Gametophytic Self-Incompatibility (GSI)
The RNase mechanism
- In gametophytic self-incompatibility (GSI), the SI phenotype of the pollen is determined by its own gametophytic haploid genotype. This is the more common type of SI, existing in the families:Solanaceae, Rosaceae, Plantaginaceae, Fabaceae, Onagraceae, Campanulaceae, Papaveraceae and Poaceae. Two different mechanisms of GSI have been described in detail at the molecular level, and their description follows
The S-glycoprotein mechanism
- The female component of GSI in the Solanaceae was found in 1989.Proteins in the same family were subsequently discovered in the Rosaceae and Plantaginaceae.Despite some early doubts about the common ancestry of GSI in these distantly related families, phylogenetic studies and the finding of shared male determinants (F-box proteins) early established homology.Consequently, this mechanism arose approximately 90 million years ago, and is the inferred ancestral state for approximately 50% of all plants.In this mechanism, pollen tube elongation is halted when it has proceeded approximately one third of the way through the style.
- The female component ribonuclease, termed S-RNase probably causes degradation of the ribosomal RNA (rRNA) inside the pollen tube, in the case of identical male and female S alleles, and consequently pollen tube elongation is arrested, and the pollen grain dies.The male component was only recently putatively identified as a member of the "F-box" protein family.The members of this group typically function as ubiquitin ligases, and they may function by recognizing the matching S-RNase molecules and sending them to proteasomal degradation.Despite some fairly convincing evidence that it may be the male component, several features also make it an unlikely candidate.
- The following mechanism was described in detail in Papaver rhoeas.In this mechanism, pollen growth is inhibited within minutes of its placement on the stigma. The female determinant is a small, extracellular molecule, expressed in the stigma; the identity of the male determinant remains elusive, but it is probably some cell membrane receptor. The interaction between male and female determinants transmits a cellular signal into the pollen tube, resulting in strong influx of calcium cations; this interferes with the intracellular concentration gradient of calcium ions which exists inside the pollen tube, essential for its elongation. The influx of calcium ions arrests tube elongation within 1-2 minutes. At this stage, pollen inhibition is still reversible, and elongation can be resumed by applying certain manipulations, resulting in ovule fertilization.
- Subsequently, the cytosolic protein p26, a pyrophosphatase, is inhibited by phosphorylation possibly resulting in arrest of synthesis of molecular building blocks, required for tube elongation. There is depolymerization and reorganization of actin filaments, within the pollen cytoskeleton within 10 minutes from the placement on the stigma, the pollen is committed to a process which ends in its death. At 3-4 hours past pollination, fragmentation of pollen DNA begins, and finally (at 10-14 hours), the cell dies.
Sporophytic self-incompatibility (SSI)
- In sporophytic self-incompatibility (SSI), the SI phenotype of the pollen is determined by the diploid genotype of the anther (the sporophyte) in which it was created. This form of SI was identified in the families: Brassicaceae, Asteraceae, Convolvulaceae, Betulaceae, Caryophyllaceae, Sterculiaceae and Polemoniaceae. Up to this day, only one mechanism of SSI has been described in detail at the molecular level, in Brassica (Brassicaceae).
- Since SSI is determined by a diploid genotype, the pollen and pistil each express the translation products of two different alleles, i.e. two male and two female determinants. Dominance relationships often exist between pairs of alleles, resulting in complicated patterns of compatibility/self-incompatibility. These dominance relationships also allow the generation of individuals homozygous for a recessive S allele.
- Compared to a population in which all S alleles are co-dominant, the presence of dominance relationships in the population, raises the chances of compatible mating between individuals. The frequency ratio between recessive and dominant S alleles reflects a dynamic balance between reproduction assurance (favoured by recessive alleles) and avoidance of selfing (favoured by dominant alleles).
- The SI mechanism in Brassica. As previously mentioned, the SI phenotype of the pollen is determined by the diploid genotype of the anther. In Brassica, the pollen coat, derived from the anther's tapetum tissue, carries the translation products of the two S alleles. These are small, cysteine-rich proteins. The male determinant is termed SCR or SP11, and is expressed in the anther tapetum (i.e. sporophytically), as well as in the microspore and pollen (i.e. gametophytically). There are possibly up to 100 polymorphs of the S-haplotype in Brassica, and within these there is a dominance hierarchy.
- The female determinant of the SI response in Brassica, is a transmembrane protein termed SRK, which has an intracellular kinase domain, and a variable extracellular domain. SRK is expressed in the stigma, and probably functions as a receptor for the SCR/SP11 protein in the pollen coat. Another stigmatic protein, termed SLG, is highly similar in sequence to the SRK protein, and seems to function as a co-receptor for the male determinant, amplifying the SI response.
- The interaction between the SRK and SCR/SP11 proteins results in autophosphorylation of the intracellular kinase domain of SRK, and a signal is transmitted into the papilla cell of the stigma. Another protein essential for the SI response is MLPK, a serine-threonine kinase, which is anchored to the plasma membrane from its intracellular side. The downstream cellular and molecular events, leading eventually to pollen inhibition, are poorly described.