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Stress-resistant or stress-tolerant plants
Stress-resistant or stress-tolerant plants exhibit the ability to adjust to environmental stresses through various biochemical and physiological processes. Discuss this remarkable plasticity of plants to adjust to their environment when plants experience the following stresses.
Salinity (high concentration of salt or water)
Salinity is abiotic stress that reduces and diminishes plant productivity and growth due to poor water quality during irrigation. It can also arise due to soil salinization. The kind of changes caused by salinity depends on the following factors; the time the stress has taken and how severe it is. The stress imposed physiological and metabolic changes in their normal processes. Stress tolerant plants overcome the stress by both biochemical and physiological mechanisms.
Physical and biochemical processes
Stress resistant plants have come up with several mechanisms on how to overcome salinity stress. Some of these mechanisms are; synthesis of polyamines, ion homeostasis and compartmentalization, hormone modulation, antioxidant enzyme/ antioxidant compounds synthesis, and nitric oxide development.
Ion homeostasis and salt tolerance
High salt concentration can not be tolerated by glycophytes as well as halophytes in the cytoplasm. Therefore, when it becomes excessive, the extra is taken to the vacuole and older tissues, which will later deplete, helping in plant protection from the stress. Normally NaCl is the primary salt found in the soil. When excess salt is noted in the cytoplasm, Na+ by use of Na+/H+ antiporter gets taken to the vacuole. There are two H+ pumps in the membrane of vacuoles; vacuolar pyrophosphatase(V-ATPase) and H+-ATPase. The most important H+ pump is V-ATPase, which plays an important role during both stressed and non-stress situations. During non-stressed times, it regulates solute homeostasis. Incorporating the three proteins(SOS1, SOS2, SOS3) found in the stress signaling pathway( Salt Overly Sensitive SOS) helps in encoding Na+/H+ antiporter, which later maintains Na+ efflux at the cell. SOS2 gets accelerated by Ca+ signal stress while SOS3 has a myristoylation site and helps in the Ca+ binding. With SOS1, when there is a Na+ concentration increase, it causes immediate intracellular Ca2+ increase, which triggers the SOS3 binding. NAF domain in SOS2 is used as a site for SOS3 binding. When Ca2+ connects the cytoplasm Na+ with SOS proteins, SOS3 leads to the formation of the SOS3-SOS2 complex, taken to the plasma membrane to phosphorylate SOS1 (Gupta and Huang). The SOS1 facilitates Na+ efflux leading to a decrease in Na+ toxicity.
Antioxidant Regulation of Salinity Tolerance
Electron transport chain(ETC) interference sometimes results due to salinity stress. The interference mostly takes place ETC processes taking place in mitochondria and chloroplast. When the distraction occurs, ROS increases in concentration due to electron acceptor, O2(Molecular oxygen). There are strong oxidizing compounds that can cause harm to cell integrity. These compounds include; hydroxyl radical (OH-), hydrogen peroxide H2O2, singlet oxygen 1O2, and superoxide radical (O2-). Salinity stress induces detoxification of ROS by the use of antioxidant enzymes(glutathione reductase(GR), catalase(CAT), ascorbate peroxidase(APX), and superoxide dismutase(SOD) as well as non-enzymatic compounds. Together with the non-enzymatic compound, these enzymes regulate salinity level by ensuring that photosynthesis, together with antioxidant machinery, are maintained and improved (Roy et al. 115–124). For example, when the rice root zone is treated with silicon, antioxidant responses get activated, reducing the salinity level.
Drought(lack of sufficient water)
Drought has become a great threat to plants by resulting in negative effects on the yield of crops/plants and their growth since it involves a long period without deficient water and precipitation. During drought, the decreased growth rate is normally caused by; enzyme suppression, water relation interference, reduced carbon dioxide assimilation, oxidative stress at the cell, and membrane distractions of some tissues. When water moves to the apoplast from the vacuole and cytosol, the given plant becomes dehydrated hence faces drought stress. Stress resistant plants can overcome this by use of the following mechanisms;
Physiological and biochemical factors
Plants can deal with the stress by; incorporating water conservation techniques through the closing of the stomata and reduction of the leaf surface is as well as canopy cover, by ensuring that they complete their life cycle before extreme water limitation arises, also by increasing the elasticity of their cell wall and osmotic adjustments.
During drought stress, electron excitation and utilization via photosynthesis are imbalanced due to carbon assimilation reduction. The lack of balance produced in the release of; hydrogen peroxide(H2O2), active oxygen species(ROS, and primarily superoxide(O2.-). Oxidative stress results through the distraction of proteins, cell membranes and nucleic acids caused by ROS. Also, Abscisic acid (ABA) is the most important stress hormone due to its crucial act in physiological and biochemical processes to ensure that the stress is controlled and aids in shoot and root growth during extreme drought conditions. When ROS concentration is increased as a result of NADP+ regeneration suppression, primarily superoxide and hydrogen peroxide. MDA is used to detect how far stress has affected the plant. Primarily superoxide gets converted to H2O2 by SOD. CAT and APX then convert the H2O2 to O2 and H2O. The three enzymatic mechanisms ensure that O2- and H2O2 are maintained at low intracellular parts concentrations (Abid et al. 21441–21447). An example of a very useful enzymatic antioxidant is glutathione(GSH), which aids in cellular defense. When the ratio of glutathione to oxidized glutathione is increased, ROS fails to damage the chloroplasts. Carotenoids protect photosynthesis by converting excess released energy to heat.
