4. During gametophytic self-incompatibility the primary response is
   (1) The interaction between the SRK and SCR/SP11 and downstream cellular and molecular events, leading eventually to pollen inhibition
   (2) S-RNase probably causes degradation of the ribosomal RNA (rRNA) inside ovule of female plant
   (3) Formation of concentric ring from Golgi
   (4) Ca²⁺-dependent signaling cascade in incompatible pollen inhibiting elongation

During gametophytic self-incompatibility, the primary response involves the specific molecular interaction between the S-receptor kinase (SRK) present on the stigma and the S-locus cysteine-rich protein (SCR/SP11) from the pollen. This interaction triggers downstream cellular and molecular events, which ultimately result in the inhibition of pollen tube growth, preventing self-fertilization.

Therefore, the correct answer is:

(1) The interaction between the SRK and SCR/SP11 and downstream cellular and molecular events, leading eventually to pollen inhibition.

Introduction

Self-incompatibility (SI) is a reproductive mechanism in flowering plants that prevents inbreeding and promotes genetic diversity by rejecting self-pollen. In gametophytic self-incompatibility, the pollen’s haploid genotype determines the incompatibility response, specifically mediated by an interaction between the female and male determinants encoded at the S-locus. This primary interaction triggers a cascade of molecular events leading to pollen tube growth inhibition.

Molecular Players: SRK and SCR/SP11

• The stigma expresses S-locus receptor kinase (SRK), a transmembrane receptor located on the surface of stigmatic papilla cells.​

• The pollen coat expresses S-locus cysteine-rich protein (SCR), also called S-locus protein 11 (SP11), acting as the male determinant of self-incompatibility.​

• SRK and SCR/SP11 are highly polymorphic, allowing precise recognition of self versus non-self pollen.​

Primary Response Mechanism

• When pollen with the same S-haplotype as the stigma lands on the stigma, SCR/SP11 binds specifically to SRK.​

• This binding activates the kinase domain of SRK via phosphorylation, initiating an intracellular signal transduction cascade.​

• The downstream signaling leads to multiple cellular responses, such as inhibition of pollen hydration, germination, and pollen tube growth.​

• The overall effect is to prevent self-fertilization and ensure only genetically diverse pollen fertilizes the ovule.

Other Proposed Responses

• S-RNases in some gametophytic SI systems degrade RNA within incompatible pollen tubes, but this acts downstream or in a different SI system.​

• Calcium-dependent signaling and other secondary pathways are involved but initiated by the SRK-SCR interaction.​

Summary Table: Key Features of Gametophytic Self-Incompatibility Primary Response

Conclusion

The primary response in gametophytic self-incompatibility is the direct molecular recognition between SRK on the stigma and SCR/SP11 on the pollen. This specific interaction activates signal cascades culminating in pollen inhibition, preventing self-fertilization and promoting genetic diversity.

FAQ

Q: What triggers the primary response in gametophytic self-incompatibility?
The binding of SCR/SP11 pollen ligand to the SRK receptor on the stigma initiates it.​

Q: What happens after SRK activation?
Phosphorylation activates downstream signals that inhibit pollen tube growth.​

Q: Does gametophytic SI involve RNA degradation?
RNA degradation occurs in some SI systems but is not the primary response; it follows receptor activation.