Plant resistance to arthropods. Flashcards
Content for final exam.
4 Types of plant traits.
Biochemical, morphological, physicochemical, and physiological.
What is phenotypic plasticity?
The ability of one genotype to produce more than one phenotype when exposed to different environments. A plant’s resistance phenotype depends on its its history of interactions with biotic and abiotic factors in the environment.
Differences between HPR and IPI literature.
HPR is concerned primarily with intraspecific variations in resistance (among plant varieties). In contrast, IPI literature deals with variation in resistance at several levels: within and among various plants and insect populations, intraspecific variation, and interspecific variation.
What is digestion?
Enzymatic and non-enzymatic degradation of chemical polymers to smaller molecules that can be absorbed through the midgut epithelium.
Describe gene-for-gene interactions.
Single plant genes encode important regulatory elements in some plant-insect systems, and activation of these genes result in a cascade of changes that result in resistance. This type of phenomenon usually involves a type of resistance called “vertical resistance”.
Successful examples of HPR as a primary means of pest control.
Grape cultivars to grape phyloxera
Wheat to Hessian fly
Rice to brown plant hopper
What is plant resistance?
Any plant trait that influences any aspect of the interaction between the plant and herbivore is potentially a basis (cause) of resistance. Different genotypes typically differ in one or more traits that can influence a plant-herbivore interaction. Therefore, plant resistance is the composite effect of all heritable plant traits that reduce the impact of potential herbivores by altering some aspect of the interaction between a plant, an herbivorous, and the natural enemies of the insect. The ultimate effect is the increased plant yield or fitness.
Antibiosis, antixenosis, and tolerance definition.
Antibiosis: plant traits that negatively affect the fitness (growth, development, fecundity) of pest insects.
Antixenosis: comprises those plant traits that negatively affect the behavior of pest insects, resulting in (generally) reduced colonization of resistant plant.
Tolerance: comprises those plant traits that allow a plant to recover from/compensate for injury, thereby reducing damage per unit injury.
How are the three types of resistance distinguished experimentally?
Antibiosis – no choice experiments. Antixenosis – choice experiments. Experimental demonstrations of tolerance are more difficult than experimental demonstrations of other types of resistance. Tolerance can be quantified in at least three ways.
I – Genotype (susceptible vs resistant) selection for endpoint (usually yield or seed production, mortality) comparison .Treatment groups with different levels of injury are established to compare the effects of injury with respect to the endpoint in different genotypes.
II – Injury response curves. Density curves are a common example in which the x-axis is some measure of injury and the y-axis is some measure of fitness or yield.
III – Analysis of covariance in which the slopes of the relationships between injury and fitness are compared among genotypes. Significant differences in slope indicate genotypic differences in tolerance.
Where does digestion take place? Role of peritrophic membrane?
The alimentary canal of insects is divided into three regions: foregut, midgut, and hindgut. Midgut, which is the primary site of digestion, contains the ventriculus (stomach), gastric caecae , and endo/ectoperitrophic spaces. Enzyme secretion, digestion, fluid secretion and nutrient absorption take place in the midgut. The main role of the peritrophic membrane is to separate the gut into ecto and endo peritrophic spaces. The peritophic membrane is porous, which may allow partially digested proteins through while excluding undigested proteins. Thus, the peritrophic membrane may allow compartmentalization of digestion and digestive enzymes.
Manifestations of post-ingestive resistance. Discuss the effects of post-ingestion on herbivores that are not adapted to resistance-related traits.
These effects can range from acute toxicity that results in high mortality to relatively small changes in developmental time or insect growth. These effects are categorized as antobiosis effects, but might have very different implications for pest management.
General considerations:
Resistant plants/secondary metabolites and other resistance-related traits have negative effects on herbivorous insects.
a) These effects are specific-specific, the same chemical may have dramatic effects on one insect but no effect on another (adapated/specialist) insect.
b) Structurally similar compounds may have divergent effect on the same insect.
c) Toxic chemicals often, but not always, have deterrent effects. Experiments often do not disentangle deterrency from toxicity. How can we disentangle these two effects?
d) On most studies on the effects of secondary metabolites on insects, the effects are assessed at the whole-insect level of analysis. The mechanisms by which secondary chemicals bring about their whole-organisms effects are often not known.
e) The negative effect of secondary chemicals can be manifested in numerous ways, determined in part by the experimental methods used.
f) A distinction can be made between compounds that interfere with digestive processes in insect guts. (i.e., never enter the body of the insect proper; the target site is in the gut) and those that pass through the epithelium and interact with a target site in the insect. The former may be called anti-nutritive compounds or digestibility reducers; the latter, toxins.
3 categories of resistant-related traits mode of actions and specific examples of each: toxin vs antinutritive vc antidigestive.
Anti-nutritive compounds or digestibility reducers interfere with digestive processes in insect guts. (i.e., never enter the body of the insect proper; the target site is in the gut. Toxins pass through the epithelium and interact with a target site in the insect.
Toxin: Tomatine – tomatine destroys membranes by interacting with cholesterol.
Canavanine – Canavanine absorbed into hemolymph, gets incorporated into tissues.
Hydrogene cyanide and glucosinolate.
Anti-nutritive compounds: polyphenol oxidase and other oxidative enzymes. Highly reactive oxidized phenolics interact with proteins and render them less nutritious. Result is less proteins
digested and absorbed.
Anti-digestive compounds: proteinase inhibitors. Reduction in amount of protein digested = anti-digestive.
What are some of the “drivers” of phenotypic plasticity?
Water stress; biotic factors such as prior herbivory, plant density, and predator/parasitoid behavior; nutrient availability.
Types of induced resistance.
Hypersensitive response Direct induced resistance Indirect induced resistance Plant stress-induced responses Interplant communication Priming
