How Crop Plants Sense the Stress: Signaling and Pathway
Abiotic Stress and its Recognition by Plant
As plants are sessile, it is tough to measure the exact force exerted by stresses and therefore in biological terms it is difficult to define stress. A biological condition, which may be stress for one plant may be optimum for another plant. The most practical definition of a biological stress is an adverse force or a condition, which inhibits the normal functioning and well being of a biological system such as plants (Jones and Jones, 1989).
A cell is separated from its surrounding environment by a physical barrier i.e. the plasma membrane. This membrane is permeable to specific lipid molecules such as steroid hormones, which can diffuse through the membrane into the cytoplasm and is impermeable to the water-soluble material including ions, proteins and other macromolecules. Primarily, the cellular responses are initiated by interaction of the extracellular material with a plasma membrane protein. This extracellular molecule is known as ligand or an elicitor and the plasma membrane protein, which binds and interacts with this molecule, is called a receptor. Various stress signals both abiotic as well as biotic serve as elicitors for the plant cell.
The stress is first perceived by the receptors present on the cell membrane of the plant (Figure 1), the signal is then transduced downstream and lead to the generation of second messengers like calcium, reactive oxygen species (ROS) and inositol phosphates.
These messengers promote the intracellular calcium level. This perturbation in cytosolic Ca2+ level is sensed by Ca2+ sensors. These sensors apparently lack any enzymatic activity and change their conformation in a calcium dependent maimer. These sensory proteins interact with their respective interacting partners initiating a phosphorylation cascade and target the major stress responsive genes or the transcription factors regulating these genes. Eventually the products of these stress genes lead to plant adaptation and help the plant to survive even under stressed conditions. Thus, plant responds to stresses as individual cells and synergistically as a whole organism. Stress induced changes in gene expression in turn may participate in the generation of hormones like ABA, salicylic acid and ethylene. These molecules may amplify the initial signal and initiate a second level of signaling that may follow the same or different components of signaling pathway. Certain molecules also known as accessory molecules may not directly participate in signaling but participate in the modification of signaling components. These proteins include the protein modifiers, which may be added cotranslationally to the signaling proteins like enzymes for myristoylation, glycosylation, methylation and ubiquitination.
Fig. 1. Plant responses to abiotic stresses
Plant Response to Drought and Salinity Stress
Drought Stress and Agriculture
Drought is one of the most significant environmental stress affecting global agricultural production and massive efforts are being made by plant scientists to improve crop yields under limiting water availability (Cattivelli et al, 2008). During the twentieth century, the world’s population will be tripled from approximately 1.65 to 5.98 billion and population projections of 8.91 and 9.75 billion are expected to occur by 2050 and 2150, respectively. Developing countries like Africa and Asia account for approximately 80% of this growth and, with an estimated 800 million people in these countries already undernourished. The FAO predicts that a 60% increased world food production is required in the next two decades to sustain these populations.
Currently, agriculture accounts for approximately >70% of global water use and irrigation for up to 90% of total water withdrawals in arid nations (FAO, 2009a). Approximately, 40% of all crops produced in developing countries are grown on irrigated arable land, which accounts for only 20% of the total arable land in these nations (FAO, 2009c). Tire water withdrawal requirement for irrigation is expected to increase by 14% in developing countries by 2030 and strategies to reduce this demand by developing crops that require less irrigation will, therefore, play a vital role in maintaining world food supply. While within a few decades, the expanding world population will require more water for domestic, municipal, industrial and environmental needs (Hanrdy et al., 2003). This trend is expected to emphasize due to global climatic change and increased aridity (Vorosmarty et al., 2000). Thus, to meet the projected food demands, more crops per drop are required (Condon et al, 2004).
Nature of Drought and Plant Response
A plant requires water to complete its life cycle consisting of at least 70% water on a fresh weight basis. When water in the plant environment becomes deficient, plant transpiration cannot frilly meet the atmospheric demand and plant water deficit evolves. Water deficit is a strain on the plant that causes damage and drives a network of gene responses. These are proportional to the rate of deficit. Exacerbate action of abiotic str ess conditions can led to gr eat losses in productivity due to crop stress. When subjected to water deficit plants go through a cascade of metabolic alterations started with reduction in photosynthetic pigments concentration. Physiological mechanisms of plant response to water stress are summarized in Figure 2. Deficient water level leads to removal of water from the membrane disrupts normal bilayer structure and results in the membrane becoming
Fig. 2: Physiological mechanisms induced by water stress
exceptionally porous when desiccated. It induces imbalances in osmotic and ionic homeostasis, loss of cell turgidity, damage to structural and functional cellular proteins and membranes as well. Consequently, water-stressed plants wilt, lose photosynthetic capacity, and are unable to sequester assimilates into the targeted plant organs. Severe drought conditions result in significant yield loss and plant death. Revelation of plant drought tolerance and response mechanisms has been compounded at variable levels and forms of drought. Drought can be spatially and temporally variable; terminal, short-term, or sporadic; severe, moderate, or minor; and can occur at rates ranging from very sudden to gradual. The effects of drought and water deficit on crop productivity vary for different crops, macro and microenvironments across a single field, plant life stages, and the plant material to be harvested. Additionally, the effects of drought on crop productivity are often compounded by associated stresses such as salt, heat and other stress. The components of drought and salt stress cross talk with each other as both the stresses ultimately result hr dehydration of the cell and osmotic imbalance. Drought and salt signaling encompasses three important parameters (Liu and Zhu, 1998) viz., 1) Reinstating osmotic as well as ionic equilibrium of the cell to maintain cellular homeostasis under str ess; 2) control as well as repair of stress damage by detoxification signaling and 3) signaling to coordinate cell division to meet the requirements of the plant proteins under stress.
This stress also leads to activation of enzymes involved in the production and removal of ROS (Zhu, 2002; Cushman and Bohnert, 2000). Reduced cyclirr- dependent kinase activity results hr slower cell division as well as inhibition of growth under water deficit condition (Schuppler et al., 1998). The physiological effects of drought on plants are the reduction in vegetative growth, in particular shoot growth. Leaf gr owth is generally more sensitive than the root growth. Reduced leaf expansion is beneficial to plants under water deficit condition, as less leaf area is exposed resulting in reduced transpiration. In accordance, many mature plants respond to drought stress by a process is known as leaf area adjustment by accelerating senescence and abscission of older leaves. Regarding root, the relative root gr owth may undergo enhancement, which facilitates the capacity of the root system to extract more water from deeper soil layers so as to cope with water stress.