163. The bacterial flagellin activates a typical MAPK cascade consisting of MEKK1-MKK4-MPK6 leading to the
activation of ACS6 enzyme involved in ethylene biosynthesis in plants. Which one of the following events
will be true in a transgenic Arabidopsis plant overexpressing the constitutively active form of MKK4?
1. Ethylene responsible genes will be less transcribed in the presence of flagellin stimuli
2. MPK6 will not get activated in the absence of flagellin
3. Flagellin stimuli are not required for the activation of ACS6
4. Flagellin stimuli will be required for ethylene biosynthesis

MAPK Cascade and Ethylene Biosynthesis in Plants

Plants use MAPK (Mitogen-Activated Protein Kinase) cascades to transduce environmental signals like flagellin, a bacterial elicitor. In Arabidopsis, flagellin activates the MAPK cascade:

• MEKK1 → MKK4 → MPK6,
  which then activates ACS6 (1-aminocyclopropane-1-carboxylic acid synthase 6), a key enzyme in ethylene biosynthesis.

What Happens in a Plant Overexpressing Constitutively Active MKK4?

A constitutively active MKK4 means the protein is always in its active form, regardless of upstream signals, such as the presence of flagellin.

As a result:

• MPK6 is continuously activated,

• ACS6 is always turned on,

• Ethylene biosynthesis proceeds independently of flagellin stimuli.

Explanation of Options

1. Ethylene responsible genes will be less transcribed in the presence of flagellin stimuli
   → Incorrect. Ethylene-related genes would not be less transcribed—if anything, they’d be unaffected or further activated depending on the system’s saturation.

2. MPK6 will not get activated in the absence of flagellin
   → Incorrect. In this transgenic line, MPK6 will be activated even without flagellin, due to the constitutive MKK4.

3. Flagellin stimuli are not required for the activation of ACS6
   → Correct. Since MKK4 is always active, ACS6 is activated without the need for flagellin.

4. Flagellin stimuli will be required for ethylene biosynthesis
   → Incorrect. The biosynthesis continues independently of flagellin in this scenario.

Correct Answer:

3. Flagellin stimuli are not required for the activation of ACS6

By bypassing upstream signaling with a constitutively active MKK4, the plant initiates ethylene biosynthesis without external bacterial triggers.