Since plants cannot move, they face many environmental stresses. Plants have to deal with changes in light, humidity, drought, or cold. And if that was not enough, they still have to fight against all kinds of pathogens. The stress in plants can be categorized as abiotic (originated by drought, cold, high light) and biotic (originated by the attack of bacteria, fungi, herbivores).
For those reasons, plants have developed an arsenal of enzymes, metabolites, and signaling pathways to face all kinds of environmental stresses. But even though a lot of research has been done in this sense, there are still some important gaps in understanding how plants interact and defend themselves.
Through this article, I will give a quick overview of the most recent findings in the stress signaling in plants and how they face abiotic and biotic stress. Furthermore, I will talk about the crosstalk in the signaling network of plant stress responses.
Oh yes! Although you might not believe it, plants get stressed very easily. And the sad part is that they cannot even escape from it. As sessile organisms (they cannot move from one place to another), plants have had to develop different mechanisms to face those stresses. The stress not only affects the plant’s productivity but also their survival. Depending on the type of stress, plants can dry out, freeze, burn or even die. Plant stress obviously matters to the plants, but also to all of humanity because plants are the primary source of food for human consumption (not to mention their importance to the many researchers who study them)! Our food security, our very lives, are endangered if plant productivity gets too low.
Researchers have studied plant stress deeply, but still, there are many still unknown. Our understanding is key to helping the plants cope with their stress, creating more robust or resistant strains, as well as our survival as a species.
The findings conclude that plants have many strategies: such as protein and metabolite production, activation of the gene expression, and signaling cascades (or transduction pathways) to face abiotic and biotic stress. Also, plants use similar strategies to respond to different types of stresses. Let me explain those mechanisms for each type of stress.
How do plants cope with abiotic stress?
When there is a water deficit in the soil or the air, plants are exposed to drought stress. Under this kind of stress, plants suppress their cell growth and photosynthesis and close their stomata to avoid water loss. Therefore, the transpiration rate (the ratio between the transpired water over the dried matter produced) is repressed.
The perception of drought stress begins with membrane receptors such as G Protein-Coupled Receptors (GPCRs), receptor-like kinases (RLKs), histidine kinases, and ion channels. These receptors create metabolic changes in the cytoplasmic Ca2+ levels and generate secondary molecules such as Abscisic Acid (ABA), and Reactive Oxygen Species (ROS). Abscisic acid (ABA) is a plant hormone that regulates numerous aspects of plant growth, development, and stress responses. ROS are molecules involved in the equilibrium of oxidative metabolism, as well as participating in DNA damage repair and the antioxidant system of the plant.
These secondary messengers initiate a protein phosphorylation cascade by various kinases, such as Calcium-Dependent Protein Kinases (CDPKs), Calcineurin-B-Interacting Protein Kinases (CIPKs), protein kinases (PKs), and protein phosphatases (PPs). In a phosphorylation cascade different enzymes are phosphorylated (a phosphoryl group is attached to the molecule). In biology, the phosphorylation of a molecule is related with the "activation" of the molecule and led to transduction pathways and transfer of chemical signals.
Some genes have been identified at this point, such as Dehydration responsive element-binding (DREB), ABRE-binding protein (AREB/ABF), which activate transcription factors such as ABA-responsive element (ABRE), and MYC recognition sequence (MYCRS). This leads to the activation of several responsive genes involved in membrane and protein stabilization and cellular homeostasis (self-regulating process to maintain cell stability), such as heat shock proteins (HSPs), late embryogenesis abundant (LEA) proteins and lipid transfer proteins (LTPs). Finally, all these allow the production of stress tolerance responses (like the closure of stomata as mentioned above) (Figure 1).
Essentially, although the plant signaling stress is still under constant study, all that is really known is the activation and participation of these genes ends in metabolic changes (like increase of flavonoids) which strength the defense system of the plants.
Calcium-Dependent Protein Kinases
Regulatory protein deciphering calcium signals triggered by various developmental and environmental stimuli.
Regulate developmental processes, hormone signaling transduction and mediate stress responses in plants
Proteins which selectively modifies other proteins by covalently adding phosphates to them (phosphorylation)
Proteins that remove a phosphate group from the phosphorylated amino acid residue of its substrate protein.
Figure 1. Drought stress signaling showing the key regulators in the pathway.
