After enjoying a nice break on Independence Day, OUTPACE kicked off Week 2 with the first lecture on bacteria called “Mechanisms of Plant Immunity to Phytopathogenic Bacterium Pseudomonas syringae” by our very own Dr. Karolina Mukhtar. During this lecture, OUTPACErs focused on what is known as the zig-zag model of plant-pathogen interactions, shown below.
Pathogens are first recognized by plants by the means of pattern recognition receptors (PRRs), which allow the plant to recognize pathogen associated molecular patterns (PAMPs). Once PAMPs are recognized by PRRs, the first part of the zig-zag model, pattern triggered immunity, is initiated in three different ways: kinase cascade that leads to transcriptional responses, calcium ion influx, and reactive oxygen production. A lot of the time, this is enough to take care of the pathogen. Sadly, that is not always the case, as will be seen in the second part of the zig-zag model.
The second part of the zig-zag model, effector-triggered susceptibility, occurs because some pathogens are able to produce effectors that can enhance their virulence. Effectors suppress the plant’s immune response and/or contribute to the pathogen’s viability. Different types of pathogens introduce their effectors in different ways. For example, while nematodes use their feeding stylets to introduce their effectors, many bacterial effectors are introduced by the means of Type-III secretion systems (T3SS).
In the third and final part of the zig-zag model, effector-triggered immunity, resistance proteins (R proteins) are involved, which are able to recognize effectors intracellularly. It is important to note that this is only biotroph-specific type of resistance. There are three ways R proteins can recognize effectors and they are: direct, guard/decoy, and bait. The guard/decoy method is currently the most popular method.
The result of R protein activation is enhanced defense. It triggers an immune response including multiple factors, such as salicylate production, that eventually kills the infected cell in a hypersensitive cell death response (HR).
Since there are no known R genes that confer resistance to necrotrophic pathogens, breeding resistance to necrotrophs is more challenging than to biotrophs. In response to necrotrophic infection, plants trigger their immune response through jasmonate production, as compared to salicylate production in biotrophic infection. What is interesting is that salicylate and jasmonate are mutually antagonistic in their production. The reason is that the pathogenicity of necrotrophs is enhanced by the hypersensitive response (HR),so HR is suppressed in the presence of necrotrophs.
Week 2 also included Lab #2 titled “Phytopathogen Media Preparation for Upcoming Pseudomonas syringae Infection Experiment.” During this lab, OUTPACErs learned to prepare solid agar plants and liquid media of four different media broadly used to cultivate various phytopathogens.
The 9 OUTPACErs were divided 3 groups of 3. Group 1 prepared the yeast peptone dextrose (YPD) medium and King B’s (KB) solid medium, and the members are shown below.
Group 2 prepared the V8 medium and KB liquid medium, and the members are shown below.
Lastly, Group 3 prepared the yeast extract-dextrose-CaCO3 (YDC) medium and another batch of KB liquid medium, and pictures are shown below.
Week 2 ended with bacteria lecture II by guest speaker Dr. Xinran Du. This lecture consisted of basic background information on bacteria, their identification, their generalized disease cycles, and symptoms of bacterial disease.
Basic background information on bacteria included the fact that bacteria are prokaryotes, and as prokaryotes, they have no membrane bound nuclei or organelles, have single circular chromosomes, lack chlorophyll, most have cell walls, and some are motile through the means of flagellum.
Bacteria could be identified according to their structure (cell shape, colony appearance, motility), mode of respiration (aerobic, anaerobic, facultative anaerobic), through biochemical methods (cell wall composition), by their nutritional requirements, pathogenicity, and through different tests like fatty acid profile analysis.
When infecting their plant host, bacteria have a disease cycle. Overseasoning can involve infected or infested seeds, infected plant parts or plants, plant residues, alternative hosts, soil, and insects. Short-distance dissemination can involve water, soil, tools and equipment (used during farming), and insects, while long-distance dissemination can involve insects, storage, shipping containers, movement of infected plant parts. Penetration can occur via natural openings (stomatas, hydathodes, nectaries) or wounds (caused by tools, equipment, insects, hail). Development and reproduction could be intercellular or intracelleular, and causes the plant to wilt.
Typical symptoms of bacterial disease are water soaking, wetwood, shoot blight, bleeding cankers, and galls (shown below). The leaf also undergoes necrosis, which results in classic spotting of leaves, and reduces photosynthesis and cell respiration of plant material (also shown below).
In conclusion, OUTPACErs had an enjoyable week of learning, and here’s to another great week!