Introduction Abiotic Stress Responses Importance

Stress is the negative effect that an organism may suffer, and can be classified as internal or external. Internal stress is that derived from mutations or abnormal cell divisions that can lead to metabolic changes. External stress can have a biotic or abiotic origin. Biotic stress may be caused by the attack of herbivores and pathogens. Biotic stress refers to the physical or chemical changes in the environment of the individual (Madlung and Comai 2004) . Among the most common abiotic stresses are those related to drought, excess salt in the soil, extremes of temperature, and the presence of toxins contaminating the environment (Bhatnagar-Mathur et al. 2008).

Plants are sessile beings, so the lack of mechanisms to escape from adverse conditions has fostered, through evolution, the development of unique and sophisticated responses to environmental stress. The chain of events that culminate in a response begins with the perception of a specific signal. This signal will generate a specific set of internal responses that lead from the changes in gene expression to changes in metabolism (Hazen et al. 2003; Shao et al. 2007; Agrawal et al. 2010). All these sets of changes represent nothing less than the effort of this sessile organism to overcome the stress situation, maintain homeostasis and adapat (Altman 2003; Hazen et al. 2003).

Survival in hostile environments involves developing mechanisms of tolerance, resistance, or avoidance. Plants that develop tolerance to a given factor can, over time, overcome the effects of this factor without injury. For instance, Anastatica hierochuntica and several species in the genus Selaginella are called "resurrection plants" because of their ability to withstand and recover from extended periods of internal water deficit. Another tolerance mechanism to avoid dehydration is the accumulation of osmolytes and changes in metabolism (Bouchabke et al. 2008).

To develop resistance means to submit to a given environment by means of counter measures. Acceleration of the plant life cycle to allow flowering before a drought period is a good example of this strategy. Many arid-land grain crops have been improved through breeding programs that allow the crop to avoid seasonal dry periods (Des Marais and Juenger 2010).

Avoidance prevents exposure to the stress (Madlung and Comai 2004). A good example of this strategy is what happens to plants subjected to osmotic stress (drought). Plants can adjust their absorption and water loss by regulating the physiological function of the roots and transpiration, respectively. Stomatal regulation is a strategy to avoid dehydration (Buckley et al. 2003). However, despite this conservative strategy, reductions of photosynthesis and growth can occur.

Plants are often unable to adjust to a certain condition and become sensitive to it (Wang et al. 2003). Depending on the degree of plasticity that a plant possesses to deal with a new environmental situation, in response to abiotic stress, morphological, anatomical, and physiological changes may occur. These changes can affect plant growth, productivity in agriculture, metabolic profile, and plant nutritional potential, for example (Altman 2003) . Therefore, plant abiotic stress has been a matter of concern for the maintenance of human life on earth and especially for the world economy.

The main aim of improvements in agricultural production is the eradication of hunger for the ever-increasing human population. It is worrying that 70% of the extremely poor who suffer from hunger live in rural areas (Sanchez and Swaminathan 2005) . It is important to improve the nutritional quality of food for 854 million people (about 14% of our population) worldwide who are chronically or acutely malnourished (FAO 2004) . It is urgent to increase agricultural productivity with the most nutritious plants; however, the predominant concern is the maintenance of a healthy environment and conservation of local biodiversity, both of which are becoming progressively degraded and subject to accelerated global climate change (Hu et al. 2010).

Specifically in agriculture and the eco-environment, abiotic stresses such as extreme temperatures, salinity, and drought have decreased productivity as much as 50% (Boyer 1982; Bray 1997) . Osmotic stress may reduce crop yields to less than half of their potential (Boyer 1982). Forecasts for the year 2050 indicate that up to 50% of farmland may become saline (Wang et al. 2003). Salinization is a problem today, and presently affects 22% of arable areas (FAO 2004). All these data are tightly linked to plant biology, because plants offer the globe its only renewable resource, not only of food, but also of building material and energy. Knowledge of plant biology is also a powerful tool to use natural resources reasonably (Agrawal et al. 2003; Beer and Tavazoie 2004).

In view of this general situation, a major question arises: how to overcome all these adverse factors? One simple answer is to study the responses to different stresses. A key challenge for plant breeding is to investigate these responses at the genetic level, identifying genes responsible for important crop traits. Studying plant responses to different abiotic stresses can reveal how some plant species overcome these adverse conditions by developing resistance or tolerance, the nature of environmental changes can be explored, and finally, tolerant and/or resistant plants can be developed (Meyerowitz 2002; Gesch et al. 2003).

Many studies in this direction have been implemented in attempts to identify stress-regulated genes. Some have shown that plants that are exposed to different stresses, have genes that are regulated in singular ways, but that nevertheless induce similar defense responses (Ozturk et al. 2002; Altman 2003). Probably this is because drought, salinity, extreme temperatures, and oxi-dative stress are interconnected, and may induce similar effects on plants. Salinity and drought, for example, cause similar responses in plant cells: membrane and protein damage and disruption in the distribution of ions (Vinocur and Altman 2005; Racz et al. 2008; Hu et al. 2010). Stress inducers from abiotic as well as biotic factors also have some common signal and response pathways in plants (Hodge 2004; Bray 2004; Chinnusamy et al. 2004; Hinsinger et al. 2005; Hongbo et al. 2005; Liu and Li 2005; Munns 2005; Leakey et al. 2006; Humphreys et al. 2006) and thereby have the potential to moderate each other's effects through cross-talking (Shigeoka et al. 2002; Shinozaki et al. 2003; Soltis and Soltis 2003; Hongbo et al. 2005). Investigation of those responses that follow a similar pattern can be useful in developing sustainable agriculture by reducing the need for chemicals (e.g., fertilizers, herbicides, insecticides, fungicides) and preserving/ optimizing natural resources (e.g., water, reclaiming wasteland for intensive agriculture) (Wang et al. 2003; Agrawal et al. 2010).

