In the course of the membrane potential elicitation, secondary messengers are initiated, leading to a downstream signaling cascade that terminates in the production of phytohormones and changes in plant metabolism, a process which can take anywhere from hours to days Maffei et al. During these cascades, calcium signaling is initiated Reddy et al. However, the initiation of a calcium signal must precede other signals since it initiates the ion fluxes responsible for electrical signals Zebelo and Maffei, Maffei et al. Interestingly, calcium signaling was only initiated as a response to biotrophic wounding and could not be recorded after artificial mechanical wounding Bricchi et al.
Calcium is not the only secondary messenger involved in fast signaling responses. Recently ROS have also been correlated with electrical signals. ROS signals can travel in the xylem at up to 0. Therefore, current hypotheses support a close connection between ROS and electrical signals. However, whether ROS activate or increase the signal intensity has yet to be uncovered.
The ROS signaling pathway is initiated by several stressors, indicating that ROS is a general signaling molecule, and diversity in its oscillations frequency and amplitude allows the plant to decipher signals Mittler et al.
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There is distinct evidence that plants have several closely interconnected signaling pathways. However, we lack information on the integrated timing of hydraulic, electrical, and chemical signaling pathways, and the importance of each pathway for information transmission. For example, while chemical signals are necessary for the initiation of metabolic responses, the long-distance communication signal has to be either electrical or hydraulic in nature Stratmann and Ryan, Conversely, some evidence supports hydraulic signals as essential for signal propagation, even though, in some cases, they are not able to initiate chemical signaling cascades independently Malone et al.
Furthermore, the connectivity among signaling pathways is shown by the inseparable connectivity of hydraulic signals and SWPs. Additionally, electrical signals can initiate chemical signals, and chemical signals in turn can intensify hydraulic signals, resulting in essentially a feedback loop of signal types. Plants are able to differentiate between both biotic and abiotic stressors and orchestrate signaling pathways despite very similar stress responses.
For example, herbivores, sucking insects, and pathogens have unique chemicals within their saliva, regurgitates, or exudates through which many plants are able to detect the identity of attacker. Bricchi et al. After wounding, only an increase in volatile compound emission in response to multiple wounding by the robotic worm and the herbivore was recorded, pinpointing that the repetition of wounding might be a trigger for stress response.
The compound fraction of emitted volatiles unveiled a difference between the robotic worm and the herbivore indicating that the plants were able to differentiate between different wounding effects Bricchi et al. Further investigation revealed that only the saliva of S. Interestingly, these results suggest that more than one signaling pathway leading to volatile emissions exists in plants.
However, experiments focused on plant wounding by different herbivores the caterpillar S. Wounding by three different biotic stressors the caterpillar S. Myzus persicae regulated a fold higher number of genes than S. These findings emphasize the ability of plants not only to identify the nature of the stressor, but also to fine-tune the stress response and match the intensity of the response with the inflicted stress intensity. Plants are able to distinguish between abiotic stresses despite a range of overlapping signal cascades. Among an array of abiotic stressors, cold, salt, and drought stress elicit similar responses by altering plant water relations, thus affecting the osmotic potential Shinozaki and Yamaguchi-Shinozaki, ; Mahajan and Tuteja, Based on the osmotic component, plants respond with overlapping signal cascades such as calcium signaling Sanders et al.
Interestingly, it is thought that ABA induction is due to changes in water potential caused by each stressor, and not the high salt concentrations occurring during salinity Zhang et al. However, salt stress has an additional ionic component which often leads to ion toxicity and thus initiation of a special signaling cascade, SOS salt overly sensitive Mahajan et al. The receptors for the stress perception have not been identified; however, the plasma membrane plays a key role in the perception and transmission of stress responses, and the cell wall participates in the triggering process because of its role in regulating tension of the plasma membrane Jia et al.
