S-RNase uptake by compatible pollen tubes in gametophytic self-incompatibility

Abstract

Many flowering plants avoid inbreeding through a genetic mechanism termed self-incompatibility. An extremely polymorphic S-locus controls the gametophytic self-incompatibility system that causes pollen rejection (that is, active arrest of pollen tube growth inside the style) when an S-allele carried by haploid pollen matches one of the S-alleles present in the diploid style. The only known product of the S-locus is an S-RNase expressed in the mature style. The pollen component to this cell-cell recognition system is unknown and current models propose that it either acts as a gatekeeper allowing only its cognate S-RNase to enter the pollen tube, or as an inhibitor of non-cognate S-RNases. In the latter case, all S-RNases are presumed to enter pollen tubes; thus, the two models make diametrically opposed predictions concerning the entry of S-RNases into compatible pollen. Here we use immunocytochemical labelling of pollen tubes growing in styles to show accumulation of an S-RNase in the cytoplasm of all pollen-tube haplotypes, thus providing experimental support for the inhibitor model.

Structural and Transcriptional Analysis of the Self-Incompatibility Locus of Almond: Identification of a Pollen-Expressed F-Box Gene with Haplotype-Specific Polymorphism

“…Despite the apparent difficulty of accommodating the high degrees of structural diversity of the S-RNases, the inhibitor model is supported by recent findings on S haplotype-independent uptake of the S-RNase by the pollen tube (Luu et al, 2000) and the essential role of pollen S products for pollen tube elongation (Golz et al, 2001). As a possible interpretation of the peculiar nature of the pollen S product in the model, it was hypothesized that a pollen S protein contains an RNase inhibitor domain and an S haplotype specificity domain and that it interacts differently with self and nonself S-RNases .…”

“…This finding suggests that the pollen S product is essential for pollen tube elongation, which is consistent with the inhibitor model but in conflict with the gatekeeper model. Immunocytological experiments showing the entry of the S-RNase into the compatible pollen tube support the inhibitor model (Luu et al, 2000). The identification and characterization of the pollen S gene is essential to understanding the molecular mechanisms of the RNase-based GSI.…”

Identification and characterization of S-RNase genes and S-genotypes in Prunus and Malus species

“…In addition, all the four SFB alleles have undergone loss of hyper-variable and/or variable regions. In all the species exhibiting S-RNase-based self-incompatibility, the acceptance of compatible pollen tubes requires the degradation or inhibition of the S-RNases produced by the pistils; in Prunus this role is thought to be played by a still uncharacterized general inhibitor (GI), and SBF proteins are supposed to protect S-RNases from degradation/detoxification by the GI (Luu et al 2000;Tao and lezzoni 2010). Thus, it is reasonable to speculate the hyper-variable and/or variable regions play an important role in protecting S-RNases from inhibition.…”

Rab11 GTPase-Regulated Membrane Trafficking Is Crucial for Tip-Focused Pollen Tube Growth in Tobacco

“…Some of these vesicles, together with endocytosed vesicles that originate from membrane around the apical and subapical regions, are retrotransported along the center of pollen tubes. Endocytic vesicles are likely to have packaged within them molecules from the extracellular matrix and, for pollen tubes elongating in the pistil, contain molecules secreted by female tissues, some of which have been localized inside pollen tubes (Lind et al, 1996;Luu et al, 2000). To date, little is understood about how anterograde and retrograde vesicle trafficking, fusion with the plasma membrane, and budding from the plasma membrane are regulated during pollen tube tip growth.…”