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Summary Genetics 215 Notes
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Genetics 215 Notes containing microbiology contents such as Mutualistic plant microbes
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Biological control of postharvest plant pathogensPostharvest Rots
The Battle between Pathogen and Fruit
- PAMPs/MAMPS - microbes associated molecular pattern
- Effectors - small secreted molecules that alter cell structure or function and that either assist in
disease development of induction of resistance
- Oxidative burst – rapid, transient, production of huge amounts of reactive oxygen species (ROS)
Postharvest Rots of Fruit
- Postharvest losses can vary from 5% - 50%
- Postharvest rots fruits and vegetables mainly due to pathogenic fungi in genera Penicillium,
Botrytis, Monilinia, Rhizopus, Alternaria, Aspergillus, Geotrichum, Gleosporium and Mucor
- Harvested fruit highly susceptible to these pathogens due to availability large amount of nutrients
and water (especially if wounds are present)
à low pH
à decrease in intrinsic decay resistance of fruit after harvest
- Some postharvest rot fungi can produce mycotoxins harmful to humans
- Development of modern fungicide and improved storage technologies 1960s & 1970s
→ improved shelf life of fruits
- Postharvest controlled atmosphere technologies = reduced losses
- Fungicides important many crops for reducing post-harvest rots
- Problems with fungicides:
→ fungicide resistance
à lack of new fungicides
à minimum residues levels (MRLs) for export fruit and market access
Postharvest Fungicide Treatment of Fruit
- Alternative methods required for control of postharvest diseases
- Biological control of postharvest fruit rots has been active research field for past 30 years
- Biological control - Lowering of inoculum density of pathogen or its disease producing activity
through one or more organisms
- Not all postharvest rots are good targets for biocontrol
• Latent infecting pathogens vs wound pathogens
Latent Infection Type Pathogen
- Some postharvest pathogens cause latent infections (infect in orchard with no symptom
development until fruit starts ripening)
• Biological control these types of pathogens very difficult
i.e. à Brown rot (Monilinia fructicola) of peaches and nectarines
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,Uniqueness of Postharvest Systems for Biological Control
- Fresh fruit has high sales price
- Only one postharvest application of biocontrol agent can provide control of rots developing in
wounds
→ application high concentration biocontrol agent economical feasible
- Biocontrol application can be done through current application systems
I.E. (“drenches”, online sprayers, online water baths)
- Biotic and abiotic postharvest environment easier to manipulate than orchard conditions
- Application antagonists in orchards often not economically feasible
Where to Search for Biocontrol Agents
- Fructoplane (fruit surface) source of most antagonists
- Phylloplane also good source of antagonists → some fructoplane microbes live for part of their life
cycle on leaves
- Popular way of screening is application wash water of fruits into surface wound and isolating
microbes from wounds that were protected against pathogen infection (pathogen artificially
inoculation)
- Sometimes antagonists obtained from some other unexpected sources e.g. plant roots, soil and sea
water
Yeasts as Biocontrol Agents
- Yeasts are of special interest as biocontrol agents since in general they do not produce toxic
secondary metabolites (antibiotics and mycotoxins)
- In general well adapted to
• Environmental conditions of storage (temperature close to 0˚C, high relative humidity, low
oxygen)
• Fruit characteristics (high sugar concentration, high osmotic pressure and low pH)
- Sufficient information on their commercial scale production including fermentation, formulation,
storage and handling
Characteristics of Yeasts Suitable for Biocontrol
- Genetically stable
- Effective at low concentrations
- Non fastidious in its nutrient requirements
- Capable of surviving under adverse environmental conditions (low/high temperature, oxidative
stress and controlled atmosphere)
- Effective against a wide range of pathogens on a variety of fruits
- Amenable to production on an inexpensive growth medium
Characteristics of Yeasts Suitable for Biocontrol
- Preparable in a formulation that can be effectively stored and dispensed
- Compatible with commercial processing procedures
- Resistant to pesticides
- Friendly to environment and non-pathogenic to host commodity
- Should not produce metabolites that are deleterious to human health
- Unable to grow at 37˚C and not associated with infections of humans
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, Mechanisms of Biocontrol
1. Competition for nutrients and space
• Considered to be primary mode of action against postharvest fungal pathogens
o Especially important in the first 24h after wound formation, other modes of action
become important after 24h
• Antagonists and pathogen have same requirement for specific nutrient or resource
o Antagonist should be able to use limited nutrients more effectively than pathogen
• Competition for iron very important since iron essential for fungal growth and pathogenesis
o Yeast produce siderophores that are effective at scavenging limited iron resources
2. Biofilm formation
• Biofilm formation is development process of microbes resulting in formation
morphologically distinct multicellular structures with microbes having altered gene
expression patterns – only occurs at high cell densities
o enhanced resistance to stresses
o ability to attach to plant surface and fungal spores and hyphae
o reduction physical space available for pathogen
o interfering with flow of nutrients to pathogen
o Interfering with germination signals from the host to the pathogen spores
3. Production of antimicrobial compounds
• Preferable that antagonists don’t produce antibiotics, due to risk in resistance development
in human microbial pathogens
• Volatile organic compounds (VOCs) with antifungal activity involved some biocontrol, but
must be carefully screened for human safety, e.g. should not be carcinogen
• Some yeasts produce killer toxins/proteins that kill species of the same type or different
species
o Must ensure that is not harmful to humans
4. Parasitism and release of hydrolases
• Parasitism is when antagonist feeds on or within the pathogen, resulting in direct
destruction of plant pathogen
• Yeast often produce hydrolytic enzymes (glucanases and chitinases) that breakdown fungal
cell walls and assist penetration of fungal plant pathogen cell
5. Induction of fruit resistance
• Some antagonists can trigger fruit tissue defence responses (hypersensitive response,
formation of structural barriers, synthesis of phytoalexins, lignification of plant cell walls,
synthesis of lytic enzymes, expression of pathogenesis-related proteins) that results
inhibition plant pathogen infection
6. Ability to survive oxidative stress in wounds
• Ability to survive and proliferate in wounded host tissues important
o Wounded fruit tissue has high reactive oxygen species (ROS) such as hydrogen
peroxide, superoxide anion and hydroxyl radical
Interactions in/on Fruit
- Complex biological interactions that can only be
Controlled to some extent
à Influence of environment
- Need to learn more about epiphytic microflora
à Fruit microbiome
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