Wound response in plants

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Plants are constantly exposed to different stresses that result in wounding. Plants have adapted to defend themselves against wounding events, like herbivore attacks or environmental stresses. [1] There are many defense mechanisms that plants rely on to help fight off pathogens and subsequent infections. Wounding responses can be local, like the deposition of callose, and others are systemic, which involve a variety of hormones like jasmonic acid and abscisic acid. [1]

Contents

Overview

There are many forms of defense that plants use to respond to wounding events. There are physical defense mechanisms that some plants utilize, through structural components, like lignin and the cuticle. [1] The structure of a plant cell wall is incredibly important for wound responses, as both protect the plant from pathogenic infections by preventing various molecules from entering the cell. [1]

Plants are capable of activating innate immunity, by responding to wounding events with damage-associated Molecular Patterns (DAMPs). [1] Additionally, plants rely on microbe-associated molecular patterns (MAMPs) to defend themselves upon sensing a wounding event. There are examples of both rapid and delayed wound responses, depending on where the damage took place.

MAMPs/ DAMPS & Signaling Pathways

Plants have pattern recognition receptors (PRRs) that recognize MAMPs, or microbe-associated molecular patterns. [2] Upon entry of a pathogen, plants are vulnerable to infection and lose a fair amount of nutrients to said pathogen. The constitutive defenses are the physical barriers of the plant; including the cuticle or even the metabolites that act toxic and deter herbivores. Plants maintain an ability to sense when they have an injured area and induce a defensive response. Within wounded tissues, endogenous molecules become released and become Damage Associated Molecular Patterns (DAMPs), inducing a defensive response. DAMPs are typically caused by insects that feed off the plant. [2] Such responses to wounds are found at the site of the wound and also systemically. These are mediated by hormones.[1]

As a plant senses a wound, it immediately sends a signal for innate immunity. [3] These signals are controlled by hormones such as jasmonic acid, ethylene and abscisic acid. Jasmonic acid induces the prosystemin gene along with other defense related genes such as abscisic acid, and ethylene, contributing to a rapid induction of defense responses. Other physical factors also play a vital role in wound signaling, which include hydraulic pressure and electrical pulses. Most of these that are involved within wound signaling also function in signaling other defense responses. Cross-talk events regulate the activation of different roles. [3]

Callose, Damaged Sieve Tube Elements, and P-Proteins

Sieve elements are very rich in sugars and various organic molecules. Plants don't want to lose these sugars when the sieve elements get damaged, as the molecules are a very large energy investment. The plants have both short-term and long-term mechanisms to prevent sieve element sap loss. [2] The short-term mechanism involves sap proteins, and the long-term mechanism involves callose, which helps to close the open channels in broken sieve plates. [2]

The main mechanism for closing damaged sieve elements involves P-proteins, which act as a plug in the sieve element pores. P-proteins essentially plug the pores that form in sieve elements. [2] They act as a stopper in the damaged sieve elements by blocking the open channels so that no additional sap or sugar can be lost. [2]

A longer-term solution to wounded sieve tube elements involves the production of callose at the sieve pores. Callose is a β-1,3 glucan synthesized by callose synthase, which is an enzyme that's localized within the plasma membrane. Callose gets synthesized after the sieve tube elements undergo damage and/or stress. The use of wound callose occurs when callose gets deposited following sieve element damage. [2] Wound callose is proven to first be deposited at the sieve plate pores, or the intracellular connections, where it then spreads to different regions. [2] Essentially, wound callose seals off the parts that were damaged, and separates them from the parts that are still healthy and not broken. Once the sieve elements get fixed, the callose is always dissipated by callose-hydrolyzing enzyme. [2] Callose is also synthesized during normal plant growth and development, and it typically responds to things like high temperatures, or allows the plant to prepare for more dormant seasons. [2]

When the sieve elements get damaged, the sap, sugar, and other molecules inside rush to the end that was damaged. If there was no mechanism to stop the sugars from leaking out the plant would lose an incredibly large amount of invested energy. [2]

