Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events. Most commonly, protein phosphorylation is catalyzed by protein kinases, ultimately resulting in a cellular response. Proteins responsible for detecting stimuli are generally termed receptors, although in some cases the term sensor is used. The changes elicited by ligand binding in a receptor give rise to a biochemical cascade, which is a chain of biochemical events known as a signaling pathway.
Arabidopsis thaliana, the thale cress, mouse-ear cress or arabidopsis, is a small plant from the mustard family (Brassicaceae), native to Eurasia and Africa. Commonly found along the shoulders of roads and in disturbed land, it is generally considered a weed.
Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the binding of Ca2+ is required for the activation of calmodulin. Once bound to Ca2+, calmodulin acts as part of a calcium signal transduction pathway by modifying its interactions with various target proteins such as kinases or phosphatases.
Flagellin is a globular protein that arranges itself in a hollow cylinder to form the filament in a bacterial flagellum. It has a mass of about 30,000 to 60,000 daltons. Flagellin is the principal component of bacterial flagella, and is present in large amounts on nearly all flagellated bacteria.
Brassinosteroids are a class of polyhydroxysteroids that have been recognized as a sixth class of plant hormones and may have utility as anticancer drugs for treating endocrine-responsive cancers by inducing apoptosis of cancer cells and inhibiting cancerous growth. These brassinosteroids were first explored during the 1970s when Mitchell et al. reported promotion in stem elongation and cell division by the treatment of organic extracts of rapeseed pollen. Brassinolide was the first brassinosteroid to be isolated in 1979, when pollen from Brassica napus was shown to promote stem elongation and cell divisions, and the biologically active molecule was isolated. The yield of brassinosteroids from 230 kg of Brassica napus pollen was only 10 mg. Since their discovery, over 70 BR compounds have been isolated from plants.
Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed, mainly, by cells of the innate immune system, such as dendritic cells, macrophages, monocytes, neutrophils and epithelial cells, to identify two classes of molecules: pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens, and damage-associated molecular patterns (DAMPs), which are associated with components of host's cells that are released during cell damage or death. They are also called primitive pattern recognition receptors because they evolved before other parts of the immune system, particularly before adaptive immunity. PRRs also mediate the initiation of antigen-specific adaptive immune response and release of inflammatory cytokines.
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.
The gene-for-gene relationship is a concept in plant pathology that plants and their diseases each have single genes that interact with each other during an infection. It was proposed by Harold Henry Flor who was working with rust (Melampsora lini) of flax (Linum usitatissimum). Flor showed that the inheritance of both resistance in the host and parasite ability to cause disease is controlled by pairs of matching genes. One is a plant gene called the resistance (R) gene. The other is a parasite gene called the avirulence (Avr) gene. Plants producing a specific R gene product are resistant towards a pathogen that produces the corresponding Avr gene product. Gene-for-gene relationships are a widespread and very important aspect of plant disease resistance. Another example can be seen with Lactuca serriola versus Bremia lactucae.
Brassinolide is a plant hormone. The first isolated brassinosteroid, it was discovered when it was shown that pollen from rapeseed could promote stem elongation and cell division. The biologically active component was isolated and named brassinolide.
Integrin-linked kinase is an enzyme that in humans is encoded by the ILK gene involved with integrin-mediated signal transduction. Mutations in ILK are associated with cardiomyopathies. It is a 59kDa protein originally identified in a yeast-two hybrid screen with integrin β1 as the bait protein. Since its discovery, ILK has been associated with multiple cellular functions including cell migration, proliferation, and adhesion.
Peptide signaling plays a significant role in various aspects of plant growth and development and specific receptors for various peptides have been identified as being membrane-localized receptor kinases, the largest family of receptor-like molecules in plants. Signaling peptides include members of the following protein families.
Resistance genes (R-Genes) are genes in plant genomes that convey plant disease resistance against pathogens by producing R proteins. The main class of R-genes consist of a nucleotide binding domain (NB) and a leucine rich repeat (LRR) domain(s) and are often referred to as (NB-LRR) R-genes or NLRs. Generally, the NB domain binds either ATP/ADP or GTP/GDP. The LRR domain is often involved in protein-protein interactions as well as ligand binding. NB-LRR R-genes can be further subdivided into toll interleukin 1 receptor (TIR-NB-LRR) and coiled-coil (CC-NB-LRR).
