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= R-genes =

Introduction
Humans have been associated with plants since ancient times, and plants in general are an important part for the whole ecosystem on earth, the understanding of plant survival up until now is something that have been an important topic for scientists.

Plants in general are in constant symbiotic relationships with fungus, and even bacteria, but there are cases these symbionts can take advantage of this symbiotic relationship in which case will result in a disadvantageous situation for both organisms. R genes protein syntheses are a way of identifying the pathogen effectors and stop their infection throughout the plant system. Molecules essential for pathogen defense are pattern recognition receptors (PRRs), wall associated kinase (WAKs), receptors with nucleotide-binding domain (NLRs) and leucine-rich repeats (LRRs) .All these R proteins play roles in detecting and recognizing pathogen effectors, initiating multiple signal transductions inside the plant cell, these signals transductions will lead to different responses that will aid in pathogen destruction and prevention of further infection. These responses are:

·        Production of Reactive Oxygen (ROS)

·        Hypersensitive Response

·        Closure of the stomata

·        Production of different chemical compounds (terpenes, phenolic, tannins, alkaloids, phytoalexins)

Note that plants have various mechanisms to prevent and detect pathogenic infections, but factors such as geography, environment, genetic, and timing can affect the recognition pattern of a pathogen or can have an effect on the recognition of avirulent (avr) pathogens in plants.

Pathogen Recognition
R genes synthesize proteins that will aid with the recognition of pathogeni effectors:

Pattern recognition receptors (PRRs):

This receptor is often composed of leucine-rich repeats (LRRs). LRRs have a wide range of bacterial (proteins), fungal (carbohydrates) and virulent (nucleic acids) recognition, this means that LRRs recognizes many different molecules but each LRRs usually has a very specific molecule it detects. The ability of PRRs to recognize various pathogenic components relies on a regulatory protein called brassinosteroid insensitive 1 –associated receptor kinase (BAK1). Once the pathogen has been recognized by PRRs the release of a kinase into the nucleus has been transduced triggering a transcriptional reprogramming.

Wall Associated Kinase (WAKs):

The plant cell wall is conformed of pectin and other molecules. Pectin has abundant galacturonic acids which is the compound that WAKs recognizes after a foreign invasion in the plant. Every WAKs (WAK1 & WAK2) has an N-terminal which interacts with pectin in the cell wall when pectin is being degraded to galacturonic acids by fungal enzymes.

Pathogen-associated molecular pattern (PAMPs) and damage-associated molecular pattern (DAMPs) are often identified by lectins which is a protein that binds specific carbohydrates.

Nucleotide-Binding Domain and Leucine-rich repeats (NLRs):

NLRs shifts its conformation from ADP state to and ATP state which allows it to send as signal transduction. The activation of NLRs is yet to be completely understood, according to current studies suggest that it is subject to multiple regulators (dimerization or oligomerization, epigenetic and transcriptional regulation, alternative splicing, and proteasome-mediated regulation)

Despite all these differences NLRs, PRRs, WAKs, effector trigger immunity (ETI) and PAMP-triggered immunity (PTI) there are certain similarities such as in the mechanism of signal transduction which includes mitogen-protein kinase (MAPK) cascades through phosphorylation which will be, calcium ion signaling.

An overall overview about the mechanical interaction about a plant defense and the ability of a pathogen to infect a plant would be for instance such a common interaction between bacterial flagellin and receptor-like kinase which triggers a basal immunity sending signals through MAP kinase cascades and transcriptional reprogramming mediated by plant WRKY transcription factors (Stephen T). Also plant resistance protein recognize bacterial effectors and programs resistance through ETI responses.

Signal Transduction
A plant defense has two different types of immune system, the one that recognizes pathogen/microbes associated molecular patterns (PAMPs), and this is also known as PAMP-triggered immunity (PTI). Plant defense mechanism depends on immune receptors found on the plasma membrane and then the mechanism can sense the pathogen associated molecular patterns (PAMPs) and microbial associated molecular patterns (MAMPs). Detection of PAMPs triggers a physiological change in the cell activated by the pattern recognition receptors (PRRs) initiating a cascade response which through the recognition of PAMPs and MAMPs lead to the plant resistance. The other type of defense is also known as effector-triggered immunity (ETI) which is the second type of defense mediated by R-proteins by detecting photogenic effectors. ETI detects pathogenic factors and initiates a defense response. ETI is a much faster and amplified system than PTI and it develops onto the hypersensitive response (HR) leading the infected host cell to apoptosis. This does not terminate the pathogen cycle, it just slows the cycle down.

Plants have many ways of identifying symbiotic or foreign pathogens; one of these receptors causes fluctuations in the calcium ions and this fluctuation in the calcium ions. A transcription factor plays an important role in defenses against pathogenic invasion.

Pathogen Invasion
The mechanisms utilized by pathogens are very specific for certain types of plants, pathogens have to be able to recognize their host through some recognition processes. Even though plants have very sophisticated mechanisms of protection, pathogens evolve ways to overcome plant defenses in order for the pathogen to infect and spread.

Pathogen elicitors are molecules that stimulate any plant defense; among these elicitors we can find two types of pathogen derived elicitors, pathogen/microbe associated molecular pattern (PAMPs/MAMPs), and also there is a second type which is produced by plants known as damage or danger associated molecular patterns (DAMPs). PTI is a way of responding against pathogen actions happening outside the cell, but a much stronger response like ETI is generated in response to effectors molecules.

Once there is an induced resistance also known as priming, the plant can react faster and stronger to a pathogen attack. A known priming inducer is called β-aminobutyric acid (BABA) which is a non-protein amino acid. There has been a study in which BABA has been integrated in plants and defends it against Hyaloperonospora arabidopsidis and Plectoesphaerella cucumerina (Baccelli, Ivan), there has also been integration of BABA in other studies such as crops.

Successful pathogens evolve changes in their chemical conformation in order to avoid detection by PRRs and WAKs.

Some viruses have mechanisms that allow them to avoid or suppress the RNA-mediated defense (RMD) that some viruses induce in non-transgenic plants. Further studies has shown that this suppression of the host defense has being done by HC-protease (HCPro) encoded in the Potyviral genome, It was later stablished that HCPro was a mechanism used to suppress post-transcriptional gene slicing (PTGs).

Another virus protein is called cucumber mosaic virus (CMV) and it’s a 2b protein, which is also a suppressor of PTGS in Nicotiana benthamiana.

Even though HcPro and the 2b protein have different protein sequence and respective to their own virus, both target the same mechanisms of defense but not in the same way.

Future prospects
Since long ago people are trying to decrease the yield loses of crops and begging to implement conventional breeding methods in order to improve their genetic resistance against multiple pathogens. Nowadays with genetic manipulation and the evolution of plants it is possible to clone these resistance genes and enhance different plants with these types of mechanisms

Scientists have taken different approaches in order to enhance pathogen resistance in plants:

1.       Expression of pathogenesis-related proteins (PR proteins), for instance, hydrolytic enzymes, antifungal proteins, antimicrobial peptides and phytoalexins.

2.       Expression of a gene product that can neutralize a pathogen's arsenal, suchs as polygalacturonase, oxalic acid, and lipase.

3.       Intentionally express these gene products in order to increase the levels of structural defenses (peroxidase & Ligning).

4.       The expression of these gene products induces the release of signal that can regulate plant defenses and this includes the production of elicitors.

5.       The expression of R genes in order to improve hypersensitive responses and interactions with avirulence factors

Reference:
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