Plant Pathology : 1. Plant growth improvement and Stages in development of a disease

 Plant Pathology

1. Plant growth improvement and Stages in development of a disease:

a. Plant growth improvement with respect to disease resistance

b. Stages in development of a disease: Infection, invasion, colonization, dissemination of pathogens andperennation

 Plant Pathology:

Plant Pathology is a branch of agricultural science that deals with the study of fungi, bacteria, viruses, nematodes, and other microbes that cause diseases of plants. 

Plants diseases and disorders make plant to suffer, either kill or reduce their ability to survive/ reproduce.

 Any abnormal condition that alters the appearance or function of a plant is called plant disease. 

The term ‘Pathology’ is derived from two Greek words ‘pathos’ and ‘logos’, ‘Pathos’ means suffering and ‘logos’ Means to study/ knowledge. Therefore Pathology means “study of suffering”. 

Thus the Plant Pathology or Phytopathology (Gr. Phyton=plant) is the branch of biology that deals with the study of suffering plants.

 It is both science of learning and understanding the nature of disease and art of diagnosing and controlling the disease.

Plant Pathology (phytopathology) is defined as the study of the organisms (infectious organisms) and environmental conditions (physiological factors) that cause disease in plants, the mechanisms by which disease occurs, the interactions between these causal agents and the plant (effects on plant growth, yield and quality). 

Plant pathology also involves the study of pathogen identification, disease etiology(the scientific study of the causes of diseases), disease cycles, economic impact, plant disease epidemiology(the scientific study of the spread and control of diseases), plant disease resistance, how plant diseases affect humans and animals, pathosystem genetics(A pathosystem is a subsystem of an ecosystem and is defined by the phenomenon of parasitism. A plant pathosystem is one in which the host species is a plant. The parasite is any species in which the individual spends a significant part of its lifespan inhabiting one host individual and obtaining nutrients from it.), and management of plant diseases.

 It also interfaces knowledge from other scientific fields such as mycology( the study of fungi, a group that includes the mushrooms and yeasts), microbiology, virology, biochemistry, bioinformatics, etc.

 Since plant pathology is directly relevant to man's need to grow enough food and fiber to sustain civilization.

1. Plant growth improvement and Stages in development of a disease:

a. Plant growth improvement with respect to disease resistance

Plants have developed a complex defense system against diverse pests and pathogens. 

Once pathogens overcome mechanical barriers to infection, plant receptors initiate signaling pathways driving the expression of defense response genes.

 Plant immune systems rely on their ability to recognize enemy molecules, carry out signal transduction, and respond defensively through pathways involving many genes and their products.

Investigation into the molecular basis of pathogen resistance reveals a suite of cellular receptors that performs direct detection of pathogenic molecules.

 Pattern recognition receptors (PRRs) within the cell membrane detect pathogen-associated molecular patterns (PAMPs) and wall-associated kinases(WAKs) detect damage-associated molecular patterns (DAMPs) that result from cellular damage during infection .

Receptors with nucleotide-binding domains and leucine-rich repeats (NLRs)detect effectors that pathogens use to facilitate infection . 

PRRs, WAKs, and NLRs initiate one of many signaling cascades that have yet to be completely elucidated.

 Mitogen-activated protein kinases (MAPKs), G-proteins, ubiquitin, calcium, hormones, transcription factors (TFs), and epigenetic modifications regulate the expression of pathogenesis-related (PR) genes This leads to various responses that prevent further infection: hypersensitive response (HR), production of reactive oxygen species (ROS), cell wall modification, closure of stomata, or the production of various anti-pest proteins and compounds (e.g., chitinases, protease inhibitors, defensins, and phytoalexins) . 

As now understood from molecular techniques, pathogen resistance in plants involves various organelles and classes of both proteins and non-protein compounds, each of which are required to regulate defense response (see Figure ).

 Factors in each of these roles affect various other signaling systems, such as growth and abiotic stress response. 

An improved understanding of plant–pathogen interaction requires that we fully describe these molecular interactions that take place when a compatible pathogen interacts with plant tissue. 

First, however, we must briefly describe the pathogens that elicit these responses, and how their evolution has led to the complex immune system that plants possess.

Figure . Components of plant disease resistance mechanisms involved in pathogen detection,

signal transduction, and defense response (detection in the upper center and progressing around

clockwise, ending in defense response in the upper left). 

