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Project/Lecture in BIOL 512
Signal Transduction of Ubiquitin in Plant Immunity

Project article page: Plant Disease Resistance

Mechanism of action
Ubiquitination is a central role in cell signaling that regulates several processes including protein degradation and immunological response. Much of the defense in plants relies on the destruction of defective or invaded materials within the cell, increasing the importance of functional proteasomes and protein targeting. Although one of the main functions of ubiquitin is to target proteins for destruction, it is also useful in signaling pathways, hormone release, apoptosis, and translocation of materials throughout the cell. Even in plants, ubiquitination is a key factor in several immune responses that are vital for the organism's survival. Without ubiquitin's proper functioning, the invasion of pathogens and other harmful molecules would increase dramatically due to weakened defenses in the plant's immunity.

The E3 Ubiquitin ligase enzyme is the main component that provides specificity in the regulation of immune signaling pathways. The E3 enzymes components are determined by which domains they contain and range from several types. These include the Ring and U-box single subunit, HECT, and CRLs. These plant signaling pathways are controlled by several feedback pathways, mainly negative feedback pathways; and they are regulated by De-ubiquitination enzymes, degradation of transcription factors, and the activation of transcription factors.

Types of plant immunity
Plants contain two branches of immunity that differ from the immunity that are present in most animals. The first branch of immunity is called PAMP-Triggered Immunity (PTI) and is the first inducible response performed by plants. It is activated by PAMPS, such as flagellin, toxins, or LPS, and it usually halts the production of pathogenic genes. This is most commonly achieved through the use of Reactive Oxygen Species (ROS) or by reinforcement of the cell wall. This production is the result of the activation of the MAPK Pathway as well as the production and use of hormones.

The second branch of plant immunity is known as Effector Triggered Immunity (ETI) and is activated by the presence of pathogenic effectors. ETI causes a hypersensitive cell death response and an increase in programmed cell death. This process is regulated by nitric oxide production, SA accumulation, potassium, protons, and intracellular calcium pathways.

Transcription factors and the hormone response
Much of a plant's activity is regulated by signaling hormones such as:


 * Salicylic acid (growth, development, transportation)
 * Jasmonic acid (response to stresses, growth inhibition, flower development)
 * Ethylene (growth regulation, signaling, etc.)

Signaling messages can also be regulated by ion signaling, degradation, or negative feedback. The transcription responses from these factors usually induce a primary gene response, which triggers a secondary gene response, and finally the cross-talk interaction of the two causes a system acquired response (SAR) in the plant. These responses are thought to be regulated by transcription pulses, which may be caused by a change in redox reactions that dictate these pulses within 24-48 hour spans. These pulses are mutually exclusive within a single cell, allowing transcription factors to be activated before inducing the SAR.

A majority of the plant's gene expression in the immunological response is regulated by degradation. This degradation can occur by either conformational changes, enhancers, or repressors of various hormones. Some examples of these hormones are:


 * Auxin: binds to target repressor for degradation
 * Jasmonic Acid: leads to degradation of JAZ
 * Gibberellic Acid: Conformational change binds Della and targets with E3 for degradation
 * Ethylene: protects regulators by degrading E3 components by an unknown E3

E3 Signaling
E3 ubiquitin ligases have various roles in the ubiquitin pathway for immune signaling, which include plant defenses. The E3 role in ubiquitination and signaling is of major importance in the immune response, and the primary functioning of the enzymes include the following:


 * Regulators of hypersensitive cell death
 * Regulators of plant resistance
 * Regulators of PAMP (PAMP-Triggered Immunity)
 * Regulators of SAR (System Acquired Response)
 * Regulators of transcription factors
 * Regulators of hormone response

Receptor-like kinase
The receptors found in plants are of similar structure to those of vertebrates and other organisms. These receptor-like kinases function in a similar way to the Receptor tyrosine kinases and partake in various enzymatic activity. The majority of the kinases involve ubiquitin ligase and culminate in the degradation of proteins. Most of the kinase pathways are activated and regulated by the presence of R proteins, phosphorylation, negative regulation, or various mechanisms that are still unknown.

Signaling pathways in innate immunity
Structure and receptor function are conserved among species such as Drosophila, mammals, and even plants. There are some differences in binding proteins and signaling pathways, but they regulate similar endpoints, with a majority of the endpoints being the MAP Kinase Pathway. The signaling for innate immunity in plants is controlled mostly by leucine-rich repeat (LRR) receptors, and it is this receptor that activates the transcription factors via the MAPK Pathway, which activates the plant's immune responses.