In: Biology
Describe if the following homozygous mutant plants will undergo the triple response in the presence and absence of ethylene. Briefly explain your answer.
A. A recessive mutation (loss-of-function) in the EIN3 gene. This recessive mutation results in EIN3 being unable to activate the expression of ethylene inducible genes.
B. A recessive mutation (loss-of-function) in one ethylene receptor gene (ETR1). This recessive mutation resulted in an ETR1 protein that is unable to activate CTR1.
C. A dominant mutation (gain-of-function) in one ethylene receptor gene (ETR1). This dominant mutation results in an ETR1 protein that is unable to bind ethylene.
D. A dominant mutation (gain-of-function) in EIN2. This dominant mutation results in an EIN2 protein that cannot be phosphorylated by CTR1.
Answer: The triple response is induced by ethylene gas in the plants under certain environmental conditions. The model organisms used to study the phenomenon are Pisum sativum and Arabidopsis thaliana. When there is no light, the plants start to produce a high concentration of ethylene gas, a phytohormone. Under experimental conditions, the seeds of the plant Arabidopsis were incubated to investigate the effect of ethylene gas production. To the surprise of investigators, it was found that there is a reduction in the length of stem as well as the inhibition of shoot induction. In contrast to these results, additionally, it was found that there was an increase in the thickness of the stem. The growth of epicotyl was found to be more. In this way, the variable phenotypic differences in hypocotyls and epicotyl led to the terminology called a triple response. The molecular mechanism underlying this behavior was studied by researchers.
(A) In this case, there is a loss-of-function mutation in the EIN3 gene resulting in the inactivation of ethylene inducible genes. This gene act downstream of ETR1 and CTR1. Here, both copies of the EIN3 gene are mutated so there would be no production of active transcription factor which can migrate to nucleus and bind to the promoter/operator sequences of ethylene inducible genes and activate their transcription. Therefore such insensitive mutants don't show triple response even in the presence of ethylene gas.
(B) In this case, when the loss-of-function mutation is reported in the ETR1 gene, it resulted in the production of ETR1 protein that was unable to bind the CTR1 gene. Here it is essential to mention that even the recessive mutation in the ETR1 gene has resulted in the production of ETR1 protein. This protein will directly migrate to the nucleus and activate the ethylene inducible genes. As a result there would be a triple response. Actually the ETR1 is the ligand that binds to ethylene receptors and as a result, the triple response would be a reality.
(C) In this case, there is a dominant mutation in the ETR1 gene leading to the inability to produce ETR1 protein. So there will be no transcription of ethylene inducible genes and as a result, no triple response be seen. The gain of function mutation in this gene also leads to many phenotypic anomalies in resulting plant progeny.
(D) In this case, the dominant or gain-of-function mutation in the EIN2 gene. As a result, the EIN2 protein becomes unable to get phosphorylated by CTR1 protein. It is noteworthy to mention that the EIN2 gene acts downstream of CTR1 and upstream of EIN3. EIN2 gene when mutated as a loss-of-function there would be insensitivity towards ethylene-induced triple response. It is the only gene with such activity. But in this case, there is an overproduction of EIN2 protein which by alternative pathway induce triple response as well as a stress response in the plant. It is well known to activate alternative plant hormone-mediated pathways.