Hi,
Please discuss the following questions in group.
1. Fatimah group
Q: Discuss the plant-pathogen interactions.
2. Melanie group
Q: Prokaryotes as plant pathogens.
3. Kenneth group
Q: Fungi, water mold, viruses as plant pathogens.
4. Quek group
Q: Discuss the existing approaches to combat plant disease.
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ReplyDeleteKenneth Group (Fungi as Plant Pathogens by rayyeow).
ReplyDeleteFungi pathogens are heterotrophic organisms that requires external source of nutrients to support growth, development and reproduction. Generally, fungi have common feature known as hyphae, the thread-like strands with filamentous growth habit. Hyphae are used by fungi to colonize plants through host surface especially through wounded part. Some fungi penetrate intact plant surface directly.
Plant pathogenic fungus are able to produce extra cellular hydrolytic enzymes to penetrate into host plant system such as :-
i) Depolymerases
ii) Glycosidase
iii) Pectinase
iv) Protease
Enzymes stated above enables physical structural penetration such as waxes, cuticle and cell wall of plants. Another type of enzyme known as pectic enzymes is very efficient in degrading plant tissues. Degradation of plant cells enables fungi to tap into the nutrient pool stored within the plant.
Most phytopathogenic fungi belongs to Ascomycetes and Basidiomycetes that produce spores easily transported through air or water. Many of these species are capable of living in the soil.
Examples of famous fugal-plant disease:
i) Ganoderma boninense - Basal Stem Rot disease caused by Bracket fungus that devastates the oil palms plantation in Malaysia (Siddiquee et al., 2009).
ii) Sclerotium rolfsii - Wide host range of at least 500 species including banana, durian and rice in Malaysia. (Jyothi, 2006).
iii) Fusarium sp. - Panama disease of banana, some species produce mycotoxins that is very immunogenic for human if carried along with crops.
iv) Phythium gramnicola - Infect sugarcane plantation in Malaysia.
References:
1. Siddiquee, S., Yusuf, U.K., Hossain, K. & Jahan, S. (2009). In vitro Studies on The Potential Trichoderma harzianum For Antagonistic Properties against Gahoderma boninense. Journal of Food, Agriculture & Environment. 7(3&4). 970-976.
2. Jyothi, K.C. (2006). Morphological and Molecular Variability Among the Isolates of Sclerotium rolfsii Sacc. From Different Host Plants. University of Agricultural Sciences, Dharwad. 7.
Kenneth Group (Virus as plant pathogen by Sii Longwin & Saw Jun Hao)
ReplyDeleteInsects are the common vectors to carry viral particles to plants because plants do not move. Viruses are transmitted from insects to plants by invading physical defense mechanism on plants such as cuticles and epidermis layer of plant during insects feeding on plants. Upon penetrating xylem and phloem tissues, insects release viral particles and viruses start to replicate within host cells.
Evolutionary resistance
Plants have evolved resistance gene (R gene) whose gene products allow recognition of adapted viral effectors or by recognition of the alteration that the effector has caused to a host protein. R genes are usually NBS-LRR genes. NBS-LRR genes encode nucleotide-binding site (NBS) and a leucine rich repeat (LRR) that allow directly binding to Avirulence gene (Avr gene) products and detect protein degradation by Avr gene. The plant R gene and the viral avirulence gene (effector gene) products must have matched specificity for that R gene to confer resistance (Mondragon-Palomino et al., 2002). Besides that, plants often produce siRNA to bind complementarily to viral RNA. Bound RNA is silenced and susceptible to enzymatic degradation (McDowell & Woffenden, 2003).
Artificial resistance
Biotechnology can help in making a virus resistant plant, and the most common way of doing so is by inserting viral gene encoding the virus’ coat protein. The plant will then be able to express the coat protein due to the inserted viral gene, cosuppressing the virus from proliferating within the plant parts as plants have their own mechanisms to shut down the protein expression of virus. The synthesis of viral coat protein will be disrupted when virus infect a certain GM plant (GMO Compass, 2006). It is also reported that GM plant infected by virus has lower viral antigen, showing that the rate of replication of virus is reduced due to cosuppression by the GM plant (Pribylova et al., 2006).
Reference:
1. GMO Compass (2006). Disease Resistance. GMO Compass: Breeding Aims. Retrieved from http://www.gmo-compass.org/eng/agri_biotechnology/breeding_aims/148.disease_resistant_crops.html on 21 November, 2011.
