title: Transmitting tissue architecture in basal-relictual angiosperms: Implications for transmitting tissue origins.
abstract: Carpel transmitting tissue is a major floral innovation that is essential for angiosperm success. It facilitates the rapid adhesion, hydration, and growth of the male gametophyte to the female gametophyte. As well, it functions as a molecular screen to promote male gametophytic competition and species-specific recognition and compatibility. Here, we characterize the transmitting tissue extracellular matrix (ECM) and pollen tube growth in basal-relictual angiosperms and test the hypothesis that a freely flowing ECM (wet stigma) was ancestral to a cuticle-bound ECM (dry stigma). We demonstrate that the most recent common ancestor of extant angiosperms produced an ECM that was structurally and functionally equivalent to a dry stigma. Dry stigmas are composed of a cuticle and primary wall that contains compounds that facilitate the adhesion and growth of the male gametophyte. These compounds include methyl-esterified homogalacturonans, arabinogalactan-proteins, and lipids. We propose that transmitting tissue evolved in concert with an increase in cuticle permeability that resulted from modifications in the biosynthesis and secretion of fatty acids needed for cuticle construction. Increased cuticle permeability exposed the male gametophyte to pre-existing molecules that enabled rapid male gametophyte adhesion, hydration, and growth as well as species-specific recognition and compatibility.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada M5S 3B2.
authors.forename: Tammy L
authors.initials: TL
authors.lastname: Sage
authors.forename: Katerina
authors.initials: K
authors.lastname: Hristova-Sarkovski
authors.forename: Veronica
authors.initials: V
authors.lastname: Koehl
authors.forename: Joelle
authors.initials: J
authors.lastname: Lyew
authors.forename: Vincenza
authors.initials: V
authors.lastname: Pontieri
authors.forename: Peter
authors.initials: P
authors.lastname: Bernhardt
authors.forename: Peter
authors.initials: P
authors.lastname: Weston
authors.forename: Shaheen
authors.initials: S
authors.lastname: Bagha
authors.forename: Greta
authors.initials: G
authors.lastname: Chiu