Molecular Biology of Biotic and Abiotic Stresses Lecture Notes PDF

By Hala Eissa  ‫رؤية كلية التكنولوجيا الحيوية‬ ‫أن تكون كلية معتمدة اكاديميا و رائدة في‬ ‫مجاﻻت التكنولوجيا الحيوية علي المستوي‬ ‫المحلي واﻷقليمي والدولى ‪.‬‬  ‫رسالة كلية التكنولوجيا الحيوية‬ ‫تلتزم كلية التكنولوجيا الحيوية – جامعة مصر للعلوم والتكنولوجيا‬ ‫بتخريج اخصائ...

By Hala Eissa  ‫رؤية كلية التكنولوجيا الحيوية‬ ‫أن تكون كلية معتمدة اكاديميا و رائدة في‬ ‫مجاﻻت التكنولوجيا الحيوية علي المستوي‬ ‫المحلي واﻷقليمي والدولى ‪.‬‬  ‫رسالة كلية التكنولوجيا الحيوية‬ ‫تلتزم كلية التكنولوجيا الحيوية – جامعة مصر للعلوم والتكنولوجيا‬ ‫بتخريج اخصائي تكنولوجيا حيوية طبقا للمعايير اﻻكاديمية المعتمدة‬ ‫يلبي احتياجات سوق العمل المحلي واﻻقليمي في القطاعات الطبيه‬ ‫والصيدﻻنية والزاعية والبيئية واجراء بحوث علمية مبتكرة وتقديم‬ ‫خدمات مجتمعية واستشارات علمية في اطار قيم ارتقائية ‪.‬‬  Intended Learning Outcomes a.1. Define Different types of plant stresses a.4.List the xerophytes strategies a.5.Interpret the plant response to drought stress at the molecular level DROUGHT  Water is arguably the single most important constituent of a plant, comprising more than 90% of the fresh weight of most herbaceous plants. Water is also a solvent with unique biophysical properties – such as high heat of vaporization high dielectric constant high surface tension.  These unique properties allow water to remain liquid over a broad temperature range and solvate a wide variety of ions, minerals and molecules. In addition to being the solvent, water acts as a reactant in a number of critical biochemical reactions including serving as the primary electron donor in photosynthesis. Finally, from a physiological perspective, water is the key component in maintaining cell turgor.  Drought or water deficit is the most severe limiting factor of plant growth and crop production Drought leading to water stress in plants is a major problem in reducing agricultural productivity especially in tropical, semi- arid and arid regions of the world  Important definitions Drought: The limitation of water over a prolonged period of time. Refers to meterological events and should be reserved for field-grown plants. In plants denotes the loss of water from tissues and cells. Dehydration: The loss of water from a cell. Plant cells dehydrate during drought or water deficit. Desiccation: The extreme form of water loss from a cell. Denotes the process whereby all free water is lost from the protoplasm  In general, drying of soil is slow, but decrease atmospheric humidity can sometimes be quick. Accordingly, plants need suitable systems both in their roots and leaves that sense environmental dryness Plant leaves close their stomata immediately on sensing an increase in leaf-air vapor pressure difference, even if the roots have sufficient water. This response is completed in several minutes  Xerophyte Strategies Drought escape Avoiders: complete life cycle during wet season Drought tolerance water savers, water stores  Xerophyte Strategies Drought escape Annual plants escape unfavorable conditions by not existing They mature in a single season, then die after channeling all of their life energy into producing seeds instead of reserving some for continued survival  Xerophyte Strategies Drought tolerance Drought tolerance (or drought dormancy) refers to a plant's ability to withstand desiccation without dying  Xerophyte Strategies Drought tolerance Plants in this category often shed leaves during dry periods and enter a deep dormancy Most water loss is from transpiration through leaf surfaces, so dropping leaves conserves water in the stems Some plants that do not normally shed their leaves have resinous coatings that retard water loss (e.g., creosote bush) Creosote bush  Xerophyte Strategies Drought tolerance The roots of drought tolerant shrubs and trees are extensive compared to those of plants in wetter climates, covering an area up to twice the diameter of the canopy. They exploit the soil at greater depth than the roots of succulents; sometimes they extend to extreme depths (e.g., mesquite). Most of a mesquite's roots, however, are within three feet (0.9 m) of the surface.  