2. McDowell, J.M. & Woffenden, B.J. (2003). Plant Disease Resistance Genes: Recent Insights & Potential Applications. Trends in Biotechnology, 21(4), 178-183. DOI: 10.1016/S0167-7799(03)00053-2
3. Mondragon-Palomino, M., Meyers, B.C., Michelmore, R.W. & Gaut, B.S. (2002). Patterns of Positive Selection in the Complete NBS-LRR Gene Family of Arabidopsis thaliana. Cold Spring Harbor Laboratory Press, 12, 1305-1315. DOI: 10.1101/gr.159402
4. Pribylova, R., Pavlik, I. & Bartos, M. (2006). Genetically Modified Potato Plants in Nutrition & Prevention of Diseases in Humans & Animals: A Review. Veterinarni Medicina, 51(5), 212-223.
Water molds, also nown as oomycetous plant pathogens, comparises of both foliar and soiborne organisms that confer disease to plants in under wet and warmt condition. Water mold has several distinguish features that sets them apart from the true fungi, such features include:
ReplyDeletea. Lack of cell walls in hypahe
b. Diploid nuclei of vegetative cells
c. Cell walls composed of beta-1,3 and beta1-6 glucans
d. Produce biflagellated swimming spores
Examples of several species of water molds that confer diseases to plants are:
a. Phytophthora infestans,
¬-Causal agent of late blight on potatoes and tomatoes
b. Phytophthora capsici,
-Causal agent of Phytophthora crown and fruit rot on tomatoes, peppers, squash, and cucumbers
c. Pseudopersonospora cubensis.
-Causal agent of downy mildew on cucumber
The wet and warm conditions promotes the inititation and development of epidemic levels of laet blight, Phytophthora crown and fruit rot, and downy milder in vegetable and potato crops.
Symptoms of water mold infections:
(Source: Amanda, J.G., 2011)
References:
Amanda, J.G. (2011). Understanding and Managing the Pathogens Causing Late Blight, Downy Mildew, and PPhytophthora Fruit and Crown Rot. Proc. Of the 2011 Winconsin Crop Management Conference. 50.
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ReplyDelete4. Quek group (Quek shiu chong, Gan kah hui, Low Benji, Low Chee Hoe, Jason Lee)
ReplyDeleteQ: Discuss the existing approaches to combat plant disease.
At present, there were three main approaches to combat with the plant based disease, included chemical approach, biological control strategies and transgenic approach.
Chemical approach:
Many plant’s disease was controlled through the use of expensive chemical which able to kill both the pathogen and the vector that carry it, this method although consider effective, but it caused negative effective toward human health as well as environment. For example, fumigant methyl bromide a highly toxic compound used to kill rodents, insects, mites, and a range of pathogenic organisms in soil, compost, and timber (Roosels et al, 1982). In USA, around 30000 ton of this chemical had been used to fumigate the soil before planting as well as fumigated the harvested crop.
Biological control strategies:
Biological control strategies were an alternative which was relatively cleaner compared to chemical approach. In this approach, microorganism was used to combat the plant’s disease, for example, various species of Agrobacterium was used to limit the occurred of crown gall disease, such as Agrobacterium rhizogenes 84.
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ReplyDeleteTransgenic approaches:
ReplyDeleteAmong all three methods, transgenic method was the preferred approaches because of its efficiency, cost and relatively less negative effect to the environment. The designed and used of the transgenic approaches was based on the plant’s natural mechanism of disease resistant, included four levels of defenses: (1) anatomical defenses, (2) pre-existing protein and chemical protection, (3) inducible system and (4) systemic response.
(1) Anatomical defenses
Before pathogen can infected the plant, it must be first getting excesses through the exterior part of the plant, which was difficult. This exterior part was the morphological structure and structural system of the plant, such as bark, waxes etc.
(2) Pre- existing protein and chemical protection
This level of defenses depend on the antimicrobial proteins produced by the plant, included the defensin and defensin-like protein. The defensin protein was an important protein which able to fight against the infectious agents, for example, some type of defensin can caused slow growth of fungus.
(3) Inducible system
Third level defence - a switch to counterattack that relies on protein systhesis de novo.
Elicitor response
The inducers, or elicitors (pathogen) that gain entry or arrives into a living cell, it may induce resistance to infection. First, the local response involves interaction with molecules released by pathogen (exogenous elicitor) or the host itself response to pathogen attack (endogenous elicitor) by the receptor molecules on the host.
This will stimulate general defense systems in both host and non-host plants OR race-specific interactions inducing defense responses to specific cultivars. Both the interactions may lead to cascade of reaction that includes hypersensitive response (HR).
a) General response
Damage of plant cell wall in first stage of infection by fungi or bacteria. They release proteolytic enzymes (e.g. oligogalacturonides) cause release wall fragments pectic-oligomers.
Thus, endogenous elicitors which act as a general response bind to specific receptors leading to induction of specific defence genes.
