After successful completion of the mode of infection stage, the pathogens enter inside the host and establish biological relationship with its cells/tissues, procure their nutrients from them, and favour disease development. For successful establishment of infection inside the host the infecting organism must be in a pathogenic state, the host must be in a susceptible condition, and the environmental conditions must be favourable. However, this is the stage when real and final struggle starts between the pathogen and the host.
The pathogen, after entering into the host, releases a number of enzymes, toxins, growth regulators, and polysaccharide substances to affect the structural integrity of the host cells and their physiological processes and, contrary to it, the host reacts with a variety of defence mechanisms to protect itself from the attacking pathogen.
If the pathogen defeats the defence mechanisms adopted by the host, the infection is established and disease manifests and, finally, the symptoms appear. And if the pathogen is defeated by the defence of the host, disease is discarded and the host plant remains healthy.
In this article, we shall discuss the various mechanisms of attack by the pathogen on the host. After successfully entering inside the host, the pathogens bring out disturbances in structural integrity and physiological processes of the host by producing certain chemical substances.
These substances, mainly enzymes, toxins, growth regulators and polysaccharides, are thought of as the chemical weapons of the pathogen by which it tries to overcome the host-defence-system and succeed in disease establishment in host. However, among the plant pathogens, all except viruses and viroids can probably produce these substances. Plant viruses and viroids do not produce any such substances themselves, but they induce the host cell to result in required injurious substances.
Substance # 1. Enzymes:
Enzymes are considered more commonly produced than any other injurious substances by the pathogens inside the host to help disease establishment. Plant pathogenic enzymes disintegrate the structural components of the host cells, breakdown inert food substances in the cell, or affect the protoplast directly and interfere with its functioning systems. In 1980s, six criteria were set out with respect to the involvement of degradative enzymes in pathogenesis or virulence, to which two other have been added more recently.
These criteria are:
1. Ability of pathogen to produce enzyme in vitro.
2. Enzymes must be detected in diseased tissue.
3. Depletion of plant material such as middle lamella.
4. Establishment of correlation between enzyme production and pathogenecity or virulence.
5. Symptom reproduction in host when added with purified enzymes.
6. Symptom reduction in vivo after the enzyme activity is inhibited.
7. Fusion of the promoter of gene specifying the enzyme of interest to a reporter gene (e.g., GUS that encodes glucuronidase).
8. Genetically engineering alterations in enzyme synthesis and manifestation of corresponding alterations in pathogenicity or virulence.
Substance # 2. Toxins:
It was Anton de Bary in the 19th century who first suggested that one way in which plant pathogens damage their hosts was to secrete toxins into them. Gaumann in 1954 went further and wrote that “microorganisms are pathogenic only if they are toxigenic”.
Although many of Gaumann’s contemporaries might not have agreed with this proposition at that time. But, remarkable progress in phytopathological research during the last three decades and achievements made therefrom, especially in the area of phytopathogenesis, stand close to Gaumann’s claim. Several toxins are known today which undoubtedly establish their significant role in disease development, and many more are awaiting experimental confirmations.
Toxins, the extremely poisonous substances effective in very low concentration, are produced by phytopathogenic microorganisms inside host during the course of their attack. Toxins so produced act directly on living host protoplast by affecting the permeability of the cell membrane or by activating or inhibiting enzymes necessary for various metabolic reactions. In this way, they result in serious damage, or killing of plant cells.
Toxins affecting plants are divided into two classes – non-host-selective or non-host-specific toxins (NSTs) and host-selective or host-specific toxins (HSTs). Non-host-selective or non- host-specific toxins (NSTs) produce all or part of the disease syndrome not only on the host plant but also on other species of plants that are not normally attacked by the pathogen in nature.
Such toxins increase the extent of disease caused by a pathogen but are not essential for the pathogen to cause disease. In contrast, host-selective or host-specific toxins (HSTs) affect only plants that are hosts of the toxin-producing microbial pathogen and are usually essential for disease development. They show little or no toxicity against non-susceptible plants.
The significance of non-host-selective toxins (NSTs) in disease development is more difficult to establish than that of host-selective toxins (HSTs) since the former usually only contribute to virulence, whereas the latter are necessary for disease development.
Evidence for the role of an NST usually relies upon one or more of the following principles:
1. The pathogen must produce the toxin in vitro as well as in vivo.
2. The purified toxin must reproduce disease symptoms after inoculation on the host.
3. There should be correlation of the pathogen with toxigenicity.
4. There should be correlation of susceptibility of the host with toxin sensitivity.
5. Virulence of the pathogen should be attenuated when genes involved in toxin synthesis are disrupted.
6. Pathogen virulence should be restored in toxin-minus mutants by transformation with the genes required for toxigenicity.
Substance # 3. Growth Regulators:
Growth regulators (auxins, gibberellins, cytokinins, ethylene, and abscisic acid) are the substances that naturally occur in plants, act as hormones therein, and regulate plant growth.
They act in extremely low concentrations, and even a slight deviation from their normal concentration generally results in serious disturbances in natural pattern of plant growth. It has been found that various phytopathogenic microorganisms, while affecting their host plants, produce growth regulators and, in this way, cause imbalance in the natural concentrations of growth regulators in plants.
This result in abnormal growth patterns in such plants. Symptoms such as stunting, overgrowth or malformation of host tissue, resetting, excessive root branching, epinasty, defoliation, witches’ broom effects, etc. represent some abnormal growth patterns in plants due to imbalance in natural concentration of growth regulators in plants.
Substance # 4. Polysaccharides:
Though variety of phytopathogens constantly releases slimy polysaccharides in the host body, the role of polysaccharides in disease development appears to be limited mainly to vascular wilt diseases. It is thought that polysaccharides released by pathogen in the xylem block the xylem vessels and thus initiate wilting. However, knowledge regarding the role of polysaccharides in disease development is still in its infancy and needs further investigations.
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