65. The following statements are made to describe auxin signal transduction pathway, from receptor binding to the physiological response:
    A. Auxin response factors (ARFs) are nuclear proteins that bind to auxin response elements (Aux REs) to activate or repress gene transcription.
    B. AUX/IAA proteins are secondary regulators of auxin induced gene expression. Binding of AUX/IAA proteins to the ARF protein blocks its transcription regulation.
    C. Auxin binding to TIR1/AFB promotes ubiquitin mediated degradation and removal of AUX/IAA proteins.
    D. Auxin binding to auxin response factors (ARFs) causes their destruction by the 26S proteasome pathway. Which one of the following combinations of above statements is correct?
    (1) A, B and C                                (2) A, C and D

(3) B, C and D                              (4) A, B and D

Correct answer: (1) A, B and C

Explanation:

• (A) Auxin response factors (ARFs) are nuclear transcription factors that bind to auxin response elements (AuxREs) on DNA and can activate or repress auxin-responsive gene transcription. This is true.

• (B) AUX/IAA proteins are transcriptional repressors that bind to ARFs and block their ability to regulate transcription, acting as secondary regulators. This is true.

• (C) When auxin binds to the TIR1/AFB receptor, it promotes the ubiquitin-mediated degradation of AUX/IAA proteins via the 26S proteasome, releasing ARFs to activate gene expression. This is true.

• (D) Auxin binding to ARFs does not cause their destruction; rather, auxin signaling leads to degradation of AUX/IAA repressors, not ARFs. This is false.

Option-wise:

• (1) A, B and C — correct.

• (2) A, C and D — D incorrect.

• (3) B, C and D — D incorrect.

• (4) A, B and D — D incorrect.

Auxin signaling in plants involves ARFs activating gene transcription, AUX/IAA proteins repressing ARFs, and auxin binding to TIR1/AFB receptors triggering ubiquitin-mediated degradation of AUX/IAA repressors, enabling auxin-responsive genes to be expressed, a critical process for plant growth regulation.