Responses to abiotic stress at the gene level fall into one of three types: (a) genes coding proteins that play an important role in signaling cascades and in transcriptional control (Zhu 2001), (b) genes whose products immediately confer protection on membranes and proteins (Bray 1997) ; and (c) those that are involved in water and ion uptake and transport, such as aquaporins and ion transporters (Blumwald 2000). Examples of the first option are MyC, MAP kinases and SOS kinase (Zhu 2001), phospholipases (Frank et al. 2000), and many transcription factors (TFs) that regulate transcription by binding, and belong to several gene families including AP2/EREBPs (APETALA2 and ethylene-responsive element-binding proteins), HSF, CBF/DREB (dehydration-responsive element/C-repeat-binding), ABF/ ABAE families, bZIP (basic-domain leucine zipper), NAC, MYB/MYC, Cys2/His2 zinc-finger, and WRKY (Umezawa et al. 2006; Hongbo et al. 2005) . Transcriptional elements can activate or suppress the transcriptional effect of corresponding genes (Beer and Tavazoie 2004; Bray 2004; Liu and Baird 2004) .

Genes that code for products that directly confer protection on membranes and proteins and therefore the function of plant cells to resist environmental stress, are those that synthesize proteins related to the support of the integrity of cellular structures, or destruction of structures damaged by osmotic stress. Late embryogenesis abundant proteins (LEA67), heat shock proteins (HSP68) (Bray 1997), antifreezing proteins, osmotic regulatory proteins, free-radical scavengers (Wang et al. 2003), and various proteinase inhibitors are examples of the latter type of proteins (Des Marais and Juenger 2010; . LEA and chaperones often have conservative sequences and polar amino acids, so they are stable and can cooperate in stabilizing the structures of proteins and cell membranes (Fu et al. 2007; Jyothsnakumari et al. 2009).

From investigations on genetic identification and/or molecular responses of stress-related plant responses, modern molecular techniques were developed to breed better crops. The principal objective in plant breeding is to obtain plants that combine higher yields, reliable yield stability, better quality, and obvious stress-resistant characters (abiotic and biotic) over different years and locations (Bray 2004; Chaves and Oliveira 2004). The identification and use of molecular markers and introgression of genomic portions (QTLs) involved in stress tolerance is one good alternative, although undesirable agronomic characteristics from the donor plants may be introduced into the target plant (Roessner and Pettolino 2007).

Techniques that are more accurate than conventional or molecular breeding, such as genetic engineering, allow the selection of genes and their overexpression and/or introduction into the genome. By these methods, new cultivars can be produced more rapidly and efficiently with less chance of failure. Genetic engineering techniques can also overcome barriers to sexual crossing, so that genes of interest arising from taxonomically distant organisms can be selected and introduced (Bhatnagar-Mathur et al. 2008).

According to data collected on the productivity of rice, wheat, and corn during the last three decades, the observed increase is related to breeding and selection of high-yielding genotypes (Wang et al. 2003). However, this improvement in productivity is not followed by an increase in the potential yield of crops. That is, even in optimum environmental conditions, without infection by pathogens and without limitation of resources, both old and new cultivars give the same yield. Therefore, better understanding of the responses of cultivars to abiotic stress, in plant breeding, can implement plant improvement at a very practical level (Wang et al. 2003) .

Some molecular responses to abiotic stress, or levels of stress, have been well established and can be used to optimize the production of more resistant individuals. Many of these studies were carried out with Arabidopsis. However, apart from specific stress responses at the gene or metabolic level, there is a common signal transduction pathway model for stress, which is shared by many higher plants. This model proceeds through the perception of the environmental signal, and subsequently the production of a secondary messenger (such as inositol phosphates and reactive oxygen species - ROS), which will regulate the endogenous levels of Ca2+. From this point, a chain of events occurs that affects protein phos-phorylation, reaching proteins linked to the protection of cellular structures or transcription factors controlling specific sets of stress-regulated genes. These genes are related to the production of regulatory molecules such as the plant hormones abscisic acid (ABA), ethylene, and salicylic acid (SA). Some of these regulatory molecules can, in turn, initiate a second round of circulation (Shao et al. 2007). From the analysis of all the data that can be generated from this model of response, from the standpoint of molecular biology as well as physiological, metabolic, and environmental stress, several questions arise. How to integrate all this available information? How to analyze the data completely? How to establish a relationship among different data sets at different levels and obtain accuracy? These questions can be answered by using techniques of the postgenomic era such as proteomics and metabolomics, which will be discussed in the next section.

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  • viviano
    Why is Abiotic Stress Resistance important?
    4 months ago

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