Low temperatures expose the plant to cold stress. Generally, cold stress affects the plant cell membranes physically and biochemically by changing the lipid composition and inducing other non-enzymatic proteins. Plants are more likely to tolerate chilling temperatures (8°C to 0°C).
Like drought stress, the initial perception of cold stress starts with membrane receptors (GPCRs, RLKs, histidine kinases, and ion channels). These receptors lead to increases in the cytoplasmic Ca2+ levels. This change initiates the activation of several kinases, in particular, CDPKs. Plants have evolved a series of survival mechanisms to adapt to diverse environmental challenges, including drought, salinity, wounding, and low temperatures. Calcium, functioning as a second messenger of plant cells, plays an essential role in various signaling transduction pathways, particularly in providing signals to help the plants be adapted to diverse environmental challenges. The changes in Ca2+ concentration is sensed by several Ca2+ sensors or Ca2+‐binding proteins like CDPKs. Thus, CDPKs play important roles in various physiological processes of plants, including growth and development, stress responses and hormone signaling.
For instance, the OsCDPK13 (An Oryza sativa CDPK) has been detected in response to cold in rice (Abbasi et al. 2004). Some other genes that have been detected in response to cold are: DREB1A, SCOF, or SGBF, which activate other genes like DRE/CRT and ABRE (Verma et al. 2013). Interestingly, both mitochondrial and nuclear genes are involved in cold stress.
Finally, the above genes led to different signaling stress responses like sugars accumulation, which decreases the temperature at which ice forms, similar to putting salt on roads (Kidokoro et al 2017) (Figure 2).
Figure 2. Cold stress signaling showing the key regulators in the pathway.
Excessive salt content accumulation in the soil leads to high salinity stress. It causes a reduction in the photosynthetic rate and arrests the growth of the plant. These changes can be caused by water stress, or a lack of humidity in the soil. When this occurs, it leads to salts in the soil becoming concentrated and induces salinity stress.
The primary result of salt stress is the induction of jasmonic acid (JA) pathway genes and genes responsive to JA and ABA.
When a plant senses salt stress, it induces JA and ABA, which leads to changes in the cytosolic Ca2+ levels. Here CDPKs are activated and induce various signal transduction pathways, including SOS (Salt Overly Sensitive) pathway with the genes SOS1, SOS2, and SOS3 (Ji et al. 2013).
Several transcription factors regulating these signaling cascades processes include DREB/CBF, AREB/ABF, bZIP, MYC/MYB. Their activation results in a further downstream signaling cascade that favors the production of proteins regulating energy metabolism, ROS scavenging, cytoskeleton stability, photosynthesis, and photorespiration.
All together these allow better ion homeostasis (the ion equilibrium within each cell), because the plant capacity to remove toxic byproducts is improved through metabolic changes and antioxidants production, which finally enhances the growth recovery (Figure 3).
Figure 3. Salinity stress signaling showing the key regulators in the pathway.
Because plants cannot move, they are more prone to suffer light stress. High light intensities can affect the plants by inhibiting their physiological and metabolic processes, including photosynthesis, antioxidant machinery, as well as their ability to fix atmospheric carbon and nitrogen. These negative effects led to reduced plant growth and even death because the photosynthetic system is hampered, and plants cannot produce adequate sugars and biomass to survive. However, this type of stress has been less understood in comparison with others.
The high incident light is sensed by several classes of photoreceptors, including phototropins, phytochromes, and cryptochromes, which are proteins that change their structure and activate once light is perceived. These photoreceptors activate a signal cascade where transcription factors such as Elongated Hypocotyl 5 (HY5) and Constitutive Photomorphogenic (COP1) are activated to initiate transcription of factors such as WD40, MYB and bHLH. Although photoreceptors are activated in normal non-stressed conditions, the production of reactive oxygen species (ROS) (which damage proteins and DNA molecules) by the light stress induce the overexpression of these genes. Their overexpression may lead to activating defense responses via quenching mechanisms (a mechanism of flow chemical energy in the photosynthetic apparatus, at electron level) for high energy and antioxidant machinery for scavenging ROS. Furthermore, low molecular weight non-enzymatic antioxidants (such as ascorbic acid, glutathione) are also involved in light responses by neutralize the ROS species (Figure 4).
Figure 4. Light stress signaling showing the key regulators in the pathway.