Pathogenic elicitors (cell components or effectors) produced by bacteria, fungi, insects, nematodes, or viruses trigger plant receptors to initiate signaling cascades.

 Activated receptors (blue) then initiate one of many signal

transduction pathways or directly act as transcription factors (TFs).

 Signal transduction pathways (yellow) include mitogen-activated protein kinase (MAPK) cascades, calcium ion signaling,hormone production, TF activity, and epigenetic regulation. These factors trigger the expression of genes associated with defense responses, such as those regulating the production of reactive oxygen species (ROS), antimicrobial enzymes, defensins(are small cysteine-rich cationic proteins across cellular life, including vertebrate and invertebrate animals, plants, and fungi), and phytoalexins(re low molecular weight antimicrobial compounds that are produced by plants as a response to biotic and abiotic stresses.).

 These defense-related compounds (red) actively inhibit pathogen reproduction or make further infection more difficult. 

Breakdown of pathogenic cell components by defense compounds leads to further release of

receptor-triggering elicitors(are pathogen signal metabolites, recognized by plant cells, which trigger plant defenses. They are produced either by the pathogen or by plant cell components, such as cell wall, upon hydrolyzing action of the pathogen.), increasing the resistance response.

 Multiple organelles are involved in defense response, including chloroplasts and peroxisomes for hormone production as well as the nucleus, endoplasmic reticulum, and Golgi apparatus for antimicrobial protein production.

PRR: Pattern recognition receptors; WAK: wall-associated kinases; NLR: nucleotide-binding domains and leucine-rich repeats; PDR: pathogen-derived resistance; HR: hypersensitive response; TIR:N-terminal Toll/interleukin-1 receptor-like; CC: coiled-coil; SA: salicylic acid; JA: jasmonic acid;ET: ethylene.

b. Stages in development of a disease: Infection, invasion, colonization, dissemination of pathogens and perennation:

Infection:

A  plant is considered to be susceptible to infection if environmental factors alter its physiological processes thus resulting in a disrupted structure, growth, functions, or other parameters.

Plant diseases are classified as infectious and non-infectious depending on the nature of a causative agent. 

The symptoms of the disease may depend on its cause, nature, and the location of the impact site. 

The factors causing plant diseases can be of biotic and abiotic nature. 

Non-infectious diseases are caused by unfavorable growth conditions; they are not transmitted from a diseased plant to a healthy one. 

Infectious diseases, on the contrary, can spread from one susceptible host to another, since the infectious agent can reproduce in the plant or on its surface. 

The signs of plant diseases include wilting, spotting (necrosis), mold, pustules, rot, hypertrophy and hyperplasia (overgrowth), deformation, mummification, discoloration, and destruction of the affected tissue

Invasion:

This occurs when the pathogen invades the plant tissue and establishes a parasitic relationship between itself and the plant. 

Viruses, bacteria, and phytoplasmas are not able to actively penetrate or enter plant host tissues. Therefore they must rely on other methods to infect plant tissues and cells.

Colonization:

Pathogen establishing itself in the host plant. 

These four types of pathogens share certain characteristics regarding the disease cycle.

Fungi, bacteria, and nematodes often survive in crop residue or in the soil. Viruses and bacteria often survive in insect vectors.

 Dissemination of pathogens :

Plant pathogens may be disseminated/transmitted in different ways. 

Fungal and bacterial pathogens may be disseminated largely by wind or water and in some cases with the help of insects. 

Viruses and phytoplasmas on the other hand, predominantly depend on vectors such as insects, mites, nematodes, and fungi for their natural spread under in vivo conditions. 

The infected seeds or vegetatively propagated seed materials form the most important primary sources of infection, irrespective of the nature of the pathogen.

 The infected seeds/seed materials may exhibit some symptoms or show the presence of the fungal or bacterial pathogens when incubated.

 But infection of seeds by viruses or viroids or phytoplasmas may not be discernible, unless special methods of detection are employed. 

Transmission of some of the viruses and viroids through pollen, leading to seed infection, is also recognized.

perennation:

perennation is the ability of organisms, particularly plants, to survive from one germinating season to another, especially under unfavourable conditions such as drought or winter.

 Common forms of perennating organs are storage organs (e.g. tubers, rhizomes and corm), and buds.

It typically involves development of a perennating organ, which stores enough nutrients to sustain the organism during the unfavourable season, and develops into one or more new plants the following year.

Reference:

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https://en.wikipedia.org/wiki/Perennation