NaCl) are elevated in soil and water1 . Saline (salt-affected) soils occur mainly in arid or semi-arid areas and can arise from natural processes like weathering of mineral rocks (called primary salinity). In these arid areas, there is often inadequate rainfall or drainage to move the salt down through the soil so it can leach away from plant roots. “But in addition to this, secondary salinity can occur from human intervention. In agricultural areas, the land is cleared of native vegetation (like perennial shrubs and trees with deep roots) and replaced with shallow-rooting crop plants” explains Dr Rana Munns from CSIRO Plant Industry, Australia2. This causes the underground water table to rise, moving salts up to the soil surface”. Irrigation water is also often saline in these areas and adds to the level of salts that the plant must tolerate. In fact in countries such as India, irrigation is a major contributor to salinity.
But why should we care about how a plant copes with excess salt? Once inside the cell, salt can cause ionic stresses, largely as Na+ (and Cl-) inhibit metabolic processes including protein synthesis. “Na+ can rise to toxic levels in older leaves, causing them to die. This reduces the leaf area available for photosynthesis and so the plant cannot sustain growth or crop yield ” says Dr Munns. So, as our population grows (estimated to increase by 50% from 6.1 billion in 2001 to 9.3 billion in 20503), salinity is shrinking the land available for growing our crops at an alarming rate.
Every continent is affected by salinized soil and water; approximately 7% of the world's land area. But it is irrigated land that is most affected, often by the previously mentioned poor agricultural practices. Roughly 30% of all irrigated land (50% in some countries) is considered economically1 unproductive . The problem is that irrigated land has at least twice the productivity of rain-fed land and produces up to one third of the world's food4.
Unfortunately crops are some of the most salt-sensitive plants (termed glycophytes). But there are some plants, called halophytes, which do manage to live in high salt conditions. Salt tolerance is defined when plants show little growth reduction at concentrations of 300mM NaCl or more5. Professor Mark Tester6 from the Australian Centre for Plant Functional Genomics says, “The agricultural salinity problem can be tackled by improving farming practices to prevent salinization in the first place (such as planting deep-rooted trees to lower the water table). However, the immediate, increasing demands on our cultivated land also mean that crops which are salt tolerant need to be generated, either by traditional breeding or genetic manipulation technologies” (the use of GM technology is discussed further here7).
Research into the mechanisms of salt tolerance, and efforts to create tolerant plants, were the focus of session C3 at the main SEB meeting in Barcelona in July, 20058. Co-organizer of the session, Professor Tim Flowers9 from the Plant Stress Unit, University of Sussex said “The aim of this session was to integrate the research approaches from single cell biologists, molecular biologists and whole plant physiologists”.
So, we know that we need to generate salt-tolerant crops for our increasing food demands, but like most important traits, salinity tolerance doesn't appear to be a simple one. Professor Flowers adds “Attempts to improve salt tolerance through traditional breeding programmes have had very limited success. Salt tolerance is complex both genetically and physiologically”. The effects of salinity appear to be dependent on the species and on the stage of the plant's development (such as germination or vegetative growth). So the first step for some researchers has been to take a lesson from nature, to find out how the halophytes survive in areas glycophytes cannot. The aim is then to move these tolerance traits into nontolerant crops. And there appear to be two tricks, essential for tolerance, that halophytes are particularly good at. The + first is excluding Na from the roots to limit its transport to the leaves. Additionally, some halophytes accumulate Na+ in the shoot, because they can store it away from vital cellular functions. Na+ is usually stored in the vacuole, the large storage compartment of plant cells. “Bread wheat has the first of these traits and barley has the second but modern durum wheat has neither” explains Dr Rana Munns. Given that pasta makers consider Australian durum wheat to be of premium quality (and it is more valuable to farmers than bread wheat), Dr Munns and colleagues are looking at trying to increase its salt tolerance without losing the desirable pasta-making characteristics. They searched through a collection of wheat that had originated in the Mediterranean, which is a salt-affected area and found a wheat line from Persia (now Iran) which excluded salt, resulting in lower levels in the leaves. This line was then crossed with modern durum wheat four times to give a new salttolerant variety. Field trials are currently underway and preliminary results from wheat breeders indicate a 20% yield increase on saline soil. Researchers have also developed a molecular marker to flag the location of genes for the trait on a particular chromosome. This will hopefully hasten the breeding of new tolerant varieties
Because Na has such adverse effects on many plants, Dr Romola Davenport10 from the University of Cambridge is studying why crop plants don't seem to have an 'off switch' where salt is concerned, and can accidentally accumulate it until it kills them. “Most plant species show no nutritional requirement for Na+, although addition of Na+ can enhance growth in some conditions” she says. “So it is surprising that two genes, apparently for Na+ specific transporters, have been identified in the salt-sensitive model plant Arabidopsisthaliana”. Transporters are proteins within the cell membrane that allow ions (such as Na+) or other small compounds to travel into the cell and between cellular compartments. “The Na+ transporters also affect potassium (K+) balance and root development, suggesting that Na+ transport may have important physiological functions under nonsaline conditions” adds Dr Davenport. “For example in morning glory flowers a Na+ /H+ transporter in the vacuolar membrane of the cell controls petal colour by alkalizing the vacuole, turning the anthocyanin pigments blue11 . A beneficial role for Na+ in normal membrane transport processes may explain why Na+ accumulates to such dysfunctional levels in plants not normally exposed to salinity”. The function of one of these proposed Na+ transporters, called SOS1 (as it was identified from a 'sodium oversensitive mutant') is being investigated by Professor Jose Pardo12 from IRNA-CSIC in Spain (who is the co-organizer of the C3 salinity session). His group aims to discover the role of sodium transporters in Na+ exclusion by cells, storage of excess Na+ in the vacuole and long distance transport of ions within the plant. He says recent results suggest that “SOS1 may play a role in sensing salinity stress” which may then help the plant respond and survive.
