Interaction of a Stylar Pectic Polysaccharide and a Basic Protein (SCA) Mediates Lily Pollen Tube Adhesion

Abstract

Though pectins are implicated in cell adhesion in plants, this has never been tested directly. We developed an in vitro assay to study pollen tube adhesion to the stylar extracellular matrix (ECM) in lily. The adhesion of pollen tubes to the ECM of the stylar transmitting tract epidermis in vivo is proposed to be essential for a proper delivery of the sperm cells to the ovary. Using the assay, we identified two stylar molecules responsible for adhesion, a small protein and a pectic polysaccharide. The combination of at least these two molecules is required for this adhesion event. The 9-kD protein is cysteine-rich with some sequence similarity to lipid transfer protein. We named it stigma/style cysteine-rich adhesin (SCA). The second molecule has been isolated from the style using an imidazole extraction method and is mostly composed of galacturonic acid (70–75 mole%) with arabinosyl, galactosyl, rhamnosyl and glucuronosyl residues. This fraction reacts strongly with JIM5 (monoclonal antibody [MAb] to low esterifed homogalacturonans) and has some reaction with JIM7, LM5 and PAM 1(MAbs to esterified homogalacturonans, β-[1–4]-D-galactans, and blocks of 30 Ga1A repeat units). Pollen tube adhesion can be significantly reduced with a pretreatment of this pectic fraction with endopolygalacturonase. All these data implicate a stylar pectic polysaccharide in lily pollen tube adhesion. In vivo immuno-localization data show that SCA and low esterified homogalacturonan are co-localized at the transmitting tract epidermal surface where the pollen tubes adhere. Binding assays reveal that pectin and SCA bind each other in a pH dependent manner and that binding is necessary to produce pollen tube adhesion in the assay. Involvement of pectic polysaccharide and proteins in cell adhesion will be discussed.