Lily ( Lilium longiflorum L.) pollen protoplast adhesion is increased in the presence of the peptide SCA

Abstract

The style of lily produces a specialized extracellular matrix (ECM) in the transmitting tract epidermis that functions to guide pollen tubes to the ovary. This adhesive ECM contains low esterified pectins and a peptide, SCA (stigma/stylar cysteine-rich adhesin). Together they form a matrix to which pollen tubes adhere as they grow through the style. Pollen tubes also adhere to each other but only when grown in vivo, not in vitro. Pollen does not produce detectable SCA, but when SCA is added to an in vitro growth medium, it binds to pollen tubes that have esterified and low-esterified pectins in their walls. Since adhesion of the pollen tube to the stylar matrix requires tip growth, we hypothesized that the pectin wall at the pollen tube tip interacted with the SCA protein to initiate adhesion with the stylar pectin [Lord (2000) Trends Plant Sci 5:368–373]. Here, we use a pollen protoplast system to examine the effect of SCA on protoplast adhesion when it is added to the growth medium in the absence of the stylar pectin. We found that SCA induces a 2-fold increase in protoplast adhesion when it is added at the start of protoplast culture. This effect is less when SCA is added to the medium after the cell wall on the protoplast has begun to regenerate. We show that among the first components deposited in the new wall are arabinogalactan proteins (AGPs) and highly esterified pectins. We see no labeling for low esterified pectins even after 3 days of culture. In the pollen protoplast culture, adhesion occurs in the absence of the low esterified pectin. The newly formed wall on the protoplast mirrors that of the pollen tube tip in lily, which is rich in AGPs and highly esterified pectins. Thus, the protoplast system may be useful for isolating the pollen partner for SCA in this adhesion event.