Abiotic stresses, such as drought, salinity, extreme temperatures, chemical toxicity and oxidative stress are serious threats to agriculture and the natural status of the environment. Increased salinization of arable land is expected to have devastating global effects, resulting in 30% land loss within the next 25 years, and up to 50% by the year 2050. Therefore, breeding for drought and salinity stress tolerance in crop plants (for food supply) and in forest trees (a central component of the global ecosystem) should be given high research priority in plant biotechnology programs. Molecular control mechanisms for abiotic stress tolerance are based on the activation and regulation of specific stress-related genes. These genes are involved in the whole sequence of stress responses, such as signaling, transcriptional control, protection of membranes and proteins, and free-radical and toxic-compound scavenging. Recently, research into the molecular mechanisms of stress responses has started to bear fruit and, in parallel, genetic modification of stress tolerance has also shown promising results that may ultimately apply to agriculturally and ecologically important plants. The present review summarizes the recent advances in elucidating stress-response mechanisms and their biotechnological applications. Emphasis is placed on transgenic plants that have been engineered based on different stress-response mechanisms. The review examines the following aspects: regulatory controls, metabolite engineering, ion transport, antioxidants and detoxification, late embryogenesis abundant (LEA) and heat-shock proteins.

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Plant responses to drought, salinity and extreme temperatures: towards genetic engineering for stress tolerance

The completion of the Arabidopsis thaliana genome sequence allows a comparative analysis of transcriptional regulators across the three eukaryotic kingdoms. Arabidopsis dedicates over 5% of its genome to code for more than 1500 transcription factors, about 45% of which are from families specific to plants. Arabidopsis transcription factors that belong to families common to all eukaryotes do not share significant similarity with those of the other kingdoms beyond the conserved DNA binding domains, many of which have been arranged in combinations specific to each lineage. The genome-wide comparison reveals the evolutionary generation of diversity in the regulation of transcription.

http://science.sciencemag.org/content/sci/290/5499/2105.full.pdf

Arabidopsis transcription factors: genome-wide comparative analysis among eukaryotes.

Summary
Experimental studies on CO2 assimilation of mesophytic C3 plants in relation to relative water content (RWC) are discussed. Decreasing RWC slows the actual rate of photosynthetic CO2 assimilation (A) and decreases the potential rate (Apot). Generally, as RWC falls from c. 100 to c. 75%, the stomatal conductance (gs) decreases, and with it A. However, there are two general types of relation of Apot to RWC, which are called Type 1 and Type 2. Type 1 has two main phases. As RWC decreases from 100 to c. 75%, Apot is unaffected, but decreasing stomatal conductance (gs) results in smaller A, and lower CO2 concentration inside the leaf (Ci) and in the chloroplast (Cc), the latter falling possibly to the compensation point. Down-regulation of electron transport occurs by energy quenching mechanisms, and changes in carbohydrate and nitrogen metabolism are considered acclimatory, caused by low Ci and reversible by elevated CO2. Below 75% RWC, there is metabolic inhibition of Apot, inhibition of A then being partly (but progressively less) reversible by elevated CO2; gs regulates A progressively less, and Ci and CO2 compensation point, Γ rise. It is suggested that this is the true stress phase, where the decrease in Apot is caused by decreased ATP synthesis and a consequent decreased synthesis of RuBP. In the Type 2 response, Apot decreases progressively at RWC 100 to 75%, with A being progressively less restored to the unstressed value by elevated CO2. Decreased gs leads to a lower Ci and Cc but they probably do not reach compensation point: gs becomes progressively less important and metabolic limitations more important as RWC falls. The primary effect of low RWC on Apot is most probably caused by limited RuBP synthesis, as a result of decreased ATP synthesis, either through inhibition of Coupling Factor activity or amount due to increased ion concentration. Carbohydrate synthesis and accumulation decrease. Type 2 response is considered equivalent to Type 1 at RWC below c. 75%, with Apot inhibited by limited ATP and RuBP synthesis, respiratory metabolism dominates and Ci and Γ rise. The importance of inhibited ATP synthesis as a primary cause of decreasing Apot is discussed. Factors determining the Type 1 and Type 2 responses are unknown. Electron transport is maintained (but down-regulated) in Types 1 and 2 over a wide range of RWC, and a large reduced/oxidized adenylate ratio results. Metabolic imbalance results in amino acid accumulation and decreased and altered protein synthesis. These conditions profoundly affect cell functions and ultimately cause cell death. Type 1 and 2 responses may reflect differences in gs and in sensitivity of metabolism to decreasing RWC.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.0016-8025.2001.00814.x

Photosynthetic carbon assimilation and associated metabolism in relation to water deficits in higher plants

