Molecular Biology of Plant Stress

1. Molecular Biology of Plant Stress Dr. B.D.RanjithaKumari Professor and Head Department of Botany

2. Molecular Biology of Plant Stress Biotic Abiotic drought, cold and salt etc. plant pathogen attacks • To adapt to these stresses, plants use diverse and sophisticated strategies for recognizing and responding to these stresses. • Sensing of environmental stresses may occur at the point of initial stress perception itself. • Plants might perceive the stresses in different ways, such as by plasma membrane located receptors, intracellular or cytoskeleton-associated proteins. • Stress perception is transmitted by signal cascades into altered gene expression programmes ultimately resulting in metabolic adjustment and altered physiological responses. Plant stresses

3. Plant Adaptations towards Stress Response of plants to lethal and sublethal level of stresses. The plant in an unfavourable environment could face the following two situations: (i) lethal stress where the plant may ultimately die due to increased senescent activities and (ii) sublethal stress or lethal stress preceded by sublethal stress where certain adaptive changes may occur, leading to survival of the plant. These adaptations could be at the molecular level involving changes in gene expression, synthesis of stress proteins, etc. and at the biochemical level. The latter changes ultimately may bring about the physiological response and finally the whole plant response.

4. Regulation of Stress Gene Expression • Plants are capable of adapting to a variety of stresses by inducing specific sets of genes that play key roles in the adaptation process of plants against diverse stimuli including biotic and abiotic stresses. • Transcriptome profiling and Phosphoproteomics can be used to uncover the signalling machinery which would help in improving stress tolerance of plants.

5. ABIOTIC STRESS • Abiotic stress is one of the primary causes of crop losses worldwide. • Much progress has been made in unraveling the complex stress response mechanisms, particularly in the identification of stress responsive protein-coding genes. • In addition to protein coding genes, recently discovered microRNAs (miRNAs) and endogenous small interfering RNAs (siRNAs) have emerged as important players in plant stress responses. • Studies have demonstrated an important functional role for these small RNAs in abiotic stress responses.

6. DROUGHT & DEHYDRATION • Drought stress induces a range of physiological and biochemical responses in plants which include stomatal closure, repression of cell growth and photosynthesis, and activation of respiration. • Plants also respond and adapt to water deficit at both the cellular and molecular levels, by the accumulation of osmolytes and proteins specifically involved in stress tolerance. • An assortment of genes with diverse functions are induced or repressed by these stresses. • Drought triggers the production of the phytohormone abscisic acid (ABA), which in turn causes stomatal closure and induces expression of stress-related genes. • Several drought-inducible genes are induced by exogenous ABA treatment, whereas others are not affected. ABA-independent and ABA-dependent regulatory systems govern the drought-inducible gene expression. • Both cis-acting and trans-acting regulatory elements functioning in ABA-independent and/or ABA-responsive gene expression induced by drought stress have been analysed at the molecular level.

7. Plant responses to Drought Stress Functions of drought stress-inducible genes in stress tolerance and response. Gene products are classified into two groups. The first group includes proteins that probably function in stress tolerance (functional proteins), and the second group contains protein factors involved in further regulation of signal transduction and gene expression that probably function in stress response (regulatory proteins).

8. ‘Water Stress Proteins’(WSPs) have been implicated in several important metabolic functions eg., water channel proteins have a role in movement of water through membranes, whereas certain enzymes such as pyrroline 5-carboxylate synthase and choline oxidase are required for the biosynthesis of various osmoprotectants. • The ‘Late Embryogenesis Abundant’ or LEA proteins, osmotin may protect macromolecules and membranes while chaperones and proteases are implicated in protein turnover and protein translocation. • The detoxification enzymes such as glutathione S-transferase, catalases, superoxide dismutase and ascorbate peroxidases are involved in protection from reactive singlet oxygen species and finally proteins involved in regulatory functions and in signal transduction, including various protein kinases and transcriptional factors have a broader role in governing stress responses.

9. SALINITY STRESS • The response of plants to salt stress is based on the transcriptional action of many defense proteins • Salinity creates the specific problem of ion toxicity, because a high concentration of sodium is bad for the cells. • High salt concentrations inhibit enzymes by impeding the balance of forces controlling the protein structure. • The overall effect of salinity on plants is the eventual shrinkage of leaf size, which leads to death of the leaf, and finally the plant. • Salinity may also cause reduced ATP and growth regulators in plants • The proteins involved in salt transport across the plasma membrane and the tonoplast i.e. proton pumps and Na+/H +-antiporters have been identified. • Progress in cloning of various types of ATPase has provided important tools for the study of the molecular mechanisms involved in ion sequestration.

