Q.53 Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: In gametophytic self-incompatible flowering plants. the direction of information to inhibit pollen
tube is from pistil to pollen.
Reason R: The female determinants localized in the pistil interact with the pollen tubes in the style or ovule to
inhibit the incompatible tube.
In the light of the above statements. choose the most appropriate answer from the options given below:
l. Both A and R are correct and R is the conect explanation of A
2. Both A and R are correct but R is NOT the correct explanation of A
3. A is correct but R is not correct
4. A is not correct but R is correct

Both A and R are correct and R is the correct explanation of A.

Detailed Explanation

Assertion A: Correct—in gametophytic self-incompatibility (GSI), inhibition signals travel from pistil to pollen tube. Female S-determinants (S-RNase in Solanaceae/Rosaceae or PrsS in Papaver) secreted by pistil tissues interact with pollen tube S-loci, blocking growth of “self” pollen (matching S-haplotype) in style.

Reason R: Correct and explains A—female determinants localized in pistil (stigma/style) bind pollen tube receptors (SLF/SFB proteins), arresting tube growth via calcium bursts (Papaver) or RNase degradation of pollen RNA (Solanaceae). Interaction occurs in style or ovule, precisely matching GSI mechanism.

Options Explained

1. Both correct, R explains A: Correct—R describes molecular basis of pistil→pollen signaling in GSI.​

2. Both correct, R doesn’t explain: Wrong—R directly details interaction mechanism behind A.​

3. A correct, R incorrect: Wrong—R accurately describes GSI S-locus recognition.​

4. A incorrect, R correct: Wrong—Direction is definitively pistil→pollen in GSI (vs. sporophytic SI).​

Introduction
In gametophytic self-incompatible flowering plants, the direction of information to inhibit pollen tube is from pistil to pollen, controlled by female determinants interacting with pollen tubes in the style. This GSI mechanism prevents inbreeding via S-locus recognition.​

GSI Molecular Mechanism

Solanaceae/Rosaceae (RNase system):

• Female: S-RNase secreted into style

• Male: S-locus F-box (SLF) fails to ubiquitinate “self” RNases

• Result: Pollen tube RNA degraded → growth arrest​

Papaver (Ca²⁺ system):

• Female: PrsS binds pollen S-protein

• Result: Actin depolymerization, Ca²⁺ burst, PCD​

GSI vs SSI Comparison

Exam Key Points

GSI pollen rejected after germination (style); SSI rejected at germination (stigma). Classic distinction!​