Final- Weeds

Question 1

Why does the industry want HR crops? (herbicide-resistant)

Answer 1

• new market for herbicide sales
• difficult to discover herbicides with broad spectrum control
• environmentally friendly
• High cost of developing & registering new herb.
• High value of HR traits

Question 2

Commercialization of HR crop requirements

Answer 2

• characterize gene
• risk assessment of gene & its products
• properties of modified plants
• stability of the introduced trait
• safety of modified plant
• interactions with wild relatives
• EPA approval

Question 3

Benefits of HR crops

Answer 3

• Good for no-till systems
• allow use of herbicides with short environmental persistence
• economically favorable to the grower

Question 4

Potential downsides of HR crops

Answer 4

• Increased seed cost
• market/consumer opinion
• Relies too heavily on a single weed management (increased selection pressure)
• Gene flow and trait transfer to: organic crops, closely related weeds of the crop

Question 5

Methods of generating HR crops

Answer 5

Mutagensis
Selection from existing germplasm
Genetic engineering

Question 6

Mutagensis process

-example

Answer 6

1. Develop callus and cell suspension cultures
2. Mutagenize using chemical mutagens
3. Rengeration of plants

Atrazine resistance in soybean

Question 7

Selection from existing germplasm process

-example

Answer 7

Conventional plant breeding was used to transfer herbicide tolerant traits from related wild species

• ALS resistant gene IDed in shattercane and transferred to sorghum (pride of K-state)
• Dicamba resistant wild mustard gene transferred to canola

Question 8

Genetic engineering transfer of HR genes from other species Process

Answer 8

1. ID the desired gene or trait in nature
2. Isolate the gene copy and transfer it to the plant (using agrobacteria or gene gun)
3. The plant is tested to ensure it is safe for people, animals, the environment
4. Years of testing and reviews by the government before it is released for widespread use.

Question 9

Mechanisms of resistance utilized in transgenic HR crops

Answer 9

• Altered site of action (most glyphosate resistant crops)
• Over expression of herbicide binding protein (some glyphosate resistant crops
• Enhanced detoxification (diacamba-tolerant soybean)

Question 10

Importance of rotating MOAs in HR crops

Answer 10

Need to be sure to rotate MOAs of HR crops to prevent putting selection pressure on the weeds for those MOAs

Question 11

RNAi use for HR weeds (roundup example)

Answer 11

Apply a mixture of glyphosate and RNAi to the plant and basically silence the resistance gene in the plant, making it glyphosate susceptible.

Will only work with the weeds resistance mechanism is over expression of the gene, not for altered target site.

Question 12

Stages of herbicide evolution

Answer 12

1. elimination of the most sensitive genotypes
2. Elimination of all genotypes except the most resistant individuals within the resistant pop.
3. Interbreeding of the survivors; segregation and recombination of gnenes tends to maximize resistance level

Question 13

Selection by herbicides changes the population ___

Answer 13

over time. year 1, 1 in a million resistant, more the second year, even more after that

Question 14

Factors that determine the evolution of herbicide resistance

Answer 14

1. Selection pressure (monoculture, repeated use of one MOA, high rate of same herbicide, long soil residual herbicide)
2. Number of genes involved in the expression of functional resistance
3. Frequency of resistance alleles in natural (unselected population)
4. Mode of inheritance of the resistance alleles
5. Reproductive and breeding characterisitics of the weed species
6. Fitness of resistant and susceptible genotypes (yield, growth cost of resistance)

Question 15

Why do herbicide mixes of slightly lower efficacy delay the evolution of resistance? As compared to 1 herbicide with 98% efficacy

Answer 15

Allows some susceptible plants survive and essentially dilute the gene pool

Question 16

Back selection

-def

Answer 16

-In the absence of herbicide pressure, a reversal in the direction of selection favoring susceptible alleles may occur (esp if fitness cost of resistance)

Question 17

Polygenic vs major gene inheritance of Herbicide Resistance

Answer 17

Major Gene
-Most HR genes inherited this way
-1 or 2 genes controlling HR in target site resistance
-As most herbicides are target site specific, they impose strong selection pressure
-Spread rapidly in the population and hard to manage
Polygenic
-small enhancements of resistance
-addititive effect from several genes

Question 18

Major source of genetic variation is

Answer 18

mutations. Most mutations are bad as a rule of thumb.

Weeds like palmer produce up to a million seeds per plant and therefore have a huge amount of genetic variation

Question 19

How reproductive and breeding characterisitics influence HR spread/evolution

Answer 19

-pollen and seed dispersal significantly move the HR allele

Question 20

Fitness assessment of resistant vs susceptible plants

Answer 20

• Done in the absence of herbicide
• Measured over the entire life cycle to look at growth, seed production, mortality.
• To do accurately need an R plant and S plant with same genome minus the resistance alleles. hard to do.

Question 21

Consequences of Evolution of Herbicide Resistant Weeds

Answer 21

• Poor weed control / crop loss
• New / more expensive herbicides
• Crop Rotation changes
• Environmental impact