The study of combinatory stresses is inherently difficult in that plants can respond in a number of ways, including a response characteristic to only one stress, an increased response intensity, or a unique response unlike any elicited by individual stressors Rizhsky et al. Generally speaking, we can look to results from gene expression studies as an indicator of typical plant responses to multiple stressors. Gene level responses can be i additive, ii synergistic more than the sum of individual stresses , iii idiosyncratic completely different from the single stress responses , iv dominant response very close to one of the stressors Prasch and Sonnewald, , or v even antagonisitc Bostock, Below we outline common stress interactions and examine typical plant responses.
Abiotic stressors are inherently tightly linked in the natural environment. Heat stress, one of the most commonly observed stress factors, has been examined extensively, mainly in light of predicted future climatic temperature increases Meehl et al. Elevated temperatures in combination with drought stress have a significantly greater detrimental effect on the growth and productivity of field crops compared with only elevated temperature or water shortage alone Heyne and Brunson, ; Craufurd and Peacock, ; Savin and Nicolas, ; Jagtap et al.
Likewise, in turf grasses, high temperature and drought drastically impact plant health due to a decline in the activity of stress antioxidant enzyme activity Jiang and Huang, It is important to note that xylem sap hormone concentrations can differ from those found within roots or leaves and may provide a more informative measurement when deciphering the effects of single stress versus combined stresses.
The difficulty arises when the stressor alters the plant response in a manner that contradicts the typical trend. Examples include mitigation of negative drought and salt stress effects when CO 2 concentrations are increased, resulting in elevated plant gas exchange, plant growth, and plant nutrition Qaderi et al. It is important to consider that in these cases the plants were exposed to the treatment drought, temperature, and CO 2 for 11 d Qaderi et al. In trees, increased temperature in combination with drought stress weakens tree defense mechanisms, resulting in a reduction in stored sugars and starch, and consequently increased tree susceptibility to herbivore attack Zvereva and Kozlov, From a whole-plant perspective, Niinemets and Valladares studied North American, European, and West Asian temperate tree and shrub species.
The authors examined waterlogging, shade, and drought tolerance, and found that only 2. Interestingly, other researchers found similar conclusions to the cross-continental observations of Niimentes and Vaaladares in carefully controlled experiments comparing gene transcription patterns between single and combined stressors Rizhsky et al. Results to date reveal an anticipated distinct set of stress responses produced by coinciding stresses. For example, sunflowers exposed to high light intensity, elevated temperature, and the combination of both expressed genes after exposure to the combination treatment while only nine of these genes were expressed upon exposure to a single stress Hewezi et al.
According to our knowledge, we are lacking information on how the transcription level of genes in roots versus the shoot differ in response to multiple stressors. The effect of abiotic and biotic stress combinations has been well summarized Prasch and Sonnewald, However, we would like to highlight general observed trends that emphasize that simultaneous abiotic and biotic stress occurrences may result in either synergistic or antagonistic interactions. For example, abiotic factors such as increased temperature benefit virus as well as bacterial growth conditions, promoting the abundance of the bacterium P.
Furthermore, plant viruses benefit from elevated temperatures by enhanced virus survival and spread, an increased availability of insect vectors, and possibly suppressed host resistance Moury et al. However, high temperatures can also increase plant defense response. Likewise, sunflowers become more resistant to parasitic herbaceous plants Orobanche cumana and O. Determining the effects of temperature becomes even more challenging when changes in plant resistance are recorded on a genotype level.
Twenty-seven lines of wheat Triticum aestivum were exposed to a range of temperatures, and the plant resistance to leaf rust Puccinia recondita was highly dependent on the plant line, with both increases and decreases with changes in temperature Dyck and Johnson, Similarly, drought stress can also affect pathogen infection both positively and negatively.
Mohr and Cahill demonstrated that Arabidopsis resistance to the bacterium P. However, in tomato plants, three cycles of drought stress fully hydrated until the plant was wilted with alternating recovery periods showed subsequent enhanced inoculation resistance to the fungus Botrytis cinerea Achuo et al. Interestingly, biotic factors can also affect plant resistance to abiotic stressors such as drought and freezing tolerance Xu et al.
Several vegetable seedlings infected with four different viruses had improved drought and freezing tolerance, suggesting that infection with viruses can actually aid the plant in retaining water Xu et al. Viruses can form mutualistic relationships with plants under extreme stress conditions, even though viruses are considered parasitic symbionts under normal conditions. Under stress conditions, virus-infected plants show higher above-ground water content as well as higher water retention abilities Xu et al.
