The Biochemical Basis of Plant Adaptations to Drought and Salinity

In a stochastic environment, plants' sessile nature means that variations in water availability have detrimental effects on the plants metabolism. The availability of water for its biological roles as a solvent and transport medium, as an electron donor in the Hill reaction, and as an evaporative coolant is often impaired by environmental conditions such as drought and salinity. The oxidative stress that results from these environmental perturbations has profound biochemical responses within the plant's genetic architecture. As both these stresses impact on the water availability of the organism, they will share many response mechanisms despite being fundamentally different stimuli.

Both drought and salinity stress the cells by increasing the concentrations of ions in the cytosol. Increased ion concentrations can have osmotic effects causing the plant to lose control over water flux; furthermore, high concentrations of ions have extremely negative impacts on the tertiary structure of proteins, which form the basis of all cellular machinery.

Therefore, plants have developed responses to protect against these negative impacts and they fall into three main categories:

1. Responses that are involved in signaling cascades and in transcriptional control

2. Responses that function directly to protect the membranes and proteins

3. Responses that involved with ion uptake and transport

Firstly, signaling cascades and transcriptional controls are the foundation by which a plant is able to respond to any stimulus. They enable a plant to interact with other cells and with other plants by hormones such as ABA and as a consequence, they can tune their metabolism to their immediate need. Cells respond to ABA by generating signaling cascades and transcriptional modifications in terms of both switching genes on and changing the relative transcription rate of genes. Furthermore, some stress-responsive genes to salinity and drought will share many of the same transcription factors, as indicated by the significant overlap of the gene-expression profiles that are induced in response to these stresses.

ABA has broad functions in plant growth and development and in response to restoring water balance within cells. Abscisic acid is produced in the roots and shoots in response to decreased soil and vacuolar water potential and other situations in which the plant may be under stress. ABA then translocates to all areas of the plant resulting in signaling cascades and genetic manipulation. Since ABA mediates so many stress responses, the initial perception of dehydration and the subsequent changes in gene expression that lead to rapid ABA biosynthesis, constitute the most important stress signal transduction pathway among all the plant responses to stresses. Although detection of theses stimuli may not only be attributed to ABA (for instance, osmotic pressure may cause water stress signals to propagate), ABA provides a necessary hormonally derived mechanism, providing communication between the different parts of the cells, which forms a good example of the in the manipulation of the genetic architecture via signaling cascades.

The calcium sensor calcineurin B-like 9 (CBL9) modulates abscisic acid sensitivity and biosynthesis in Arabidopsis. The CBL9 protein appears to function as a negative regulator of ABA signalling that leads to inhibition of seed germination and ABA biosynthesis.

The S1P in this illustration acts a secondary messenger for guard cell ABA responses, transiently causing their closing when ABA is present which consequently increases the water holding potential of the leaves thus decreasing the concentration of ions which may have detrimental affects on the proteins and cellular machinery. These cascades are not fully understood but provide the basis of all cellular consequences. Early results of ABA signalling showed that an elevation of cytosolic Ca2+ is an important step and serves as a second messenger in changing the status of stomatal guard cells in their ability to prevent transpiration and a loss of water from the leaves in times of stress derived from both salinity and drought, which may or may not act in tandem with S1P. SLN1 senses the osmotic stress and passes the signal to MAPK cascades: a sensing mechanism independent from ABA, which is used to detect such stresses. However, some apparent ABA-independent pathways may require ABA for full response as a result of cross talk between ABA and stress response pathways.

The consequences of these signal transductions are what ultimately affect the histological processes. The pivotal role of ABA in plant stress responses is evidenced by the fact that many of the drought-inducible genes studied to date are also induced by ABA. Two TF families: bZIP and MYB are involved in ABA signalling and gene activation. Once again, the fundamentals of the stimulus, cascade and genetic response have not been well developed and how they are connected is poorly grasped. However, one of the main consequences resulting from these transcription factors is the up-regulation of proteins. I have classed these two types of protective proteins into scavenging (or antioxidant) proteins and chaperone proteins: those that remove the reactive oxygen species (ROS), and those that protect the macromolecules directly, repectively.

Drought and salinity are usually accompanied by the formation of ROS such as O2, H2O2, and OH-). These damage the membranes and the macromolecules. Upon stress stimulus by osmotic receptors or ABA, the antioxidant proteins are up regulated. There are many of these proteins, for example, catalases, peroxidises, reductases and dismutases. These help to metabolise the reactive oxygen species and help prevent their interactions with proteins whereby they may remove electrons and alter protein tertiary structure, decreasing their effectiveness. These molecules therefore help reduce the stress associated with drought and salinity.

The other class of protective proteins are the chaperone proteins. These include the heat-shock proteins and late embryogenesis abundant (LEA)-type proteins: two major types of stress-induced chaperone proteins that accumulate upon drought and salinity. They have been shown to act as molecular chaperones, which are responsible for protein synthesis, targeting, maturation and degradation in a broad array of normal cellular processes. Furthermore, molecular chaperones function in the stabilization of proteins and membranes, and in assisting protein refolding under these stress conditions. Furthermore, as they are proteins they can help stabilise other proteins without influencing the concentrations of ions and therefore don't alter the osmotic potential.

Hydrophilicity is a common characteristic of LEA- type and other osmotic stress-responsive proteins. LEA proteins have been grouped together with other osmotic stress-induced proteins from Saccharomyces cerevisiae and Escherichia coli into a class of proteins termed hydrophilins, based on criteria of high hydrophilicity index (>1.0) and glycine content (>6%). The functions of LEA-type proteins are largely un-known, nevertheless, their considerable synthesis during the late stage of embryogenesis, their induction by stress and their structural characteristics (hydrophilicity, random coils and repeating motifs) permits the prediction of some of their functions. It has been suggested that LEA-type proteins act as water-binding molecules, in ion sequestration and in macromolecule and membrane stabilization

The third set of genes activated by these signal transduction pathways are the membrane proteins involved with the ion uptake and transporter. Theses proteins are at high levels during non-stressful conditions (unlike that of the heat-shock proteins) as they are necessary for the everyday maintenance of homeostasis. For instance, the Na+/H+ anti-porters catalyze the exchange of Na+ for H+ across membranes and function to regulate cytoplasmic pH, sodium levels, nutrient-uptake ability and cell turgor. For instance, in Arabidopsis the plasma membrane Na+/H+ anti-porter, encoded by the SOS1 gene, was suggested to be essential for salt tolerance, and recently reported that overexpression of SOS1 improves salt tolerance in transgenic Arabidopsis. The sos1 (or salt overly sensitive mutant 1) for instance has very low tolerance for salt as it fails to export it from the cells.

In conclusion, far less knowledge is known about the responses to osmotic stress as are for other hormone-derived biochemical mechanisms. This may be because of the complex interactions of drought and salinity in an environment that to some extent always exerts these stresses, making it difficult to define the response away from the organism's 'norm'. However, progress has extended far in understanding how genes protect against the consequences of stress. This has been demonstrated by transgenic manipulation and incredible tolerances to stress have been demonstrated in these transgenic organisms. Furthermore, over production of some of the transporters have demonstrated the necessity of these proteins in protection under severe stress. The next step in this field will be to have a general understanding of the holistic way that plants respond to drought and salinity stress, and this unified theory will eventually be able to be improved and transgenically added to crop plants to maintain high yields in the uncertain times ahead.

Article Source: Mark A T Ramotowski

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