Arabinogalactan-proteins (AGPs) are a class of proteoglycans found in cell secretions and plasma membranes. Attention is currently focused on their structure and their potential role in plant cell growth and development. An acid arabinogalactan has previously been shown to be the major component of the cashew nut tree (Anacardium occidentale) gum exudate [Menestrina, J.M. et al., 1998 (Phytochemistry, 47: 715-721)]. Steric exclusion chromatography revealed that this arabinogalactan has a molecular weight of 11.2 x 103 of which 0.5 % (w/w) consists of protein. Analysis of the amino acid composition of the polysaccharide protein moiety showed that it has a high lysine content (82.1%), a low serine (9.5%) and alanine (2.2%) content, and also undetectable amounts of proline or hydroxyproline. Significant (p=0.05) reduction on the number of somatic embryos, regardless differentiation stage, was found for carrot (Daucus carota) cells grown in a culture medium enriched with this arabinogalactan. However, no significant change on the number of somatic embryos, at the cotiledonary stage, was found for carrot cells grown in this arabinogalactan-enriched culture medium. No significant (p=0.05) difference was found for fresh weight of carrot cells grown in the arabinogalactan-enriched culture medium. However, significant reduction on the cellsís fresh weight was noticed when the cells were grown in a culture medium containing a sulfated (D.S. = 1.13) form of the arabinogalactan. Based on these results, we hypothesize that the hydroxyprolin, serine, and alanine-rich protein moiety, which characterizes AGPs, is not a requirement for the arabinogalactans to be able to change the plant cell growth (fresh weight) or differentiation (embryogenetic potential) pattern.

Key words: Arabinogalactan, cell differentiation, Daucus carota, proteoglycans, somatic embryogenesis