Cashew-nut tree exudate gum: Identification of an arabinogalactan-protein as a constituent of the gum and use on the stimulation of somatic embryogenesis

Abstract

Arabinogalactan-proteins (AGPs) are a class of plant proteoglycans. In this work, we show that gel permeation chromatography of the cashew-nut tree exudate gum (CNTG), previously characterized as an acidic arabinogalactan (type II), yielded a fraction named fraction III. SDS-PAGE analysis of CNTG has an AGP-like smear, weakly stained by the β-glucosyl Yariv reagent. CNTG and fraction III reacted strongly with the JIM13 anti-AGP monoclonal antibody and presented a protein content of 1.0 and 0.2% (w/w), respectively. CNTG and fraction III showed a high Hyp content (∼50%), and Ser, Pro, Thr and Leu as the most abundant amino acids. Analysis of the carbohydrate structure, its extracellular location and low protein content, and the amino acid composition indicate that CNTG contains at least one fraction that fits the diagnostic features of a “classical” AGP. CNTG-enriched (0.1 μg ml −1) culture medium stimulated carrot somatic embryogenesis after 2 and 3 weeks of culture. The conversion of somatic embryos into plantlets was also enhanced in CNTG-enriched culture media, making this gum a potential tool for the improvement of conversion rates in systems in which low conversion rates are a constraint to micropropagation and breeding programs.