59. A researcher has obtained an Arabidopsis mutant defective in strigolactones (SLs), a novel plant hormone. The following statements were made regarding the mutant phenotype:
    A. Shoot branching gets enhanced in the mutant plant.
    B. Hyphal branching of arbuscular mycorrhizal fungi (AM-fungi) gets enhanced during colonization in the mutant plants
    C. Shoot branching gets inhibited in the mutant plants
    D. Germination of seeds of parasitic plant is prevented near the mutant plant

Which one of the following options represents the combination of correct statements?
(1) A and B             (2) B and C
(3) B and D            (4) A and D

Correct answer: (4) A and D

Explanation:

• (A) Shoot branching gets enhanced in the strigolactone-defective mutant plant. Strigolactones normally act as inhibitors of shoot branching. In their absence or deficiency, increased shoot branching or excessive lateral bud growth occurs.

• (B) Hyphal branching of arbuscular mycorrhizal fungi (AM-fungi) is reduced or impaired in strigolactone-deficient mutants because strigolactones act as signaling molecules that stimulate hyphal branching for symbiotic colonization. So, B is incorrect.

• (C) Shoot branching gets inhibited by strigolactones, so in mutants deficient in SLs, this inhibition is lifted; statement C is incorrect.

• (D) Strigolactones exuded from roots stimulate germination of parasitic plant seeds like Striga. In SL-deficient mutants, germination of parasitic seeds near these plants is prevented or reduced, so this statement is true.

Option-wise:

• (1) A and B — B is incorrect.

• (2) B and C — both incorrect.

• (3) B and D — B incorrect.

• (4) A and D — correct.