Despite the recognition of H2O2 as a central signaling molecule in stress and wounding responses, pathogen defense, and regulation of cell cycle and cell death, little is known about how the H2O2 signal is perceived and transduced in plant cells We report here that H2O2 is a potent activator of mitogen-activated protein kinases (MAPKs) in Arabidopsis leaf cells Using epitope tagging and a protoplast transient expression assay, we show that H2O2 can activate a specific Arabidopsis mitogen-activated protein kinase kinase kinase, ANP1, which initiates a phosphorylation cascade involving two stress MAPKs, AtMPK3 and AtMPK6 Constitutively active ANP1 mimics the H2O2 effect and initiates the MAPK cascade that induces specific stress-responsive genes, but it blocks the action of auxin, a plant mitogen and growth hormone The latter observation provides a molecular link between oxidative stress and auxin signal transduction Finally, we show that transgenic tobacco plants that express a constitutively active tobacco ANP1 orthologue, NPK1, display enhanced tolerance to multiple environmental stress conditions without activating previously described drought, cold, and abscisic acid signaling pathways Thus, manipulation of key regulators of an oxidative stress signaling pathway, such as ANP1/NPK1, provides a strategy for engineering multiple stress tolerance that may greatly benefit agriculture

/pdf/functional-analysis-of-oxidative-stress-activated-mitogen-4fommzy9to.pdf

Functional analysis of oxidative stress-activated mitogen-activated protein kinase cascade in plants.

Water stress substantially alters plant metabolism, decreasing plant growth and photosynthesis1,2,3,4 and profoundly affecting ecosystems and agriculture, and thus human societies5 There is controversy over the mechanisms by which stress decreases photosynthetic assimilation of CO2 Two principal effects are invoked2,4: restricted diffusion of CO2 into the leaf, caused by stomatal closure6,7,8, and inhibition of CO2 metabolism9,10,11 Here we show, in leaves of sunflower (Helianthus annuus L), that stress decreases CO2 assimilation more than it slows O2 evolution, and that the effects are not reversed by high concentrations of CO212,13 Stress decreases the amounts of ATP9,11 and ribulose bisphosphate found in the leaves, correlating with reduced CO2 assimilation11, but the amount and activity of ribulose bisphosphate carboxylase-oxygenase (Rubisco) do not correlate We show that ATP-synthase (coupling factor) decreases with stress and conclude that photosynthetic assimilation of CO2 by stressed leaves is not limited by CO2 diffusion but by inhibition of ribulose biphosphate synthesis, related to lower ATP content resulting from loss of ATP synthase

Water stress inhibits plant photosynthesis by decreasing coupling factor and ATP

The heat-shock response is a conserved reaction of cells and organisms to elevated temperatures (heat shock or heat stress). Whereas severe heat stress leads to cellular damage and cell death, sublethal doses of heat stress induce a cellular response, the heat-shock response, which (a) protects

Regulation of the Heat-Shock Response

Phosphorylation is one of the mechanisms controlling the activity of heat-shock transcription factors in yeast and mammalian cells. Here we describe partial purification, identification, and characterization of a protein kinase that phosphorylates the Arabidopsis heat-shock factor AtHSF1 at multiple serine residues. The HSF1 kinase forms a stable complex with AtHSF1, which can be detected by kinase pull-down assays using a histidine-tagged AtHSF1 substrate. The HSF1 kinase interacts with the cell-cycle control protein Suc1p and is immunoprecipitated by an antibody specific for the Arabidopsis cyclin-dependent CDC2a kinase. Phosphorylation by CDC2a in vitro inhibits DNA binding of AtHSF1 to the cognate heat-shock elements, suggesting a possible regulatory interaction between heat-shock response and cell-cycle control in plants.

Phosphorylation by a Cyclin-Dependent Kinase Modulates DNA Binding of the Arabidopsis Heat-Shock Transcription Factor HSF1 in Vitro.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/S0014-5793%2898%2901152-1

Interaction between the Arabidopsis thaliana heat shock transcription factor HSF1 and the TATA binding protein TBP.

The heat shock (hs) response, the synthesis of heat shock proteins (HSPs) and the acquisition of stress tolerance, in plants is environmentally induced by heat stress and several other stresses including drought, heavy metal, and oxidative stresses. Thus HSPs, probably acting as molecular chaperones, are important determinants for developing tolerance against heat stress and probably also against other environmental stresses. Interestingly, HSPs are also synthesised in the absence of detectable environmental stress in the zygotic embryo of many plant species during seed maturation and during microspore embryogenesis. The HSPs therefore seem to play a key role in plant growth and development. The central regulator of the hs response is the hs transcription factor (HSF) whose activity for binding to the conserved hs promoter elements (HSE) and subsequent transcription of hs genes is induced upon hs. In plants multiple HSF-like genes have been isolated on the basis of HSE binding activity and conserved sequences. HSF genes are the targets for both, studying the regulation and manipulation of the hs response and hence stress tolerance.

Andreas Reindl

Papers

Regulation of the Heat-Shock Response

Phosphorylation by a Cyclin-Dependent Kinase Modulates DNA Binding of the Arabidopsis Heat-Shock Transcription Factor HSF1 in Vitro.

Interaction between the Arabidopsis thaliana heat shock transcription factor HSF1 and the TATA binding protein TBP.

Molecular and applied aspects of the heat stress response and of common stress tolerance in plants