Objectives: An understanding of the current state of knowledge of sporophytic and gametophytic self-incompatibility.
In the last lecture we investigated pollen development and pollen tube growth through transmitting styles. In pollination, the pollen grains traveled via animals or air currents to other flowers in the vicinity or simply fell onto the stigma of the same flower from which the pollen was produced (self-fertilization). Regardless, the grains must alight on a stigma from a flower of the same species in order to have a chance of passing on the genetic information they contain to the next generation. Some flowers are capable of fertilizing their own ova with the pollen produced on adjacent anthers. Some species even enhance this self-fertilization by pollinating the stigma prior to the flower opening (cleistogamy) (e.g. arabidopsis). Other species are not capable of self-fertilization and are termed “self-incompatible”. It is this phenomenon and the mechanisms behind it that we are going to talk about in this lecture.
There are two major self-incompatibility (SI) systems distinguished by sporophytic or gametophytic determination of the self-incompatible reaction. Both are determined by the S-locus, a single, multiallelic locus that imparts to pollen a specific phenotype recognized by the stigma and style of potential receptor plants. In the gametophytic system, the phenotype of the pollen is determined by the gamete’s genotype. In the sporophytic system, the phenotype of the pollen is determined by the diploid parent’s genotype. This difference between the two systems has been elegantly explained by assuming that gamete phenotype is determined by factors expressed after (gametophytic system) or before (sporophytic system) the advent of meiosis. If the S-allele becomes active prior to meiosis then the gametes obtain and retain both self-incompatibility factors present in the paternal genotype (sporophytic). If the S-allele is silent until after meiosis is complete, then only one of the paternal incompatibility factors is present per gamete (gametophytic) (Fig. 1).
The gametophytic self-incompatibility system:
In this system fertilization does not occur any time the S-allele of the gamete matches an active S-allele of the plant to be fertilized. Typically, the pollen complete germination on the stigma but, at some point as the pollen tube traverses the transmitting tissue, its progress is terminated and the pollen tube dies, probably through uptake of RNases present on the stigma and in the style (Fig. 1).
Figure 1: The proposed mechanism leading to the development of the gametophytic- or sporophytic-self-incompatibility systems. The green symbols are the proteins produced from the “S-alleles” (really S-haplotype genes; see below).
The sporophytic self-incompatibility system:
This system is dependent on the ability of the stigma and style to differentiate between pollen produced by the same plant or genetically related plant and pollen from a plant, of the same species, that is not genetically related. Typically this inhibition is rapid resulting in the inability of the genetically related pollen to complete germination on the stigma and/or the inability of the protruded pollen tube to penetrate the papillar cell wall. The phenotype of the pollen is determined sporophytically by the diploid genome of the parent plant (Fig. 1). Typically, in the Brassicaceae, the stigma becomes self-incompatible only 1 day prior to the flower opening.
Mechanisms of self-incompatibility:
There is a wide variety of alleles associated with self-incompatibility (upwards of 60 identified in Brassica oleracea) and there are codominant and/or dominant allelic interactions that determine the intensity of the self-incompatibility response manifested. This information, combined with the realization that there are at least three genes responsible for the SI response, has led to the adoption of the term “S haplotype” rather than the classical “S-allele” to refer to the inherited information determining this complex response. Nevertheless, the term “S-allele” is quite entrenched in the literature and we will continue to use these two terms interchangeably. There are two major recognized intensities of S-haplotype responses distinguished based on the number of successful self-pollen germinations occurring in heterozygous plants. Class I responses permit the germination and successful pollen tube development of between 0 and 10 per self-pollinated stigma while Class II responses permit between 10 and 30.
In contrast to previous belief, there are three tightly linked, transcribed genes that participate in determining self-incompatibility and hence, the S-locus in both Sporophytic and Gametophytic self-incompatibility. One gene is the S-locus glycoprotein (SLG) which is a stigma-transcribed gene producing an extracellularly excreted protein that acts as an assistant receptor protein in the sporophytic self-incompatibility response. The second is highly related in amino-acid homology to SLG over that part of its length associated with the receptor portion. In addition to the receptor, it also encodes a membrane spanning domain, and a cytosolic protein kinase domain with phosphorylating activity (a serine/threonine kinase). This is the S-locus receptor kinase (SRK for S-locus Receptor Kinase). This membrane associated receptor is only present in the stigma. The third is the S-locus pollen protein 11 (SP11) or the S-locus Cysteine-Rich protein; SCR), two names for the same protein. The SP11 gene exhibits pollen specific gene expression. This protein is deposited in the pollen coat and leaches out onto the stigma upon the pollen binding to it. The SRK determines the S-haplotype specificity of the stigma while the SCR/SP11 determines the S-haplotype specificity of the pollen. SLG along with SRK form a high-affinity receptor in the membrane of the stigma for SCR/SP11. SLG is not necessary for SRK to bind SCR/SP11 (resulting in a Class II response) but in those species where it is functional, it can heighten the stringency of the SI response to a Class I response.
It is currently thought that, at least in the Brassicaceae, the SLG arose by duplication of the SLK and selective deletion of exons and introns to leave a highly expressed (very active promoter) gene producing a secreted protein, SLG. The SLK has an extracellular domain that is thought to interact with the SLG to enhance pollen incompatibility.
The SRK is usually inhibited from autophosphorylating itself on the cytoplasmic face of the membrane by a Thioredoxin H-like protein (THL) bound to it. If the SRK is not phosphorylated it is inactive. In self-incompatible plants, SRK (with or without the help of SLG) binds SRC/SP11 from the incompatible pollen and THL immediately disassociates from the kinase domain. Autophosphorylation activates SRK which recruits another kinase, Modifier Locus Protein Kinase (MLPK) which together activate an Armadillo Repeat Containing Protein (ARC). ARC recognizes pollen growth promoting proteins, poly-ubiquitinates them and has them destroyed via the 26S proteasome.
In Gametophytic SI, the Style produces RNases that are capable of entering the pollen tube. In compatible interactions, once the RNases are in the tube they are immediately inhibited by a pollen-produced RNase inhibitor (RNasin) and then ubiquitinated and destroyed. In incompatible interactions, at least one of the RNases, upon entering the tube, is bound by a pollen factor that protects the RNase from the RNasin. The protected RNase attacks the ribosomal RNA and destroys it, the ribosomes, and thus, the protein production capacity of the tube. Growth is arrested and death soon follows.