Question

Short Essay: Briefly, explain the molecular mechanism of the sporophytic self-incompatibility (Plants)

Answer 1

Molecular mechanism of sporophytic plants self-incompatibility:

• Sporophytic plants includes most of the land plants and multicellular algae. The sporophyte represents a diploid multicellular stage in plants (angiosperms-flowering plants, gymnosperms-seed plants) or algae exhibiting alternation of generation or alternation of phases.
• The mechanism of self-incompatibility (SI) is essential, to prevent self-fertilization or inbreeding. SI also aids in outbreeding and genetic diversity.
• SI locus genes are highly polymorphic.
• The SI is determined by product of S allele. The S-locus comprises of multiple (like S1 and S2 locus), tightly linked genes.
• Two major types of SI exhibited are:

1. Gametophyte SI (GSI):

• Most common form of SI, occurring in most angiosperms, like Solanaceae, beets, grasses, roses, etc.
• S- allele product is functional after meiosis. SI controlled by single S-allele.
• Initially both compatible and incompatible pollens germinate into pollen tubes and begin to grow.
• The fate of the gamete is determined by interaction of pollen tube and the style.
• The either pathway may result in preventing self or compatible pollen tube:

a. Gametophyte SI (GSI) based on based on selective degradation of pollen RNAs- Within the style, S-loci synthesize, ribonuclease (S- RNAse), which on entering the pollen tube destroys pollen RNA. This results in halting of pollen tubes.

b. Gametophyte SI (GSI) based on based on Calcium dependent signaling cascade (MARK)- signaling cascade causing pollen cell death.

2. Sporophyte SI (SSI):

• Observed in Asteraceae, Brassica, etc.
• Self-pollen rejection controlled by diploid genotype.
• Three tightly linked gene loci are involved:
• . SLG- S- locus glycoprotein, encodes proteins (receptors) forming part of stigma cell wall. SLG enables SRK activity.

b. SRK- S- locus receptor kinase, encodes transmembrane protein embedded in stigma cell membrane.

c. SCR- S-locus cysteine rich, encodes soluble ligand similar to that secreted by pollens, binding to same receptors.

1. ARC1- Armadillo repeat 1, degradation of stigma proteins.
2. . MLPK – M-locus protein kinase Positive SI mediator.

• Incompatibility reaction occur in the stigma papilla cells, by recognizing self-proteins.
• As observed in S1-S2 system (S1S2 pollen cannot germinate on S1S3 stigma)
• Pollen will not germinate if the stigma contains either of the two alleles. S1S2 pollen, produces both SCR1 and SCR2. If either SCR bind to receptor on the pistil, the kinase activities are triggered, leads to subsequent events like proteasome destruction by ubiquitination of proteins.