The cell walls of different Chara species (Charophyceae) are characterized by branched galactans rich in 3-O-methylgalactose and absence of arabinogalactan-proteins

Streptophyte algae are the closest relatives of land plants and their latest common ancestor performed the most drastic adaptation that happened in plant evolution around 500 million years ago: the conquest of land. Beside other adaptations, this step required changes in cell wall composition. Today knowledge on cell walls of streptophyte algae and especially presence of arabinogalactan-proteins (AGPs), which are important signaling molecules of all land plants, is limited. To get deeper insights in cell walls of streptophyte algae, especially of the Charophyceae, we performed sequential cell wall extractions of four Chara species. The three species Chara globularis, Chara subspinosa and Chara tomentosa revealed comparable cell wall compositions with pectins, xylans and xyloglucans, whereas Chara aspera was outstanding with higher amounts of uronic acids in the pectic fractions and lack of reactivity with antibodies binding to xylan- and xyloglucan epitopes. Search for AGPs in the four Chara species and also Nitellopsis obtusa revealed presence of galactans with pyranosidic galactose in 1,3-, 1,6- and 1,3,6-linkage, which are typical galactan motifs of land plant AGPs. A unique feature of these branched galactans were high portions of 3-O-methylgalactose. Only Nitellopsis contained substantial amounts of Ara. Bioinformatic search for prolyl-4-hydroxylase necessary for biosynthesis of AGPs revealed one possible functional sequence in the genome of Chara braunii, but no hydroxyproline could be detected in the four Chara species and Nitellopsis obtusa. We conclude that AGPs typical for land plants are absent at least in these members of the Charophyceae.