9. Which one of the following transcription factors is important for delimiting the meristematic and elongation zones of roots?
   (1) SCARECROW (SCR) (2) SHORT ROOTS (SHR)
   (3) PLETHORA (PLOT) (4) SPEECHLESS (SPCH)

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Introduction

The root system’s proper growth depends on balanced cell division and elongation in discrete zones. The meristematic zone where cells divide transitions to the elongation zone where cells expand, and this boundary is tightly regulated at the molecular level. PLETHORA (PLT) transcription factors are key regulators establishing and maintaining the root meristem and controlling the transition to elongation.

PLETHORA Function in Root Development

• PLETHORA genes encode AP2-domain transcription factors expressed in a gradient in the root tip, highest at the quiescent center and meristem, decreasing towards the elongation zone.

• This gradient controls a gene regulatory network balancing stem cell maintenance with differentiation and elongation.

• PLTs promote the expression of cell division and growth genes and repress differentiation genes within the meristematic zone.

• Mutations in PLT genes result in loss of organized root meristem structure and premature cell differentiation.

Relationship to Other Factors

• SHORT ROOT (SHR) and SCARECROW (SCR) work in parallel to PLT in root patterning but primarily regulate radial patterning and stem cell niche positioning, not the meristem-elongation boundary.

• SPEECHLESS (SPCH) regulates stomatal development and is unrelated to root zone delimitation.

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Summary Table: Roles of Key Transcription Factors in Root Development

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Conclusion

PLETHORA transcription factors are central to delimiting the boundary between the meristematic and elongation zones in plant roots by orchestrating gene expression patterns that regulate cell division and differentiation. This makes option (3) the correct choice.