58. Which one of the following plant derived signalling molecule induces hyphal branching of arbuscular mycorrhizal fungi, a phenomenon that is observed at the initial stages of colonisation of these fungi?
(1) Salicylic acid     (2) Abscisic acid
(3) Strigolactones   (4) Systemin

Correct answer: (3) Strigolactones

Explanation:

• Strigolactones are plant-derived signaling molecules that induce hyphal branching of arbuscular mycorrhizal (AM) fungi at the initial stages of colonization. This branching is crucial for the establishment of symbiotic relationships between the fungi and plant roots.

• Salicylic acid mainly functions in systemic acquired resistance and defense signaling rather than fungal symbiosis initiation.

• Abscisic acid is involved in stress responses and seed dormancy, not in inducing fungal hyphal branching.

• Systemin is a peptide involved in wound response signaling and defense, but it does not regulate AM fungi colonization.

Option-wise:

• (1) Salicylic acid — defense hormone, not for hyphal branching.

• (2) Abscisic acid — stress hormone, no effect on fungal branching.

• (3) Strigolactones — correct; promote hyphal branching and AM fungi colonization.

• (4) Systemin — defense-related peptide, not involved in AM fungal symbiosis.

  Strigolactones act as key rhizospheric signals, stimulating hyphal branching in AM fungi at very low concentrations, thus promoting successful symbiosis formation that benefits plant nutrient uptake and growth.