BRI1-associated receptor kinase 1 is an important plant protein that has diverse functions in plant development.
Mitogen-activated protein kinase (MAPK) networks are the pathways and signaling of MAPK, which is a protein kinase that consists of amino acids serine and threonine. MAPK pathways have both a positive and negative regulation in plants. A positive regulation of MAPK networks is to help in assisting with stresses from the environment. A negative regulation of MAPK networks is pertaining to a high quantity of reactive oxygen species (ROS) in the plant.
Leucine-rich repeat receptor like protein kinase are plant cell membrane localized Leucine-rich repeat (LRR) receptor kinase that play critical roles in plant innate immunity. Plants have evolved intricate immunity mechanism to combat against pathogen infection by recognizing Pathogen Associated Molecular Patterns (PAMP) and endogenous Damage Associated Molecular Patterns (DAMP). PEPR 1 considered as the first known DAMP receptor of Arabidopsis.
EF-Tu receptor, abbreviated as EFR, is a pattern-recognition receptor (PRR) that binds to the prokaryotic protein EF-Tu in Arabidopsis thaliana. This receptor is an important part of the plant immune system as it allows the plant cells to recognize and bind to EF-Tu, preventing genetic transformation and protein synthesis in pathogens such as Agrobacterium.
Botrytis–induced kinase 1 (BIK1) is a membrane-anchored enzyme in plants. It is a kinase that provides resistance to necrotrophic and biotrophic pathogens. As its name suggests, BIK1 is only active after being induced by Botrytis infection. When Botrytis cinerea is present, the BIK1 gene is transcribed so that the kinase is present to defend the cell. BIK1 functions to regulate the amount of salicylic acid (SA) present in the cell. When Botrytis cinerea or Alternaria brassicicola or any other necrotrophic pathogen is present, BIK1 is transcribed to regulate the pathogen response mechanisms. When BIK1 is present, SA levels decrease, allowing the nectrotrophic response to take place. When nectrotrophic pathogens are not present, BIK1 is not transcribed and SA levels increase, limiting the necrotrophic resistance pathway. Only the pathogenic defense response that is initiated by BIK1 is dependent on SA levels. Non-pathogenic cellular functions occur independently. In terms of non-pathogenic cellular functions, BIK1 is described as a critical component of ET signaling and PAMP-triggered immunity to pathogens.
PEPR 1 and PEPR2 are homolog kinases that act as enzymes on other proteins. They attach a phosphate group to specific proteins, called phosphorylation. These reactions can cause the function of the phosphorylated proteins to change. Both PEPR 1 and PEPR 2 can be classified as receptor kinases, which serve an important role in immunity in plants. Receptor kinases have the ability to change the conformation of receptors by adding the phosphate group. These specific receptor kinases serve as a pattern recognition receptor, or PRR, that can quickly and efficiently recognize many different molecular patterns or signatures that are unique to each pathogen. They can also detect different danger signals released from the host and respond accordingly. More specifically, the proteins contain leucine-rich repeat segments that interact outside of the cell. This leucine-rich repeat is a structural motif present in some proteins that has specific functions due to its folded structure. This fold can contain many repeating amino acids, but the most common is the hydrophobic leucine, hence the name. PEPR1 and PEPR2 are present in plants and are involved in several immune system processes. Their ability to change the conformation of receptors can have an effect on signaling processes within plants, allowing the plant to have a system of immunity in place in case of an infection or pathogen.
FLS genes have been discovered to be involved in flagellin reception of bacteria. FLS1 was the original gene discovered shown to correspond with a specific ecotype within Arabidopsis thaliana. Even so, further studies have shown a second FLS gene known as FLS2 that is also associated with flagellin reception. FLS2 and FLS1 are different genes with different responsibilities, but are related genetically. FLS2 has a specific focus in plant defense and is involved in promoting the MAP kinase cascade. Mutations in the FLS2 gene can cause bacterial infection by lack of response to flg22. Therefore,FLS2’s primary focus is association with flg22 while its secondary focus is the involvement of promoting the MAP kinase cascade in plant defense.
Ethylene signaling pathway is a signal transduction in plant cells to regulate important growth and developmental processes. Acting as a plant hormone, the gas ethylene is responsible for promoting the germination of seeds, ripening of fruits, the opening of flowers, the abscission of leaves and stress responses. It is the simplest alkene gas and the first gaseous molecule discovered to function as a hormone.