Specific defense genes involve coding of:
(1) Enzymes – synthesize structural components for cell wall thickening (to repair damage)
(2) Enzymes for secondary metabolism
(3) Lectins (multimeric sugar-binding pt that agglutinate cells)
(4) And many pathogenesis-related (PR) proteins.
b) Race-specific response
The pathogens which carried the avirulence gene (Avr) codes protein recognized by a specific receptor proteins in plant cell, encoded by resistance gene (R). However, if there is absent of one of the gene, this gene-gene system will cause disease in the plant.
This interaction induces
(1) the HR, which is manifested as a local necrosis
(2) many pathogenesis-related (PR) proteins
c) Elicitor receptors (R proteins)
They are fit into five basic structural groups. The biggest group having a nucleotide-binding site and a leucine-rich repeat region (NB-LRR proteins). However, the other four groups are structurally diverse.
Plant use receptors of similar structure comparison for the sensing of different types of elicitor that have evolved from a limited number of ancient receptors. Many elicitor proteins are key factors in the biology of the pathogen, favour plant defence mechanisms. Some plant R proteins come from mutation of LRR domain.
continue...
(4) Systemic responses
ReplyDeleteThe induction of local defense pathways may lead to the induction of intercellular signals that produce a systemic response which called systemic acquired resistance (SAR).
SAR has two phases :
(1) the initiation phase (cells at the foci of the infection release signal molecules, transported to target cells where SAR genes are expressed)
(2) the maintenance phase (quasi-steady-state resistance against virulent pathogens)
Induced systemic resistance (Other disease resistance mechanisms)
Induced by avirulent pathogens through pathway which induce ethylene or jasmonic acid (JA) as messengers thus, result in induction of different classes of defence proteins.
Host Resistance (HR)
This is the plant innate immune system. The host resistance is dependent upon the interaction of an R gene with a type III protein-secretion system (TTSS) protein leads to HR reaction
Non-host Resistance
The non-host resistance is more complex and composed of various defence genes including R proteins
2 types
(1) Does not induce an HR reaction and involves general defence mechanism eg the physical barriers of the cell wall, antimicrobial compounds, secondary metabolites followed by the response to pathogen-associated molecular patterns (PAMPs).
(2) Produce a non-host HR reaction
When the pathogen enters the cell, it will
(a) Response to cell surface elicitors, (fungi or oomycetes)
(b) TTSS-R-gene interaction (bacteria)
References
Roosels D, R U Van Den Oever, and Lahaye D. (1982). Actual hazard of methyl bromide fumigation in soil disinfection. Br J Ind Med. 39(2): 140–144.
Slater, A, Scott, NW & Fowler, MR. Plant Biotechnology: The Genetic Manipulation of Plants. 2nd ed., Oxford University Press, 2008.
Melanie's group (Prokrayotes as plant pathogens)
ReplyDeletePart A
Prokaryotes, including bacteria and mollicutes as mycoplasma-like organism, cause important disease in plants. Hence, some prokaryotes are as plant pathogens. Prokayotes can be divided into two major categories, bacteria and archea. They are unicellular microorganisms with genetic material. They have thin peptidoglycan cell wall and lipid membrane to enclose the genetic material (DNA).
For bacteria, it is the largest group of plant pathogenic prokaryotes. Most plant pathogenic bacteria are rod-shaped. Most phytopathogenic bacteria are either Gram-negative or Gram-positive bacteria. Bacteria that associated with plant can be beneficial of detrimental. Only around 100 species of bacteria is pathogenic whereas most of the bacteria are having saprotrophic (epiphytes or endophytes) relationship with plants. They grow mostly as parasites in their host plants, on plant surface and partly in plant debris. In the lab they can be seen when they grow as colonies on the surface of nutrient agar. A bacteria population of 106 CFU (colon- forming units/milliliter) or higher are needed to cause infectious disease. There are five main types of bacterial pathogenicity factors, cell wall degrading enzymes, toxins, effector proteins, phytohormones, exopolysaccharides. In general, the route of plant infection by pathogenic bacteria is through the physical damaged on plant. For instance, wound on plant will release chmoattratants and provide an opening for entry of bacteria. Bacteria can enter through cuts our other areas of damage that occur in the leaves or stems due to wind bending the plant, objects hitting the plant, aphids sucking the plant sap or animals grazing on it. Some bacteria produce enzymes that break down the cell walls of plants anywhere in the plant. They do this to break open to cell to gain access to the nutrients inside but the plant cells affected die quickly, causing parts of the plant to start rotting. Some bacteria produce toxins that are damaging to plant tissues generally, usually causing early death of the plant. Others produce large amounts of polysaccharide sugars that have long chains and are very sticky. As these travel in the water carrying vessels, the xylem, they block the narrow channels, preventing water getting from the plant roots up to the shoots and leaves, again causing rapid death of the plant. Finally, some bacteria produce proteins that mimic plant hormones. These lead to overgrowth of plant tissue and tumours form.