Other Abiotic Stresses
High temperature and heavy metal toxicity are other stresses that affect the plants but are less studied. However, with the advent of Next-Generation Sequencing technologies, researchers have performed experiments to get insights into those stresses.
For instance, in the case of high temperature, transcriptomic studies in rice and tomato revealed that heat shock factors (HSFs) are key proteins that are highly expressed when plants are exposed to high temperature (Mittal et al. 2009) (Bita et al. 2011). These proteins are indicatives of heat stress and lead to signaling cascades that help the plant to recover from radiation delivery through quenching mechanisms. The authors concluded that these factors are critical for the transcriptional induction of heat shock genes.
Regarding heavy metals, a NGS analysis revealed differential expression of several genes encoding cytochrome P450 family proteins, heat shock proteins (HSPs), glutathione S-transferase, protein kinases, ion transporters and transcription factors such as DREB and NAC when rice plants were exposed to cadmium stress (Ogawa et al. 2009). These genes are previously associated to be express under stressed conditions.
Extracellular surface receptors present in the plant cell membrane recognize when plants are attacked by bacteria, fungi, herbivores or other microorganisms, and respond by inducing pathogen-associated molecular patterns (PAMPs).
The early recognition of a microorganism at the cell surface is important in order to prevent pathogens advancing further into the plant. The plant initiates a sequence called PAMP-triggered immunity (PTI), a series of different signal cascades to stop any further invasion of the pathogen. PAMP-triggered immunity (PTI) constitutes the first layer of plant immunity that restricts pathogen proliferation. In this cascade, different host cell surface-localized pattern-recognition receptors (PRRs) recognize the pathogen and activate plant immunity.
However, some pathogens have learned to overcome the first cell membrane barrier and PTI by synthetizing effector proteins, which alter the plant resistance response to the pathogen. In other words, the effector proteins synthesized by the microorganism hide the pathogen’s entrance; therefore, plants do not know they have been attacked.
Fortunately, plants have developed an even more specialized mechanism to detect the effector proteins of the pathogens, a response called effector-triggered immunity (ETI). Here, some plant receptors proteins are synthesized by resistance genes (or R genes), which recognize the effectors proteins from the pathogens and induce a signaling network that includes the production of reactive oxygen species (ROS). It further induces a response known as the hypersensitive response (HR). HR is a type of programmed cell death thought to limit access to the plant by killing the tissue portion where the pathogen is present (Figure 5).
Figure 5. Biotic stress showing the main two pathways of PTI and ETI.
Crosstalk between stresses
The relationship between different stresses can be understood as the interconnection between signaling pathways which share similar members (e.g., genes and molecules). This means that a plant can use similar components (such as MAPK kinases) to develop defense responses against many different types of stresses. However, although the plant uses similar players for different stresses, the final output or tolerance response will differ. This is known as "specificity".
For instance, Ca2+ is stimulated by various abiotic, biotic, developmental and hormonal signals and, therefore, is a major point of signaling crosstalk. However, the intensity, concentration, duration, and subcellular location of the Ca2+ determines the specific output according to the type of sensed stress signals.
Another example is seen with MAPKs. For instance, the AtMPK4 is a gene activated by cold, low humidity, osmotic stress, touch, and wounding in Arabidopsis. Therefore, MAPKs signaling cascade offers strong evidence for crosstalk during abiotic stress signaling. Again, the binding between different MAPKs, intensity, magnitude, and duration of the expressed gene can lead to a specific tolerance response against specific stresses.
Regarding the biotic stress, plants execute defense responses by generating the alarm signals of Salicylic Acid (SA), Jasmonic acid (JA), and ethylene (ET) for different pathogens. It has been reported that biotrophic pathogens (those fed with live plant tissue) are more sensitive to SA-mediated induced defenses. In contrast, necrotrophic pathogens (fed with dead plant tissue) and herbivorous insects are susceptible to JA/ET-mediated defense. Again, an array of signaling pathways that generate different defense responses are generated against partially distinct classes of attackers.
The knowledge of plant signaling stress has allowed researchers to produce improved plants that are resistant to specific stresses. For instance, genetically modified Glycine max (soybean) and Zea mays (corn) plants have been developed and approved for drought tolerance (Khan et al 2019). In the future, continued research about these signaling pathways will open new frontiers in plant science in order to produce plants with desired specialized traits such as pathogen and abiotic factor resistance to better feed our world.
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