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EXAM QUESTION FEEDBACK EXAMPLE
MODULE &
ASSESSMENT
INSTRUMENT
COMMENTS
Exam Question:
1. Explain how expression of Ti plasmid T-DNA genes can lead to
overproduction of plant growth regulators by genetically modified
plant cells.
Specimen Answer:
T-DNA Gene 1 Tryptophan-2- Monooxygenase converts Tryptophan to
Indole-3- Acetamide, in turn converted by Gene 2 Indole-3-Acetamide
Hydrolase into highly active auxin Indole-3-Acetic Acid. Nett effect of
expressing these two T-DNA genes (Gene 1 and Gene 2) is to increase active
auxin concentrations in the plant cells by up to 1,500x. Causes excessive
rootiness and/or the formation of fluffy white roots at the site of expression.
T-DNA Gene 4 involved in complex cascade leading to overproduction of
Zeatin-type cytokinins, by up to 1,200x in genetically modified plant cells.
Causes excessive shootiness and cell division. Overproduction of the two
together causes massive uncontrolled cell proliferation, leading to growth of
characteristic crown gall tumour in dicotyledonous plant hosts.
Narrative
Commentary:
Students answered this question generally very well, with the vast majority
being able to explain the differing effects of expressing Genes 1 & 2 (for
Auxin production) and Gene 4 (for Cytokinin production) at high levels in
plant cells. A minority of students appeared to get Auxins and Cytokinins
mixed up in their answers and therefore were suggesting that the wrong
type of plant growth regulator promoted rooty or shooty phenotypes. A very
small number of students were unable to provide any meaningful
information on this topic.
For Future Learning:
Students may wish to consider enhancing their knowledge of T-DNA
structure and function, for example by using the hand-outs provided on GCU
Learn for Lecture 1 of this segment, or by reading one of the recommended
references, such as Agrobacterium Protocols, Chapter 7. Students are
reminded that it is the balance (or ratio) of Auxins: Cytokinins that will
determine whether a dicot plant cell is programmed towards trying to divide
and form root-, or divide and form shoot- initiating cells. Cells expressing
high concentrations of both types of plant growth regulators will enter a
phase of uncontrolled cell division, as known as a crown gall tumour, from
which the term ‘crown gall disease’ of dicot plants is derived. Use of an
appropriate diagram may also be helpful, to prompt recall of thoughts about
these genes and the effects of their expression upon plant cells.