Additionally, virus-infected plants showed increased SA content as well as increased levels of osmoprotectants and antioxidants which serve as defense mediators Xu et al. Considering the contrasting effect of stressors on plants, in a recently published review, the authors caution against the generalization of abiotic stress factors including high temperature, humidity, drought, and salinity in weakening plant defenses Sharma et al.
As with other stress combinations, plants are often attacked by multiple herbivores and are capable of producing an integrated response Bostock, ; Bruxelles et al. This kind of plant signal integration can not only occur during simultaneous biotic stressor encounters but is also evident when biotic stressors are temporally separated.
In other words, individual biotic agents can induce a signaling cascade in favor of future stressors. When the mold fungus Sclerotium rolfsii Saccodes infects peanut plants, the plants change their volatile compound signature to increase their attractiveness to BAW, thus making the plants more attractive to BAW larval parasitoids Cotesia marginiventris as well. Additionally, previously attacked plants were able to alter their resistance in subsequent attacks to other similar stressors Bruce and Pickett, Rice plants damaged by the white-backed plant hopper Sogatella furicifera increased resistance to the fungus, with rice blast Magnaporthe grises only when the plants were previously exposed to the hopper Kanno and Fujita, ; Kanno et al.
Pest resistance to three different biotic agents on tomato plants found that depending on the agent combination, pest resistance could increase or impair the development of other pathogens Stout et al. Even though the combination of stressors can determine overall stress susceptibility Atkinson and Urwin, , it is important to recognize that there are several other physiological factors influencing plant stress responses. Plant nutritional state, plant age, and the time of the day at which the plants are stressed can influence, enhance, or dampen stress responses in plants.
The shallow root systems of tree saplings are restricted to the upper soil layers and therefore have a higher dependence on precipitation events. Older, larger trees, on the other hand, have deeper root systems allowing access to deeper ground water and a greater uncoupling from minor drought events Dawson, ; Drake et al.
However, as trees grow and gain in height, greater light interception leads to higher leaf temperatures and potential photoinhibition for a review, see Niinemets and Valladares, Consequently, leaf cooling through transpiration is increased, requiring more water, a decrease in leaf water potential, and probably an increase in cavitation events if the water supply is not sufficient. Trees within distinct ontogenic stages are therefore prone to different stress combinations.
The simultaneous occurrence of drought, photoinhibition, heat stress, and nutrient limitation is expected to increase with increasing plant size, while shading and drought stress interactions are expected to be more prevalent in seedlings and saplings Valladares and Pearcy, , ; Niinemets and Valladares, ; Niinemets, The effect of nutrients on plant signaling relate to their role in supporting enzyme production and functioning Ranieri et al. The composition of herbivore-induced volatiles strongly depends on other abiotic environmental factors, such as nitrogen and phosphorus availability Schmelz et al.
However, in addition to the chemical stress response, nutrients can have a contrary effect on the plant defense status. Predicting the plant susceptibility to different pathogens remains challenging, because plant stress resistance is dependent on several factors including plant—pathogen interaction, environmental conditions, plant developmental stage, as well as nutrient availability, whereby each nutrient element can impact the plant differently Huber and Haneklaus, ; Dordas, Additionally, land management practices can affect nutrient availability for both the plant and the pathogen, thus affecting plant disease severity Huber and Graham, A recent meta-analysis comparing the severity of fungal pathogen infection relative to the addition of commercial fertilizer in herbaceous plants revealed that in general, the addition of N fertilizer increased the severity of fungal pathogen infection, suggesting that a slightly depleted nutrient status would benefit plants.
However, species-specific differences exist and there are some plants, such as potato Solanum spp.
A less explored field in plant signaling and stress response is the influence of circadian rhythm on plants Piechulla, The circadian clock is involved in a multitude of physiological processes: stomatal conductance, photosynthesis, stem elongation, flowering time, leaf movement, pathogen and herbivore resistance, response to abiotic stress, and plant immunity Zhang et al. Covington et al. Furthermore, plant hormones are also up- or down-regulated depending on the time of day.
