The AMOR Arabinogalactan Sugar Chain Induces Pollen-Tube Competency to Respond to Ovular Guidance

Abstract

Precise directional control of pollen-tube growth by pistil tissue is critical for successful fertilization of flowering plants [1-3]. Ovular attractant peptides, which are secreted from two synergid cells on the side of the egg cell, have been identified [4-6]. Emerging evidence suggests that the ovular directional cue is not sufficient for successful guidance but that competency control by the pistil is critical for the response of pollen tubes to the attraction signal [1, 3, 7]. However, the female molecule for this competency induction has not been reported. Here we report that ovular methyl-glucuronosyl arabinogalactan (AMOR) induces competency of the pollen tube to respond to ovular attractant LURE peptides in Torenia fournieri. We developed a method for assaying the response capability of a pollen tube by micromanipulating an ovule. Using this method, we showed that pollen tubes growing through a cut style acquired a response capability in the medium by receiving a sufficient amount of a factor derived from mature ovules of Torenia. This factor, named AMOR, was identified as an arabinogalactan polysaccharide, the terminal 4-O-methyl-glucuronosyl residue of which was necessary for its activity. Moreover, a chemically synthesized disaccharide, the β isomer of methyl-glucuronosyl galactose (4-Me-GlcA-β-(1→6)-Gal), showed AMOR activity. No specific sugar-chain structure of plant extracellular matrix has been identified as a bioactive molecule involved in intercellular communication. We suggest that the AMOR sugar chain in the ovary renders the pollen tube competent to the chemotropic response prior to final guidance by LURE peptides.

The Molecular Dialog between Flowering Plant Reproductive Partners Defined by SNP-Informed RNA-Sequencing

“…A pair of genetically identical sperm develops within a pollen grain and is transported within the cytoplasm of the pollen tube, a highly polar extension of the pollen cell (Berger and Twell, 2011). The pollen tube penetrates and extends through floral tissue and responds to guidance cues (Márton et al, 2005;Okuda et al, 2009;Higashiyama, 2012, 2016;Mizukami et al, 2016;Wang et al, 2016) that direct it into an ovule, where the pollen tube encounters a synergid cell, one of two supportive cells that flank the egg. The pollen tube then bursts just as one of the two synergids degenerates (Denninger et al, 2014;Ngo et al, 2014;Leydon et al, 2015); pollen tube rupture propels the pair of sperm to a position where they contact the egg and central cell (Hamamura et al, 2011;Ngo et al, 2014).…”

Structure-Activity Relation of AMOR Sugar Molecule That Activates Pollen-Tubes for Ovular Guidance

“…In the previous work, we designed a methyl etherified (C-4 position) benzyl glucosyluronate trichloroacetimidate as the glycosyl donor and benzyl protected Gal with unsubstituted hydroxy group at C-6 position as the glycosyl acceptor. With these two glycosylation partners (glycosyl donor and acceptor), we successfully obtained the 1,6-glycoside with a good b-selectivity (Mizukami et al, 2016). Both donor and acceptor can be synthesized from readily available Glc and Gal via classical carbohydrate synthesis.…”

“…4A; Mizukami et al, 2016). A hand-pollinated pistil (15 mm) is excised and cultivated in a thin layer of agar growth medium containing the synthesized disaccharide.…”

“…In our efforts to identify the female molecule responsible for competency control, we discovered an ovular arabinogalactan polysaccharide, named 4-O-methyl-glucuronosyl arabinogalactan (AMOR), which induces competency of the pollen tubes to respond to ovular attractant LURE peptides in Torenia (Mizukami et al, 2016;Fig. 1A).…”

“…A chemically synthesized disaccharide, the beta isomer of 4-O-methylglucuronosyl Gal (4-Me-GlcA-b(1,6)-Gal (G1; Fig. 1B) showed AMOR activity and conferred competency to respond to ovular attractant LUREs (Mizukami et al, 2016).…”