LIGNIN AND SECONDARY CELL WALLS IN RED ALGAE LEND BIOMECHANICAL SUPPORT

PATRICK T. MARTONE, JOSÉ M. ESTEVEZ, FACHUANG LU, JOHN RALPH, MARK W. DENNY & CHRIS SOMMERVILLE

USA

Simposio; Northwest Algal Symposium; 2007

Lignified secondary cell walls are widely considered to be key innovations in the evolution of terrestrial plants from aquatic ancestors, preventing collapse of water-conducting vessel elements and lending biomechanical support to upright stems.  Here we report our discovery of lignified secondary cell walls in the red alga Calliarthron cheilosporioides (Corallinales, Rhodophyta).  Unlike fleshy seaweeds, Calliarthron fronds are calcified with flexible joints, called genicula, that allow fronds to bend and go with the flow.  Genicula play a critical role in the mechanical survival of calcified fronds in wave-swept intertidal habitats.  TEM micrographs have revealed that, as genicula mature, genicular cells develop secondary cell walls, ultimately resembling tracheids in terrestrial xylem.  Using the DFRC method, we have detected three distinct monolignols in Calliarthron tissue – subunits of terrestrial lignins never described in marine algae.  Histological stains suggest that lignins found in gymnosperms and angiosperms are also found in genicular tissue, but are absent from calcified intergenicular tissue.  Lignified cell walls may underlie the unique material properties of genicular tissue and may have evolved convergently in Calliarthron and terrestrial plants as common adaptations to mechanical stress.  Contrary to the current paradigm describing a terrestrial origin of lignin, our data suggest that the developmental pathways for lignin may have evolved in a common ancestor of red and green algae more than 1 billion years ago.