68. Examples of many factors that regulate plant height in response to gibberellic acid (GA) are listed below:
    A. Binding of a GA bound repressor to the promoter of the DELLA domain containing GRAS protein gene and blocking its expression.
    B.Binding of the GA receptor complex to GRAS.
    C. Directing GRAS for ubiquitination and degradation by the 26S proteasome.
    D. Micro RNA directed down regulation of the GRAS protein expression.
    Which one of the following combinations is correct?

(1) A and B                                (2) B and C

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

Correct answer: (2) B and C

Explanation:

• (A) Binding of a GA-bound repressor to the promoter of DELLA domain containing GRAS protein gene and blocking its expression is not a described mechanism in GA signaling. DELLA proteins themselves function as repressors of GA responses.

• (B) The GA receptor complex (GID1) binds to the DELLA domain containing GRAS proteins (negative regulators of GA signaling). This interaction is essential for GA signal transduction.

• (C) Upon binding of GA to its receptor and interaction with DELLA proteins, these DELLA proteins are targeted for ubiquitination and degradation by the 26S proteasome, which derepresses GA responses allowing plant growth.

• (D) There is limited or no well-characterized evidence for microRNA directed regulation of GRAS protein expression in the GA pathway.

Option-wise:

• (1) A and B — A false.

• (2) B and C — correct.

• (3) C and D — D false.

• (4) A and D — both false.

Gibberellin hormone controls plant height by enabling the GA receptor to bind DELLA repressor GRAS proteins, marking them for degradation via the 26S proteasome. This degradation lifts growth repression, promoting elongation and development in plants.