Plants can as well interfere with water relations to help in sustaining cellular functions. The osmotic adjustment can be obtained through synthesis and increase in compatible solutes, including proline, free amino acid and sugars. This adjustment ensures that plants retain turgor pressure with low amounts of water. It also ensures that the cell volume contains low water to sustain cellular activities. Plants such as peanuts have another way of responding to stress, ensuring that the rate of nodulation is decreased. Also, it ensures biological nitrogen fixation reduction (BNF). Rehydration is very crucial after the plant has recovered to help in metabolism process restoration.
Extreme temperatures (hot and cold)
Extreme temperature changes cause severe effects on plants by reducing plant growth, reducing metabolism, and even decreased plant productivity. Responses of plants to these changes differ with the plant type, the extent of damage to the plant and its time. Several techniques that incorporate antioxidants, proteins, osmoprotectants, ion transporters, and other signaling processes and transcriptional control are accelerated due to high temperature to aid in the physiological and biochemical distractions.
Physiological and biochemical responses
Heat stress
Heat stress leads to disruptions in plant physiological processes, plant growth, germination, plant development and even plant productivity. Its primary effect on plants is excess production of ROS, which leads to oxidative stress. Plants respond to this by releasing matching solutes, which aids in arranging the proteins and cellular structures, ensuring that osmotic adjustment helps maintain the cell turgor and ensure that the cellular redox balance is reestablished through modifying antioxidant systems. Plants prevent ROS damages through synthesizing enzymatic and non-enzymatic ROS scavenging and detoxification. Their ability to aid in resolving the stress condition increases with temperature. Enzymes like CAT, SOD, and APX were found to increase at the beginning before reducing to 500 degrees. At the same time, GR and peroxidase(POX) had their temperatures between 20-50 degrees throughout the str. Research proves that antioxidant activity occurs at temperatures between 35-40 degrees. Metabolites such as tocopherol and ASA also aid in reducing oxidative stress. Acclimation avoidance includes; cooling of transpiration, the lipid composition of the plant membrane disruptions, changing leaf organization. Plants in desert areas avoid the stress by ensuring there’s minimum absorption of solar radiation through the formation of tormentose (Hasanuzzaman et al. 9643–9684). Plants also engage in rolling their leaf blades, completing their cycle during cooler periods, leaf abscission, and intensive transpiration.
Cold stress
Cold stress causes low productivity in plants, disruption of crop plant spatial distribution and plant growth. Chilling affects plants at temperatures between 0-20 degrees, while freezing takes place at temperatures below 20 degrees. Plants have developed several physiological and biochemical mechanisms to help plants survive during extreme conditions. Short avoidance of this stress is obtained by primming a treatment. The plant itself responds at a molecular level. Increased ROS is seen as a result of cold stress that leads to cell structures’ distraction (Czarnocka and Karpiński 122:4–20). MDA is also damaged. Antifreeze substances together with antioxidants include; proline(PRO), peroxidase(POD), sugar(SS) and catalase(CAT). Response takes place at the transcription level. Transcription factors (TFs) that help positively activate cold-resistant genes are; C-repeat binding factor/dehydration/-responsive element-binding factors (CBF/DREB). The TFs are activated by ICEs inducers by binding to cis-elements at the promoter. Therefore the CBF COR pathway plays a great role in overcoming low-temperature stress. CBF3/DERB1A, CBF3/DREB1A, and CBF2/DERB1C are members of CBFs found in Arabidopsis. They help in ensuring that AP2/ERF DNA binding proteins are encoded. In grapes, cold induces CBF4.
Work Cited
Abid, Muhammad, et al. “Physiological and Biochemical Changes during Drought and Recovery Periods at Tillering and Jointing Stages in Wheat ( Triticum Aestivum L.).” Scientific Reports, vol. 8, no. 1, Nature Publishing Group, Mar. 2018, pp. 1–15.
Czarnocka, Weronika, and Stanisław Karpiński. “Friend or Foe? Reactive Oxygen Species Production, Scavenging and Signaling in Plant Response to Environmental Stresses.” Free Radical Biology & Medicine, vol. 122 July 2018, pp. 4–20.
Gupta, Bhaskar, and Bingru Huang. “Mechanism of Salinity Tolerance in Plants: Physiological, Biochemical, and Molecular Characterization.” International Journal of Genomics and Proteomics, vol. 2014, Hindawi, Apr. 2014, doi:10.1155/2014/701596.
Hasanuzzaman, Mirza, et al. “Physiological, Biochemical, and Molecular Mechanisms of Heat Stress Tolerance in Plants.” International Journal of Molecular Sciences, vol. 14, no. 5, May 2013, pp. 9643–84.
Roy, Stuart J., et al. “Salt Resistant Crop Plants.” Current Opinion in Biotechnology, vol. 26, 2014, pp. 115–24, doi:10.1016/j.copbio.2013.12.004.