39. Consider the following facts regarding the control of shoot apical meristem (SAM) size in Arabidopsis
(a) Loss of the CLAVATA1 (CLV1) gene leads to bigger SAM
(b) Loss of the CLAVATA3 (CLV3) gene leads to bigger SAM
(c) Loss of the WUSCHEL (WUS) gene leads to smaller SAM
(d) Loss of both CLV1 and WUS leads to smaller SAM
(e) Loss of both CLV3 and WUS leads to smaller SAM
(f) Loss of both CLV1 and CLV3 leads to bigger SAM
(g) Over expression of CLV3 leads to smaller SAM
(h) Over expression of CLV3 in the loss of function mutant of CLV1 leads to bigger SAM.
Based on the above information, which of the following genetic pathways describes the

relationship among CLV1, CLV3 and WUS most appropriately

The genetic pathway that best describes the relationship among CLAVATA1 (CLV1), CLAVATA3 (CLV3), and WUSCHEL (WUS) in controlling shoot apical meristem (SAM) size in Arabidopsis is characterized by a feedback loop where:

• CLV3, a small secreted peptide, binds to the receptor kinase CLV1.

• Activation of CLV1 by CLV3 limits WUS expression.

• WUS, a homeodomain transcription factor, promotes stem cell identity and positively regulates CLV3 expression.

• Loss of CLV1 or CLV3 leads to an enlarged SAM due to overexpression of WUS.

• Loss of WUS leads to a smaller SAM since stem cells fail to maintain.

• Loss of both CLV1/CLV3 and WUS results in smaller SAM because WUS function is essential.

• Overexpression of CLV3 reduces SAM size by repressing WUS.

• Overexpression of CLV3 in a CLV1 mutant background cannot suppress the large SAM phenotype, indicating CLV1 is required for CLV3 function.

This model integrates all the listed observations and encapsulates the negative feedback loop controlling SAM size.

Introduction

The maintenance of stem cell populations in the shoot apical meristem (SAM) of Arabidopsis is paramount for sustained plant growth and organogenesis. The CLAVATA-WUSCHEL pathway forms a pivotal feedback regulatory circuit controlling SAM size through intercellular signaling and transcriptional regulation.

The CLV-WUS Pathway Dynamics

• CLV3 Peptide: Expressed in stem cells at the SAM surface, CLV3 encodes a diffusible peptide that signals to repress WUS expression, limiting stem cell number.​

• CLV1 Receptor Kinase: CLV1, expressed in a deeper SAM region, binds CLV3 and initiates a cascade restricting WUS transcript and protein accumulation.​

• WUSCHEL (WUS): Produced in organizing center cells below the stem cell niche, WUS promotes stem cell identity and self-renewal and simultaneously activates CLV3 expression, forming a feedback loop.​

Genetic Evidence and Phenotypes

• Loss of CLV1 or CLV3: Causes SAM enlargement because of unchecked WUS activity, leading to excess stem cell proliferation.​

• Loss of WUS: Results in a smaller SAM unable to maintain stem cells.​

• Loss of Both CLVs and WUS: Phenotype resembles wus mutants with a small SAM, necessitating WUS for stem cell maintenance.​

• Overexpression of CLV3: Inhibits WUS expression and reduces SAM size.​

• Overexpression of CLV3 in CLV1 Mutant: Fails to reduce SAM size, highlighting CLV1’s essential role in CLV3 signaling.​

Summary Table: Genetic Interactions in CLV-WUS Pathway

Conclusion

The CLAVATA1-CLAVATA3-WUSCHEL regulatory loop governs SAM size in Arabidopsis by balancing stem cell proliferation and differentiation, with CLV3 signaling via CLV1 restricting WUS activity, and WUS promoting CLV3 expression. This negative feedback loop explains the phenotypes observed in various mutations and overexpression scenarios.