165. In CLAVATA (clv) mutant of Arabidopsis, Shoot Apical Meristem (SAM) size and expression of WUSCHEL
(WUS) is increased. SAM size is reduced in wus mutant plants. Choose the correct statement regarding the
function of these two genes:
1. CLV positively regulate WUS expression and negatively regulate SAM size
2. CLV negatively regulate WUS expression and WUS positively regulate SAM size
3. WUS negatively regulate SAM size and CLV expression
4. WUS and CLV independently regulate SAM size
CLAVATA and WUSCHEL: Key Regulators of SAM in Arabidopsis
The shoot apical meristem (SAM) in Arabidopsis thaliana is a group of undifferentiated cells that enable continuous plant growth. Two central regulators of SAM size and activity are:
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WUSCHEL (WUS) – a homeobox transcription factor that promotes stem cell identity.
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CLAVATA (CLV) – a group of genes (mainly CLV1, CLV2, CLV3) that restrict WUS expression to regulate SAM size.
The WUS–CLV Regulatory Feedback Loop
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WUS Expression:
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WUS is expressed in the organizing center (OC) of the SAM.
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It promotes stem cell fate and activates CLV3 expression in the stem cell population.
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CLV Function:
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CLV3 encodes a small peptide ligand that binds to the CLV1/CLV2 receptor complex.
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This interaction represses WUS expression, creating a negative feedback loop.
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Balance of Expression:
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WUS positively regulates stem cell proliferation and CLV3 expression.
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CLV3, in turn, negatively regulates WUS to maintain proper SAM size.
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Explanation of Options
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CLV positively regulate WUS expression and negatively regulate SAM size
→ Incorrect. CLV negatively regulates WUS, not positively. -
CLV negatively regulate WUS expression and WUS positively regulate SAM size
→ Correct. This matches the established feedback loop model. -
WUS negatively regulate SAM size and CLV expression
→ Incorrect. WUS positively regulates SAM size and activates CLV3 expression. -
WUS and CLV independently regulate SAM size
→ Incorrect. They are interconnected in a regulatory loop.
Correct Answer:
2. CLV negatively regulate WUS expression and WUS positively regulate SAM size
This WUS-CLV loop is fundamental to stem cell maintenance and proper development of the shoot apical meristem in Arabidopsis.