The authors speculate that the difference in transcriptome abundance over the course of the day reflects the plant ABA level and therefore also the activity of the ABA signaling pathway. The rhythm of gene expression can be highly affected by temperature. Under non-stressed conditions, bPRP expression follows a circadian rhythm, which becomes disturbed at low temperatures, demonstrating that stress can affect the circadian rhythm of gene expression.
This regulation has been shown for oxidative stress and freezing tolerance in Arabidopsis plants Nakamichi et al. Deltapine 50 Rikin, These experiments collectively show that plants use their resources efficiently by limiting signaling pathways at certain times of the day, emphasizing the importance of including different time points in experiments focusing on plant stress responses Rienth et al.
Undoubtedly plants respond rapidly and distinctly to changing environmental conditions and biotic assaults despite their sessile lifestyle. Here, we presented the mode of action of each signaling pathway, the signaling speed, and potential interconnections between signaling pathways. Yet, many unknowns remain regarding signaling pathways, ranging from the importance of each pathway individually to the integration of signals.
Specifically, hydraulic signals seem to play an important role in information transmission Malone et al.
However, the lack of information on hydraulic signaling events makes integration of hydraulic signals into the plant response signaling pathway timeline challenging. Even though there is still a debate as to whether chemical signals are suitable as long-distance signaling pathways, the importance and presence of hydraulic, electric, and chemical pathways is beyond dispute.
Additional investigation into the integration of all three signaling pathways along with deciphering the role of each signal type in information transfer remains an area in great need of future attention. Furthermore, it is important to include environmental variation and a more diverse spectrum of plant species into the experimental design in order to account for unique species responses to changing environmental conditions.
High variability of stress resistance to abiotic stressors exists even within lines from the same species Dyck and Johnson, , and susceptibility towards different stressors can vary depending on the ontogenetic stage of plants Niinemets, In the field, plants are rarely exposed to a single stressor, but instead face a combination of stressors that probably vary in intensity. Gene transcription analyses have emphasized the uniqueness of stress responses to combined stressors. Finally, the severe lack of information on combined stressors hinders our ability to predict stress responses under changing environmental conditions.
The future of efficient crop production is highly dependent on our ability to predict stresses accurately. Therefore, addressing plant responses to different stress combinations will help researchers and farmers manage plant responses in order to minimize resource outputs and maximize productivity. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account.
Frontiers | Dynamics of long-distance signaling via plant vascular tissues | Plant Science
Sign In. Advanced Search. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents. Signal types. Hydraulic, chemical, and electrical signal integration: understanding signal speed. Other factors influencing stress response interaction. Future directions. Long-distance plant signaling pathways in response to multiple stressors: the gap in knowledge Annika E Huber. Oxford Academic. Google Scholar. Taryn L Bauerle. Correspondence: bauerle cornell. Cite Citation. Permissions Icon Permissions. Abstract Plants require the capacity for quick and precise recognition of external stimuli within their environment for survival.
Abiotic stress , biotic stress , chemical signal , defense response , electrical signal , hydraulic signal , long-distance signaling , stress combinations. View large Download slide. Table 1. Molecules involved in chemical signaling pathways and the respective references.
View Large. Table 2. Propagation speed and mode of action of hydraulic, chemical, and electrical signals. Lopez et al. Influence of drought, salt stress and abscisic acid on the resistance of tomato to Botrytis cinerea and Oidium neolycopersici. Search ADS. Substantial hydraulic signals are triggered by leaf-biting insects in tomato.
The effect of potassium nutrition on pest and disease resistance in plants. Indeed, priming of defenses i. Such rapid signal propagation throughout the plant body has been proposed to occur through both symplastic cytoplasmic and apoplastic extracellular pathways. Thus, mobility alone does not necessarily reveal a messenger carrying specific systemic information. However, there are many cases where a role for phloem mobile signals in the regulation of distant target site activity has been demonstrated.