Melanie's group (Prokrayotes as plant pathogens)
ReplyDeletePart B
Plant disease caused by bacteria is more prevalent in subtropical and tropical regions of the world. Bacteria are among the microbes that successively colonize plants as they mature. As a plant pathogen, it can cause many serious diseases throughout the world but fewer than fungi or viruses and they cause relatively less damage and economic cost. Although plant pathogenic bacteria exist throughout the world, most economic and wild plants have established innate immunity or resistance to many pathogens. Plant pathogenic bacteria impact innumerable and valuable agricultural crops, causing hundreds of millions of dollars in damage each year. However, throughout the exploration of biotechnology, some plant pathogens have been proven to be useful in agriculture and food production. The thickening agent, xanthan gum, is an extra-cellular polysaccharide derived from the plant pathogen Xanthomonas campestris pv. campestris and is found in an enormous variety of products. Transformation or genetic engineering of plants is best carried out by disarmed vectors (plasmids) of Agrobacterium tumefaciens. The elimination of a gene from a nonpathogenic Pseudomonas syringae that codes for ice formation at relatively high temperatures made history in an ice-minus derivative that prevents frost damage when applied to plants. Other properties await discovery and exploitation.
On the other hand, plant associated archaeal pathogen has not been identified yet. There is a general assumption that archeal pathogens do not exist. However, archea have the potential to be discovered as pathogens.
References:
Vidaver, A.K. & Patricia, A.P. (2004). Bacteria as plant pathogens. The plant Health Instructor. Retrieved from: http://www.apsnet.org/edcenter/intropp/PathogenGroups/Pages/Bacteria.aspx
Cavicchioli, R., Curmi, P.M., Saunders, N. & Thomas, T. (2003). Pathogenic archaea: do they exist? Bioessays. Retrieved from: http://onlinelibrary.wiley.com/doi/10.1002/bies.10354/abstract;jsessionid=B89E4FB41FCCC073B02F7AE74FBDF926.d01t03
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ReplyDelete1. Fatimah's group (Discuss the plant-pathogen interactions)
ReplyDeleteBasically, the plant-pathogen interaction can be categorized into four types, which are:
1. Symbiotic relationship between plant and pathogen
2. Pathogen cause disease to the plant
3. Plant resistant to pathogen and no infection occurred
4. Plant tolerance to the infection while the pathogen is able to grow and replicate with minimum symptoms of infection
Besides, there are two types of resistance which are host and non-host resistance.
Non-host resistance is the resistance towards a specific parasite or pathogen in the all plant species. Such resistance is acquired across the conserved regions of the species throughout many generations. Meanwhile, the host resistance can be defined as a particular plant cultivar resistance towards specific pathogen. The protein inside the plant is interacts with the protein of pathogen, which stimulate the hypersensitive response and block the spread of the pathogen.
Furthermore, certain pathogens can cause disease to plants whereas certain pathogens can’t. This is because, plants usually have the ability to recognize the invading pathogens and set up defenses but sometimes certain pathogens are able to evade the plant detection system or suppress the host defense mechanism or even both.
It is known that some plants contain a single-resistant R gene that particularly recognizes the pathogens that contains the complementary avirulent genes. This recognition results in induction of the gene expression and followed by the inhibition of pathogen growth. If the host plant does not contain the R gene, the pathogen can still produce the plant disease although it has the presence of the avirulent gene.
The interaction of R-gene of the plant and the avirulent gene of the pathogen causes many types of plant defense mechanism to be activated such as localized activation of programmed cell death (PCD). The cell death response is also accompanied by the induction of several kinds of anti-microbial defenses such as pathogenesis-related (PR) proteins which are glucanases, chitinases and also phytoalexins. The PCD which occurs during host resistance is followed by increase in the production of reactive oxygen intermediates (ROI) such as H2O2 which mediates the PCD. Furthermore, ROI also acts as a signal transduction agent that leads to induction of other defense mechanisms such as PR proteins, salicylic acid (SA), biosynthesis and systemic acquired resistance.
Reference:
Borras-Hidalgo, O. (2004). Basic insight in plant-pathogen interaction. Biotechnologia Aplicada, 21, 1-4.
Slater, A, Scott, N.W. & Fowler, M.R. (Eds.). (2008). Plant Biotechnology: The Genetic Manipulation of Plants. (2nd ed.). New York: Oxford University Press.