69. A researcher developed a mutant of Arabidopsis plant where the function of SLEEPY 1 (SLY1) containing SCF complex has been disrupted. Which one of the following statements is INCORRECT in the developed mutant in relation to gibberellic acid (GA) signal transduction?

(1) GA will bind to GA^–insensitive dwarf 1 (GID1) protein.
(2) A complex of GA-GID1 and DELLA protein will be formed
(3) The DELLA protein will be ubiquitinated.
(4) The DELLA protein will not be degraded.

Correct answer: (3) The DELLA protein will be ubiquitinated.

Explanation:

• SLEEPY1 (SLY1) is a crucial F-box protein component of the SCF (Skp1–Cullin–F-box) E3 ubiquitin ligase complex that mediates the ubiquitination of DELLA proteins in gibberellin (GA) signaling.

• (1) GA can still bind to its receptor GID1, so statement (1) is correct.

• (2) The GA-GID1-DELLA complex will still form because DELLA proteins can bind to GA-bound GID1 regardless of SLY1 disruption, so (2) is correct.

• (3) However, due to the disruption of SLY1-containing SCF complex, DELLA protein will not be ubiquitinated properly, because SLY1 is necessary for recruiting DELLA proteins for ubiquitination, so this statement is incorrect.

• (4) Consequently, the DELLA proteins will not be degraded, causing accumulation and leading to dwarf phenotypes because growth repression is not lifted; (4) is correct.

Option-wise:

• (1) Correct

• (2) Correct

• (3) Incorrect — correct answer.

• (4) Correct

In Arabidopsis mutants lacking functional SLY1, GA binding and formation of GA-GID1-DELLA complexes occur, but DELLA proteins fail to be ubiquitinated and degraded, leading to dwarfism due to continuous repression of GA responses.