Xerophytes adaptations 1. Thick cuticle 2. Stomata hidden in crypts or depressions in leaf surface (less exposure to wind and sun) 3. Reduction in size of transpiration surface (lower leaf only) 4. Increased water storage 5. Thicker leaves and stems, or leaves reduced in, or leaves drop off during dry season 6. Leaves covered with silvery hairs (creates wind break & light reflective surface) 7. Deep taproots or wide spreading fibrous roots near the soil surface  Xerophytes adaptations Adaptation How it works Example thick cuticle stops uncontrolled evaporation through leaf cells small leaf surface area less surface area for evaporation conifer needles, cactus spines low stomata density smaller surface area for diffusion sunken stomata maintains humid air around stomata marram grass, cacti stomatal hairs (trichores) maintains humid air around stomata marram grass, couch grass rolled leaves maintains humid air around stomata marram grass, extensive roots maximise water uptake cacti  DROUGHT: cellular level The cellular water deficits results in: The concentration of solutes increased Loss of turgor Change in cell volume Disruption of water potential gradients Change in membrane integrity Denaturation of proteins and several physiological and molecular components  A Variety of Functions of Drought-Inducible Genes Various genes respond to drought stress in various species, and functions of their gene products have been predicted from sequence homology with known proteins Many drought-inducible genes are also induced by salt stress and low temperature, which suggests the existence of similar mechanisms of stress responses  A Variety of Functions of Drought-Inducible Genes Two Classes of Drought-Inducible Genes Genes induced during drought-stress conditions are thought to function not only in protecting cells from water deficit by the production of important metabolic proteins but also in the regulation of genes for signal transduction in the drought stress response  The first group includes proteins that probably function in stress tolerance, such as: Chaperones LEA (late embryogenesis abundant) proteins osmotin, antifreeze proteins mRNA binding proteins key enzymes for osmolyte biosynthesis water channel proteins sugar and proline transporters detoxification enzymes and various proteases  LEA proteins, chaperones and mRNA binding proteins have been analyzed biochemically and shown to be involved in protecting macromolecules like enzymes, lipids and mRNAs from dehydration Proline, glycine betaine and sugars function as osmolytes and in protecting cells from dehydration  Water channel proteins, sugar transporters and proline transporters are thought to function in transport of water, sugars and proline through plasma membranes and tonoplast to adjust osmotic pressure under stress conditions  Detoxification enzymes such as glutathione S-transferase, superoxide dismutase, and soluble epoxide hydrolase are involved in protection of cells from active oxygens. Proteases including thiol proteases, Clp protease, and ubiquitin are thought to be required for protein turnover and recycle of amino acids. The second group contains protein factors involved in further regulation of signal transduction and gene expression that probably function in stress response, such as Protein kinases Transcription factors Enzymes in phospholipid metabolism  Protein kinases, such as: MAP kinases, Calcium dependent protein kinases (CDPK) SNF1 related protein kinase Ribosomal S6 kinase These protein kinases and phosphatases may be involved in modification of functional proteins and regulatory proteins involved in stress signal transduction pathways Transcription factors function in further regulation of various functional genes under stress conditions Phospholipid, such as inositol-1,4,5-triphosphate, diacylglycerol and phospahtidic acid are believed to be involved in stress signaling processes in plants It is hypothesized that at least four independent signal transduction pathways function in the activation of stress-inducible genes under dehydration conditions: Two are ABA-dependent (Pathways I and II) Two are ABA-independent (Pathways III and IV).  