Jasmonic Acid

Jasmonic acid (JA) is a plant hormone that increases in concentration in response to insect herbivore damage. The rise in JA induces the production of proteins functioning in plant defenses. JA also induces the transcription of multiple genes coding for key enzymes of the major pathways for secondary metabolites. Its structure and synthesis show parallels to oxylipins, which function in inflammatory responses. [2] JA is synthesized by the octadecanoid pathway, which is activated in response to wound-induced signals. [4] It is a derivative of the most rich fatty acid in the lipids of leaf membranes, alpha-linolenic acid. When plants experience mechanical wounding or herbivory, JA is synthesized de novo and induces genome-wide changes in gene expression. [5] JA travels through plants via the phloem, and accumulates in vascular tissue. [6] JA acts as an intracellular signal in order to promote responses in distal tissues. [6] The perception of jasmonate in distal responding leaves is necessary for recognition of the transmissible signal that coordinates responses to wounding stress. [5] JA mutants, which lack the gene encoding jasmonic acid, are killed by insect herbivore damage that would otherwise not harm normal-type plants. Upon the application of JA to the same mutants, resistance is restored. [7] Signaling agents such as ethylene, methyl salicylate, and salicylic acid can pair with JA and enhance JA responses. [7]

Protections Against Abiotic Stress

Morphological Changes

Plants can protect themselves from abiotic stress in many different ways, and most include a physical change in the plant’s morphology. Phenotypic plasticity is a plant’s ability to alter and adapt its morphology in response to the external environments to protect themselves against stress. [2] One way that plants alter their morphology is by reducing the area of their leaves. Though large and flat leaves are favorable for photosynthesis because there is a larger surface area for the leaf to absorb sunlight, bigger leaves are more vulnerable to environmental stresses. For example, it is easier for water to evaporate off of large surface areas which can rapidly deplete the soil of its water and cause drought stress. Plants will reduce leaf cell division and expansion and alter the shape to reduce leaf area. [2]

Another way that plants alter their morphology to protect against stress is by changing the leaf orientation. [2] Plants can suffer from heat stress if the sun’s rays are too strong. Changing the orientation of their leaves in different directions (parallel or perpendicular) allows plants to reduce damage from intense light. Leaves also wilt in response to stress, because it changes the angle at which the sun hits the leaf. Leaf rolling also minimizes how much of the leaf area is exposed to the sun. [2]

Constitutive structures

Trichomes are small, hair-like growths on plant leaves and stems which help the plant protect itself. Although not all trichomes are alive (some undergo apoptosis, but their cell walls are still present) they protect the leaf by keeping its surface cool and reducing evaporation. [2] In order for trichomes to successfully protect the plant, they must be dense. Oftentimes, trichomes will appear white on a plant, meaning that they are densely packed and are able to reflect a large amount of light off of the plant to prevent heat and light stress. Although trichomes are used for protection, they can be disadvantageous for plants at times because trichomes may reflect light away from the plant that can be used to photosynthesize. [2]

The cuticle is a layered structure of waxes and hydrocarbons located on the outer layer of the epidermis which also helps protect the plant from stress. [2] Cuticles can also reflect light, like trichomes, which reduces light intensity and heat. Plant cuticles can also limit the diffusion of water and gases from the leaves which helps maintain them under stress conditions. Thicker cuticles have been found to decrease evaporation, so some plants will increase the thickness of their cuticles in response to drought stress. [2]

Symbiotic Relationships

Plants are also further protected from both abiotic and biotic stresses when plant growth promoting Rhizobacteria (PGPRs) are present. [8] Rhizobacteria are root-colonizing and non-pathogenic, and they form symbiotic relationships with plants that can elicit stress responsive pathways. PGPRs also improve key physiological processes in plants such as water and nutrient uptake, photosynthesis, and source-sink relationships. [8] Bacteria will respond to substances secreted by plant roots and optimize nutrient acquisition for the plant with their own metabolic processes. Though dependent on the strain, most Rhizobacteria will produce major phytohormones such as auxins, gibberellins, cytokinins, abscisic acid (ABA) and ethylene, which stimulate plant growth and increase the plant’s resistance to pathogens. [9] Other substances are also released by Rhizobacteria, including nitric oxide, enzymes, organic acids, and osmolytes. [9]

See also

Related Research Articles

<span class="mw-page-title-main">Plant hormone</span> Chemical compounds that regulate plant growth and development

Plant hormones are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, including embryogenesis, the regulation of organ size, pathogen defense, stress tolerance and reproductive development. Unlike in animals each plant cell is capable of producing hormones. Went and Thimann coined the term "phytohormone" and used it in the title of their 1937 book.

