Immunity in Plants

We take antibiotics, vaccines, and other treatments to save ourselves from bacterial, viral, and other pathogens. Have you ever thought about how plants can save themselves? The answer to the above question is that plants have strong immune responses to cope up with bacterial and viral infections. The next question that arises in my mind is, Do they have a memory to recognize a secondary attack? I am sure most of your answers would be a big ‘No’. However, this is not the case. Plants do have a memory to recognize a pathogen that has previously attacked.

EXPERIMENTS CONDUCTED TO PROVE IMMUNE RESPONSE IN PLANTS

The presence of immunity in plants is experimentally proven. On the 27th of March,1961, a publication by A. Frank Ross of Cornell University, titled: Systemic Acquired Resistance Induced by a localized viral infection in plants proved the presence of the immune system in plants. Several experiments were conducted on a transgenic Tobacco plant, deliberately infecting it with Pseudomonas syringae. There were two setups: one of the plants had previous exposure to the bacteria, and the other was not exposed, acted as control setup. There was a significant difference in the rate of infection shown by the two plants. The plant which had previous exposure to the bacteria did not get infected for the second time. This concluded that either the pathogen became weak or the plant learned from its earlier experiences.

Another experiment conducted in which only one leaf of the plant was infected by bacteria deliberately. The rest of the leaves did not get affected on second exposure to the bacteria. In the next setup, the phloem was removed from the leaf stem by girdling. It was astonishing to note that, upon second exposure to the bacteria, the other leaves got infected. Thus it was concluded that the primary inoculated leaves send signals to the rest of the plant to generate infection memory.

Systemic Acquired Resistance Induced by a localized viral infection in plants.

There is a significant difference between the two leaves. The left one was plucked from a plant that was earlier exposed to the bacteria and the right one is plucked from a plant that was exposed to the bacteria for the first time.

Another interesting observation was that, if the pathogen exposed leaf was detached from the plant within a few hours, then the leaf is unable to send signals to the rest of the plant. It takes about 2 to 3 days to generate a memory response. These responses are graft transmissible and are passed through seeds. The chemical nature of the transmission factor can be detected by examining the petiole exude by HPLC(High-performance liquid chromatography). If the pathogen exposed leaf is removed from the plant after 3 days, then the response can spread. The rest of the plant can generate infection memory even after the removal of the pathogen exposed leaf.

TYPES OF PLANT PATHOGENS

There are 3 types of plant pathogens

1) Necrotroph- They first kill the plant host by secreting cell wall degrading enzymes.

2) Biotroph- They try to keep the plant host alive, they avoid the plant cell from elicitating defense responses.

3)Hemibiotroph- They initially infect the plant as biotroph but later switch themselves as necrotrophs.

The various steps of Plant-Pathogen interaction are – Infection, Recognition, Local resistance, Systemic Acquired resistance.

A Bacteria injecting Effector molecules into the cell( Effector mediated recognition) and Recognition by Molecular patterns operating side-by-side. 

Infection: The only pathogens that can enter a plant cell are the viruses, others like bacteria, nematodes inject toxins into the cell by disintegrating the plasma membrane. The Type III secretion system in bacteria releases Virulence factors.

Recognition:  Pathogen recognition occurs by two methods

1) Recognition of pathogens by Molecular patterns: MAMPS and PAMPS stand for Microbe and pathogen-associated molecular patterns respectively. In these cases, the proteins present on the surface of pathogens when recognized by receptors on the plant cell wall elicit defense response by signal transduction mechanism.

2)Effector mediated recognition: The release of effector molecules by microbes triggers immunity. Effector molecules released are by microbes to suppress an immune response. Here comes Gene for a gene Hypothesis in which the product of one gene of the pathogen should be compatible with the product of the plant gene. For example the R-Avr gene interaction. The binding of Avirulence protein present on the pathogen to the R-protein present on the plant cell wall elicits defense response in plants.

Local Resistance: local resistance is provided by the cell wall, cuticle covering, callose deposition, accumulation of secondary metabolites like terpenoids, alkaloids, and stomatal closure.

Systemic Acquired Resistance: Immune response can be regulated at the transcriptional level by epigenetic control of the mobile molecules injected by the pathogen. The secretion of Pathogenesis related proteins, Hypersecretory responses leading to accumulation of hormones like Jasmonic acid and Salicylic acid.  These PR proteins epigenetically control the gene expression thereby suppressing some and expressing some other region. These responses have faster induction kinetics and such responses are non-specific(broad-spectrum). The epigenetic modifications include Histone modification, DNA methylation, DNA acylation, Sulfonation, Ubiquitination, and phosphorylation.

Hope I can convince all the readers of this article that like animals, plants also have an immune response. Although they lack a circulatory system, they do have memory recognition immunity.