Professor Hans Bohnert13 from the University of Illinois, and colleagues, are investigating which genes make it possible for some plants to survive in high salt and drought while others die. “We are comparing a very salt-sensitive species (Arabidopsis thaliana) to a very salt-tolerant species (salt cress, Thellungiella halophila) which are closely related (90-95% DNA sequence identity). Salt cress is particularly interesting as a model plant because although it is an extremophile that can still reproduce after exposure to high salinity (500mM NaCl) and cold (-15o C), it does not have salt glands or other morphological alterations (like other halophytes), which are evolutionary recent.” This suggests that salt cress may be using the same tolerance equipment that non-tolerant plants also possess (such as the same genes and proteins). But to be successful in high salt conditions, salt cress could be using this equipment in a different way, and this does appear to be the case. Using microarrays14, his group has found that stress genes common to both species are expressed (switched on) at a noticeably higher basal level in unstressed salt cress plants. This gene expression is then rapidly increased under stress, which may mean that tolerant plants are constantly in a 'pre-activated state' so that when they do encounter high salt they are better prepared. In addition to this, it appears that the salt cress has some unique tolerance genes too. By 'knocking out' the function of salt cress tolerance genes, Bohnert and his group are trying to understand their role when they are operating normally. “We hypothesize that salt cress' stressrelevant genes have undergone significant adaptive changes after reproductive separation from the clade in which Arabidopsis is found. These evolutionarily adaptations appear to have generated the observed fitness to extreme environments”.
Because, in many plants, a large component of tolerance to long-term salt exposure is to exclude Na+ from the 6 shoot, Professor Mark Tester and his group are investigating the route sodium takes to get into the plant, in the hope that manipulation of this route will generate more salt tolerant plants. “Many components of salinity tolerance of a whole plant require particular functions in specific cells. To facilitate Na+ exclusion from the shoot, Na+ would need to be pumped out of the cells in the outer part of the root, back into the soil. But Na+ would also need to be moved into cells in the inner part of the root (adjacent to the xylem), to maintain low Na+ in the xylem and thus low delivery to the shoot. But the genes that encode pathways for Na+ influx are still not known”. To understand the role of cells in the root, his group are generating Arabidopsis plants whose gene expression is randomly activated in specific cell types. The plant shoots are then analysed for Na+ content (and that of other ions) to discover which genes in which cells are important for Na+ transport in the plant. Recently the Tester group has also employed this approach with rice, with a view to creating salt tolerant varieties of this important crop plant.
Although there have been some steps towards understanding and creating salt tolerant plants, the basis of this complex trait is still elusive. But remember, the next time you suffer sunburn or any other environmentally-inflicted ailment, unlike the plant, you could probably have avoided it! We should admire plants' coping strategies and have a lot to learn about how they survive the toxic experiences they can't escape.
University of Cambridge
4. R. Munns, 2002. Comparative Physiology of Salt and Water Stress. Plant, Cell and Environment, 25, 239-250
5. M. Tester and R. Davenport, 2003. Na+ Tolerance and Na+ Transport in Higher Plants. Annals of Botany, 91, 503-527
11. Fukada-Tanaka et al. 2000, Nature 407:581
R. Munns, 2002. Comparative Physiology of Salt and Water Stress. Plant, Cell and Environment, 25, 239-250.
M. Tester and R. Davenport, 2003. Na+ Tolerance and Na+ Transport in Higher Plants. Annals of Botany, 91, 503-527.
T.J. Flowers, 2004. Improving Crop Salt tolerance. Journal Exp Botany, 55, No396, p307-319.