Apoplastic glycosidases active against xyloglucan oligosaccharides of Arabidopsis thaliana.

All four glycanases necessary for the degradation of xyloglucan oligosaccharides (alpha-fucosidase, alpha-xylosidase, beta-galactosidase and beta-glucosidase) were found in the apoplastic fluid of Arabidopsis thaliana. These activities acted cooperatively on xyloglucan oligosaccharides (XLFG), leading to the sequential formation of XXFG, XXLG, XXXG, GXXG and XXG, as identified by matrix-assisted laser desorption ionization time of flight (MALDI-TOF). AtFXG1 (At1g67830) and AtXYL1 (At1g68560) had been previously identified as the Arabidopsis genes coding for alpha-fucosidase and alpha-xylosidase, respectively. As for the genes coding for beta-galactosidase activity, we identified in phylogenetic trees 12 candidates from family 35 of glycoside hydrolases. Similarly, four genes from family 3 were selected as possible beta-glucosidases active on xyloglucan. The expression level of all the selected genes was studied in different plant regions (young and mature rosette leaves, apical and basal region of the inflorescence stem, roots, flower and siliques) using quantitative real-time reverse transcription-PCR. The expression patterns were very diverse as well as their relationship with growth rates, showing a very complex situation. This could lead to highly varying proportions of the different xyloglucan oligosaccharides in different plant regions and developmental stages.