<span class="mw-page-title-main">Trichome</span> Fine hair-like growth on plants

Trichomes are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a plant is an indumentum, and the surface bearing them is said to be pubescent.

<span class="mw-page-title-main">Jasmonate</span> Lipid-based plant hormones

Jasmonate (JA) and its derivatives are lipid-based plant hormones that regulate a wide range of processes in plants, ranging from growth and photosynthesis to reproductive development. In particular, JAs are critical for plant defense against herbivory and plant responses to poor environmental conditions and other kinds of abiotic and biotic challenges. Some JAs can also be released as volatile organic compounds (VOCs) to permit communication between plants in anticipation of mutual dangers.

<span class="mw-page-title-main">Abscisic acid</span> Plant hormone

Abscisic acid is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to environmental stresses, including drought, soil salinity, cold tolerance, freezing tolerance, heat stress and heavy metal ion tolerance.

<span class="mw-page-title-main">Phytoalexin</span> Class of chemical compounds

Phytoalexins are antimicrobial substances, some of which are antioxidative as well. They are defined not by their having any particular chemical structure or character, but by the fact that they are defensively synthesized de novo by plants that produce the compounds rapidly at sites of pathogen infection. In general phytoalexins are broad spectrum inhibitors; they are chemically diverse, and different chemical classes of compounds are characteristic of particular plant taxa. Phytoalexins tend to fall into several chemical classes, including terpenoids, glycosteroids, and alkaloids; however, the term applies to any phytochemicals that are induced by microbial infection.

<span class="mw-page-title-main">Apoplast</span> Extracellular space, outside the cell membranes of plants

The apoplast is the extracellular space outside of plant cell membranes, especially the fluid-filled cell walls of adjacent cells where water and dissolved material can flow and diffuse freely. Fluid and material flows occurring in any extracellular space are called apoplastic flow or apoplastic transport. The apoplastic pathway is one route by which water and solutes are transported and distributed to different places through tissues and organs, contrasting with the symplastic pathway.

<span class="mw-page-title-main">Methyl jasmonate</span> Chemical compound

Methyl jasmonate is a volatile organic compound used in plant defense and many diverse developmental pathways such as seed germination, root growth, flowering, fruit ripening, and senescence. Methyl jasmonate is derived from jasmonic acid and the reaction is catalyzed by S-adenosyl-L-methionine:jasmonic acid carboxyl methyltransferase.

<span class="mw-page-title-main">Jasmonic acid</span> Chemical compound

Jasmonic acid (JA) is an organic compound found in several plants including jasmine. The molecule is a member of the jasmonate class of plant hormones. It is biosynthesized from linolenic acid by the octadecanoid pathway. It was first isolated in 1957 as the methyl ester of jasmonic acid by the Swiss chemist Édouard Demole and his colleagues.

<span class="mw-page-title-main">Plant defense against herbivory</span> Evolutionary mechanism

Plant defense against herbivory or host-plant resistance is a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores. Many plants produce secondary metabolites, known as allelochemicals, that influence the behavior, growth, or survival of herbivores. These chemical defenses can act as repellents or toxins to herbivores or reduce plant digestibility. Another defensive strategy of plants is changing their attractiveness. Plants can sense being touched, and they can respond with strategies to defend against herbivores. Plants alter their appearance by changing their size or quality in a way that prevents overconsumption by large herbivores, reducing the rate at which they are consumed.