Pathway I : requires protein biosynthesis Pathway II : does not require de novo protein biosynthesis. Cis- and trans-acting factors involved in ABA-induced gene expression  One of the ABA-independent pathways overlaps with that of the cold response (Pathway IV). There are several drought-inducible genes that do not respond to either cold or ABA treatment, which suggests that there is a fourth pathway in the dehydration stress response (Pathway III).  Recently, based on genetic analysis of Arabidopsis mutants with the rd29A promoter—luciferase transgene, the existence of drought-, salt- and cold- specific signaling pathways in stress- response was suggested, but crosstalks between these signaling pathways were also observed  Pathway I Roles of MYC and MYB Homologs in ABA-Dependent Gene Expression that Requires Protein Biosynthesis Biosynthesis of novel protein factors is necessary for the expression of ABA-inducible genes in one of the two ABA-dependent pathways A 67 bp region of the rd22 promoter is essential for this ABA-responsive expression, and contains several conserved motifs of DNA binding proteins, two MYC and one MYB recognition sequences, but this region has no ABREs  First MYC and MYB recognition sequences are essential for the ABA- and drought responsive expression of the rd‫ذ‬22 gene. The ATMBY2 gene that encodes a MYB-related protein is induced by dehydration and ABA treatment. Recombinant ATMYB2 protein binds to the MYB recognition sequence in the 67bp region of the rd22 promoter. These MYC and MYB proteins transactivate the rd22 promoter GUS fusion gene in transient expression system using leaf protoplasts.  Major ABA-Independent Regulatory System of Gene Expression during Drought and Cold Stress (Pathway IV): A number of genes are induced by drought, salt, and cold in aba (ABA-deficient) or abi (ABA-insensitive) Arabidopsis mutants. This suggests that these genes do not require ABA for their expression under cold or drought condition.  Major ABA-Independent Regulatory System of Gene Expression during Drought and Cold Stress (Pathway IV): Among these genes, the expression of a drought- inducible gene for rd29A/lti78/cor78. A 9 bp conserved sequence, TACCGACAT, named the dehydration responsive element (DRE), is essential for the regulation of the induction of rd29A under drought, low-temperature, and high-salt stress conditions, but does not function as an ABA- responsive element Major ABA-Independent Regulatory System of Gene Expression during Drought and Cold Stress (Pathway IV): The rd29A promoter contains ABRE, which functions in ABA-responsive expression. DRE-related motifs have been reported in the promoter regions of many cold- and drought-inducible genes. DRE-related motifs including C- repeat (CRT) and low temperature responsive element (LTRE), which contain a CCGAC core motif, are involved in drought- and cold- responsive but ABA-independent gene expression  Drought-Specific ABA-Independent Regulatory System (Pathway III) There are several drought-inducible genes that do not respond to either cold or ABA treatment, which suggests the existence of another ABA-independent pathway in the dehydration stress response These genes include: rd19 and rd21 that encode different thiol proteases, and erd1 that encodes a Clp protease regulatory subunit. The ERD1 protein is targeted to chloroplasts. The catalytic subunit of the Clp protease (Clp P) is encoded on the chloroplast genome. The RD19 and RD21 proteins seem to function in cytoplasm.  Major ABA-Dependent Regulatory System (Pathway II) Important Roles of ABRE Cis-Acting Element and its bZIP DNA Binding Proteins Most drought-inducible genes are upregulated by exogenous ABA treatment. The levels of endogenous ABA increase significantly in many plants under drought and high salinity conditions. In one of the ABA-dependent pathways (Pathway II), drought-stress inducible genes do not require protein biosynthesis for their expression. These dehydration-inducible genes contain potential ABA-responsive elements (ABREs; PyACGTGGC) in their promoter regions. ABRE functions as a cis-acting DNA element involved in ABA-regulated gene expression. ABRE was first identified in wheat Em and rice rab genes, and its DNA-binding protein EmBP1 was shown to encode a bZIP protein. Transcriptional regulatory networks of abiotic stress signals and gene expression