8. Which would be the result of mutation in SCARECROW genes responsible for radial patterning in roots of higher plants?
   (1) No apical root formation
   (2) Root hair will fail to develop
   (3) Roots with missing cell layer that normally generates cortex and endodermis.
   (4) Roots will be positively geotropic

Mutation in SCARECROW genes responsible for radial patterning in roots of higher plants results in roots with missing cell layers that normally generate cortex and endodermis.

Therefore, the correct answer is:

(3) Roots with missing cell layer that normally generates cortex and endodermis.

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Introduction

The SCARECROW (SCR) gene plays a pivotal role in root development by regulating asymmetric cell division critical for establishing the radial organization of tissues in the root, particularly the cortex and endodermis. Mutations in SCR disrupt this patterning, resulting in roots lacking the normal layered structure, as seen in Arabidopsis and other higher plants.

Function of SCARECROW Gene in Root Development

• SCR encodes a transcription factor that regulates asymmetric division of cortex/endodermis initial cells during root development.

• Proper SCR function ensures formation of two distinct ground tissue layers: the cortex (outer) and endodermis (inner).

• SCR expression is localized in the endodermal cell layer and the quiescent center during root radial patterning.

Effects of SCR Mutation

• Mutations in SCR lead to the loss of one ground tissue layer, with the remaining layer exhibiting characteristics of both cortex and endodermis.

• This results in roots with defective radial patterning and compromised tissue function related to nutrient transport and structural integrity.

• SCR mutants display altered cell division planes and abnormal root architecture.

Other Options Explained

• Root hair development (Option 2) is affected by other genes specifically governing epidermal patterning and is not the primary result of SCR mutation.

• Apical root formation (Option 1) involves additional regulatory genes, including SHR and PLT families.

• Root geotropism (Option 4) is mainly regulated by hormone signaling and statolith sedimentation, not SCR.

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Summary Table: Effects of SCARECROW Mutation on Root Structure

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Conclusion

The SCARECROW gene is essential for root radial patterning by regulating the asymmetric cell divisions that form the cortex and endodermis layers. A mutation in SCR results in roots with missing or combined ground tissue layers, confirming option (3) as the correct answer.