10. A transgenic lettuce plant was generated by over expressing isopentenyl transferase (IPT) gene under the control of the promoter of senescence activator gene (SAG12). Following are some statements regarding this transgenic plant. The transgenic plants
11. exhibit delayed senescence.
12. exhibit fast senescence.

C. have higher amount of cytokinin during senescence.
D. have higher amount of gibberellins during senescence.
Which one of the following combinations of above statements is correct?
(1) A and B               (2) A and C
(3) B and D                       (4) C and D

Correct answer: (2) A and C

Key idea

The IPT gene encodes isopentenyl transferase, a rate‑limiting enzyme of cytokinin biosynthesis.​
Putting IPT under the senescence‑activated SAG12 promoter means the gene is switched on when senescence starts, locally boosting cytokinin in senescing tissues.​
Cytokinin delays senescence, so such SAG12‑IPT transgenic lettuce shows delayed leaf senescence with elevated cytokinin during senescence.​

Therefore:

• A (delayed senescence) – true

• C (higher cytokinin during senescence) – true

Evaluation of each statement

A. “Exhibit delayed senescence.”

• In SAG12::IPT lettuce and other crops, leaves remain green longer and show postponed chlorophyll loss and protein breakdown compared with wild type.​

• This is the classic “stay‑green” phenotype created by senescence‑driven cytokinin production.
  → A is correct.

B. “Exhibit fast senescence.”

• Opposite of what is observed; extra cytokinin slows, not accelerates, senescence.

• SAG12::IPT lines are used specifically to delay developmental and postharvest senescence.​
  → B is incorrect.

C. “Have higher amount of cytokinin during senescence.”

• When SAG12 becomes active in senescing tissue, it drives IPT expression, increasing local cytokinin (tZ/iP‑type) levels.​

• Elevated CK correlates with delayed leaf yellowing and extended functional lifespan.
  → C is correct.

D. “Have higher amount of gibberellins during senescence.”

• IPT specifically boosts cytokinin biosynthesis, not gibberellin synthesis.

• No direct evidence that SAG12::IPT lines accumulate GA during senescence as a primary effect; the key hormonal change is cytokinin.​
  → D is incorrect.