118. TIR1, an auxin receptor, is a:
(1) F-box protein
(2) MAP kinase
(3) His kinase
(4) Ser/Thr kinase
Detailed Explanation:
Question:
TIR1, an auxin receptor, is a:
✅ Correct Answer: (1) F-box protein
What is TIR1?
TIR1 (Transport Inhibitor Response 1) is a key component of the auxin signaling pathway in plants. It acts as a receptor for the plant hormone auxin, which is essential for numerous processes including cell elongation, division, and differentiation.
Why is TIR1 an F-box Protein?
F-box proteins are part of the SCF complex (Skp1–Cullin–F-box), a type of E3 ubiquitin ligase that tags proteins for degradation via the 26S proteasome.
🧬 Functions of TIR1 as an F-box Protein:
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TIR1 binds auxin directly, making it an auxin receptor.
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Upon auxin binding, TIR1 targets Aux/IAA proteins (auxin response repressors) for ubiquitination and degradation.
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This degradation activates auxin-responsive gene expression by freeing up ARF (Auxin Response Factor) transcription factors.
Why the Other Options Are Incorrect:
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(2) MAP kinase: A signaling molecule in phosphorylation cascades, not related to auxin binding.
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(3) His kinase: Typically involved in two-component signaling systems in bacteria and some plants, not relevant here.
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(4) Ser/Thr kinase: Enzymes that phosphorylate serine or threonine residues, not directly functioning as auxin receptors.
Biological Importance of TIR1:
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Regulates plant development, tropisms, apical dominance, and organ patterning.
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Integral to auxin perception and signaling transduction.
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Forms the basis for several genetic and transgenic studies in plant biology.
Conclusion:
TIR1 plays a central role in auxin signaling by acting as a F-box protein that mediates the degradation of auxin response repressors, thereby enabling appropriate gene expression for plant growth.
✅ Correct Answer: (1) F-box protein