Critically, inhibiting FT movement prevents the transfer of flowering information reviewed in Ham and Lucas, Indeed, a suite of such mobile signals has been defined that trigger systemic response to local stimuli. That is, what are the challenges to a systemic signal that must traverse tens of cell lengths per second? In addition, many multicellular animals use a parallel signaling network, where rapid signaling is accomplished using cellular networks highly specialized to rapidly transmit electrical signal over long distances, i.
For example, because of the predicted size exclusion limit that is imposed by its constituent polysaccharide networks, the cell wall is likely to significantly limit or prevent extracellular vesicle transport between cells. The cell wall may also alter the chemistry of some small molecules that are secreted into the apoplast due to the presence of peroxidases, oxidases and other enzymes associated with it.
PD provide a symplastic connection for the transfer of ions, metabolites, hormones, proteins, RNAs and other molecules. Even macromolecules such as large proteins can move through PDs to control developmental programs. In the endodermis, SCR subsequently induces the expression of microRNAs and , which are then translocated from the endodermis to pith tissues in the stele through PDs. The accumulation of callose in this region is thought to constrict the size of the PD, and thereby restrict or block intercellular movement through the symplast.
For example, PDs show accumulation of sterols and sphinogolipids that could play a role in defining novel membrane microdomains. Thus, both small and large signaling molecules can use the PD as a means to travel systemically from cell to cell, although we are still far from fully understanding the extent to which movement of these kinds of molecules via PDs contributes to systemic response throughout the plant.
There are three potential paths for the propagation of these signals from cell to cell. The second is via symplastic connections provided by PD, either through the cytosolic cavity, or traversing the endoplasmic reticum ER membranes that permeate the PD. However, in moving this ROS wave from one cell to the next, it is not clear how the wave propagates across the distance that separates neighboring cells.
Cell walls represent a significant barrier, not only because of the distance created between cells, but also because the apoplast can provide a high antioxidative capacity that can quench a ROS signal. However, quenching can also be a factor in ROS diffusing through a symplastic connection.
An alternative model for propagating the ROS wave from cell to cell is that a different signaling molecule is used as a relay to traverse the symplastic or apoplastic connections. A third alternative is the propagation of an electrical signal along the PM connection through the PD.
Further research is needed to address this and all other questions outlined above. In addition to the currently open question as to the precise route that transfers rapid systemic signals between adjacent plant cells, understanding the role that the plant cell types or tissues play in mediating these systemic signals also holds promise to help reveal mechanism and function.
From the standpoint of number, size and PD characteristics, phloem tissue, companion cells and the epidermis contain a high number of cellular connections and could be a good pathway for the transfer of different systemic signals that propagate through both the apoplast and PD. Developmental biology 6. Sunderland, Mass. Molecular biology of the cell 4th ed. New York: Garland Science. Washington, D. The Cell: A Molecular Approach 2nd ed. Sunderland MA : Sinauer Associates. The Journal of Biological Chemistry. Physiological Reviews.
Bibcode : PNAS Annual Review of Microbiology. Infection and Immunity. Bibcode : Sci Current Opinion in Microbiology. Trends in Microbiology. The Journal of Clinical Investigation. Die Naturwissenschaften. Bibcode : NW Genome Biology. Essays in Biochemistry. Cellular Endocrinology in Health and Disease FEBS Letters. Cell surface Intracellular Co-receptor.
Signal transducing adaptor protein Scaffold protein. Intracrine action Neurocrine signaling Synaptic transmission Chemical synapse Neuroendocrine signaling Exocrine signalling Pheromones Mechanotransduction Phototransduction Ion channel gating Gap junction.
Metabolism map. Carbon fixation. Photo- respiration. Pentose phosphate pathway. Citric acid cycle. Glyoxylate cycle. Urea cycle. Fatty acid synthesis. Fatty acid elongation. Beta oxidation. Glyco- genolysis. Glyco- genesis. Glyco- lysis. Gluconeo- genesis. Pyruvate decarb- oxylation. Keto- lysis. Keto- genesis. Light reaction. Oxidative phosphorylation. Amino acid deamination. Citrate shuttle. MVA pathway.
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