10. Initial phase- water deficit lasting for a few days or weeks • Plants have several processes to respond to salt stress. This overview of plants’ response to salt stress broadly categorizes the cellular mechanisms • Two phase model Final phase- ion toxicity initiates leaf death • This overview of plants’ response to salt stress broadly categorizes the cellular mechanisms. • Adaptation of plants to Salinity: • One means of eliminating the salt that accumulates in plant cells is through storage of the salt ions in vacuoles. • Another method is allowing the salt to build up outside the cells, in the intracellular space. • This leads to a gradient of water moving out of the cells to accommodate the change in ion concentration, and eventually too much water leaves the cell and the cell becomes dehydrated (Volmar et al., 1998). This will lead to cell death. • Salt regulated proteins include osmotin, LEA, ‘Responsive to ABA’ or RAB proteins/dehydrins, salT, NP24 .

11. UV STRESS • UV-B radiation (280-320 nm) is an integral component of sunlight. • UV-B can cause damage to macromolecules, including DNA and generate reactive oxygen species. • It affects the growth, development, reproduction and survival of many organisms including plants. • Exposure to UV-B is obligatory for higher plants because of the need to maximize light capture for photosynthesis • Exposure to high amounts of UV-B causes tissue necrosis and induces the expression of stress-associated genes in part through activation of pathogen-defense and wound-signaling pathways. • At ambient UV-B levels, crosstalk between wound and UV-B signaling pathways modifies plant-insect interactions

12. Importantly, exposure to low nondamaging levels of UV-B has numerous regulatory effects on plant morphology, development, physiology, and biochemical composition. • Low rates of UV-B promote the expression of a range of genes involved in UV-B protection. • These include genes concerned with the production of flavonoids and other phenolic compounds that accumulate in the epidermal layers and provide a UV-absorbing sun screen. • Other UV-B-induced genes are involved in ameliorating oxidative stress and repairing UV damage. • Mutants lacking UV-protective components, such as the flavonoids and sinapic acid esters, are highly sensitive to ambient levels of UV-B. • It has often been speculated that UV-B may be perceived by a novel class of photoreceptor, but no such molecule has ever been identified and no UV-B-specific signaling pathway has been defined. Thus, understanding of the mechanisms of plant UV-B responses lags well behind knowledge of light responses mediated by the phytochrome, cryptochrome, and phototropin photoreceptors.

13. OSMOTIC STRESS • Upon exposure to osmotic stress, plants exhibit a wide range of responses at the molecular, cellular and wholeplant levels. • These include morphological and developmental changes (e.g. life cycle, inhibition of shoot growth and enhancement of root growth), adjustment in ion transport (such as uptake, extrusion and sequestration of ions) and metabolic changes (e.g. carbon metabolism, the synthesis of compatible solutes). • Some of these responses are trigged by the primary osmotic stress signals, whereas others may result from secondary stresses/ signals caused by the primary signals. • These secondary signals can be phytohormones [e.g. abscisic acid (ABA), ethylene], reactive oxygen species and intracellular second messengers (e.g. phospholipids). • Some of these secondary signals may not be confined to the primary stress sites such as the root and their ability to move to other parts of the plant contributes to the co-ordination of whole-plant responses to the stress conditions.

14. Osmotic stress Signal transduction • At the beginning, water deficit from either drought or high salinity may have an ionic, osmotic, or even a mechanical impact on the cell. • It is likely that all these different signals have their own cognate receptors • Changes in receptor occupancy or receptor clustering under water stress may initiate the ionic stress pathways. • Cell shrinkage from osmotic stress causes mechanical stimulation to the cell. • In Escherichia coli , upon hypoosmotic stress, mechanosensitive channels (MscL), which are repressed under hyperosmolarity are rapidly activated and result in substantial solute/water efflux. • Ion channels that sense mechanical changes are found in various organisms including plants.

15. Alterations in turgor may generate a signal that could trigger conformational changes in membrane proteins and result in the initiation of a signaling cascade. • As the stress progresses, secondary signals such as second messengers, stress hormones and ROS may be produced and subsequently activate signalling pathways. • These secondary signals can act as ligands and bind to cognate receptors to initiate signalling cascades. • These receptors include the receptor-like protein kinases, two component sensor kinases and G-protein associated receptors

16. Hypothetical model of plant stress responses (1)Stress perception may involve specific components, about which not much is known. (2), Following the ‘sensing’ of the stress, stress signal is possibly amplified and transduced through the signal transduction machinery, which may involve protein kinases, phosphatases.. (3)Through activated signaling intermediates, the stress signal is transduced inside the nucleus where the genes encoding the stress transcription factors (STF; e.g. dreb, myc, myb, cbf and hsf) are possibly synthesized/activated. (4) They bring about the transcriptional activation of stress responsive promoters. (5)Stress responsive genes (SRG) are transcribed and translated on the cytoplasmic ribosomes, leading to the synthesis of the stress proteins. (6) These stress proteins initiate a biochemical response and subsequently , (7) cellular response, which would then bring about the (8) physiological and finally the whole plant response.

17. References • Plant Physiology by Taiz and Zeiger. 2010. 5th edition. Sinauer Associates, Inc.; • https://www.slideshare.net/floradelaterra/ plant-response-to-stress-presentation. • https://en.wikipedia.org/wiki/Biotic_stress • https://en.wikipedia.org/wiki/Abiotic_stress