<span class="mw-page-title-main">Systemin</span> Plant peptide hormone

Systemin is a plant peptide hormone involved in the wound response in the family Solanaceae. It was the first plant hormone that was proven to be a peptide having been isolated from tomato leaves in 1991 by a group led by Clarence A. Ryan. Since then, other peptides with similar functions have been identified in tomato and outside of the Solanaceae. Hydroxyproline-rich glycopeptides were found in tobacco in 2001 and AtPeps were found in Arabidopsis thaliana in 2006. Their precursors are found both in the cytoplasm and cell walls of plant cells, upon insect damage, the precursors are processed to produce one or more mature peptides. The receptor for systemin was first thought to be the same as the brassinolide receptor but this is now uncertain. The signal transduction processes that occur after the peptides bind are similar to the cytokine-mediated inflammatory immune response in animals. Early experiments showed that systemin travelled around the plant after insects had damaged the plant, activating systemic acquired resistance, now it is thought that it increases the production of jasmonic acid causing the same result. The main function of systemins is to coordinate defensive responses against insect herbivores but they also affect plant development. Systemin induces the production of protease inhibitors which protect against insect herbivores, other peptides activate defensins and modify root growth. They have also been shown to affect plants' responses to salt stress and UV radiation. AtPEPs have been shown to affect resistance against oomycetes and may allow A. thaliana to distinguish between different pathogens. In Nicotiana attenuata, some of the peptides have stopped being involved in defensive roles and instead affect flower morphology.

<span class="mw-page-title-main">Callose</span> Plant cell wall polysaccharide

Callose is a plant polysaccharide. Its production is due to the glucan synthase-like gene (GLS) in various places within a plant. It is produced to act as a temporary cell wall in response to stimuli such as stress or damage. Callose is composed of glucose residues linked together through β-1,3-linkages, and is termed a β-glucan. It is thought to be manufactured at the cell wall by callose synthases and is degraded by β-1,3-glucanases. Callose is very important for the permeability of plasmodesmata (Pd) in plants; the plant's permeability is regulated by plasmodesmata callose (PDC). PDC is made by callose synthases and broken down by β-1,3-glucanases (BGs). The amount of callose that is built up at the plasmodesmatal neck, which is brought about by the interference of callose synthases (CalSs) and β-1,3-glucanases, determines the conductivity of the plasmodesmata.

<span class="mw-page-title-main">Leucyl aminopeptidase</span> Class of enzymes

Leucyl aminopeptidases are enzymes that preferentially catalyze the hydrolysis of leucine residues at the N-terminus of peptides and proteins. Other N-terminal residues can also be cleaved, however. LAPs have been found across superkingdoms. Identified LAPs include human LAP, bovine lens LAP, porcine LAP, Escherichia coli LAP, and the solanaceous-specific acidic LAP (LAP-A) in tomato.

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β-Aminobutyric acid Chemical compound

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WRKY transcription factors are proteins that bind DNA. They are transcription factors that regulate many processes in plants and algae (Viridiplantae), such as the responses to biotic and abiotic stresses, senescence, seed dormancy and seed germination and some developmental processes but also contribute to secondary metabolism.

Induced systemic resistance (ISR) is a resistance mechanism in plants that is activated by infection. Its mode of action does not depend on direct killing or inhibition of the invading pathogen, but rather on increasing physical or chemical barrier of the host plant. Like the Systemic Acquired Resistance (SAR) a plant can develop defenses against an invader such as a pathogen or parasite if an infection takes place. In contrast to SAR, which is triggered by the accumulation of salicylic acid, ISR instead relies on signal transduction pathways activated by jasmonate and ethylene.

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References

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  6. 1 2 Howe, Gregg A. (September 2004). "Jasmonates as Signals in the Wound Response". Journal of Plant Growth Regulation. 23 (3): 223–237. doi:10.1007/s00344-004-0030-6. ISSN   0721-7595. S2CID   5018217.
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  8. 1 2 Ilangumaran, Gayathri; Smith, Donald L. (2017-10-23). "Plant Growth Promoting Rhizobacteria in Amelioration of Salinity Stress: A Systems Biology Perspective". Frontiers in Plant Science. 8: 1768. doi: 10.3389/fpls.2017.01768 . ISSN   1664-462X. PMC   5660262 . PMID   29109733.
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