The best studied mechanisms of SI act by inhibiting the germination of pollen on stigmas, or the elongation of the pollen tube in the
pistils. These mechanisms are based on
protein-protein interactions, and the best-understood mechanisms are controlled by a single
locus termed
S, which has many different
alleles in the
species population. Despite their similar morphological and genetic manifestations, these mechanisms have evolved independently, and are based on different cellular components; therefore, each mechanism has its own, unique S-
genes. The
S-locus contains two basic protein
coding regions – one expressed in the
pistil, and the other in the
anther and/or pollen (referred to as the
female and
male determinants, respectively). Due to their physical proximity, these are genetically
linked, and are inherited as a unit. The units are called
S-
haplotypes. The
translation products of the two regions of the
S-locus are two proteins which, by interacting with one another, lead to the arrest of pollen germination and/or pollen tube elongation, and thereby generate an SI response, preventing fertilization. However, when a female determinant interacts with a male determinant of a different haplotype, no SI is created, and fertilization ensues. This is a simplistic description of the general mechanism of SI, which is more complicated, and in some species the
S-haplotype contains more than two protein coding regions. Following is a detailed description of the different known mechanisms of SI in plants.
Gametophytic self-incompatibility In gametophytic self-incompatibility, the SI
phenotype of the pollen is determined by its own
gametophytic haploid genotype. This is the most common type of SI. Two different mechanisms of gametophytic self-incompatibility have been described in detail at the molecular level, and their description follows.
The RNase-based SI mechanism 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 protein, 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. By 2000, proteins involved in gametophytic self-incompatibility belonging to the same RNase gene family were also found to cause pollen rejection in species of
Rosaceae and
Plantaginaceae. and the finding of shared male determinants (
F-box proteins) strongly supported
homology across
eudicots. Therefore, this mechanism likely arose approximately 90 million years ago, and is the inferred ancestral state for approximately 50% of all plant species. Predictions about the wide distribution of this mechanism of SI were confirmed in the early 21st century, placing additional support of its single ancient origin. Specifically, a style-expressed T2/S-RNase gene and pollen-expressed F-box genes are now implicated in causing SI among the members of
Rubiaceae,
Rutaceae,
Cactaceae, and
Primulaceae. Therefore, other mechanisms of SI are thought to be recently derived in eudicots plants, in some cases relatively recently. One particularly interesting case is the SI expressed in Prunus species, which functions through self-recognition (the cytotoxic activity of the S-RNases is inhibited by default and selectively activated by the pollen partner S-haplotype-specific F-box protein (SFB) upon self-pollination), while SI in the other species with S-RNase functions through non-self recognition (the S-RNases are selectively detoxified upon cross-pollination).
The S-glycoprotein mechanism In this mechanism, pollen growth is inhibited within minutes of its placement on the stigma, and the described underlying molecular mechanism detailed for
Papaver rhoeas so far appears restricted to the plant family
Papaveraceae, but with only a narrowly confined taxonomic search outside this single species. 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. 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
apoptotically.
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 SI mechanism in Brassica 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 as well as in the
microspore and pollen (i.e. sporophytically). 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. A downstream signaling cascade leads to proteasomal degradation that produces an SI response. ==Other mechanisms of self-incompatibility==