Crosstalk of arabinogalactan protein, auxin, gibberellin, and callose in Al-treated Tea seedlings

Arabinogalactan proteins (AGP) are a class of cell surface plant peptidoglycans which have been implicated in root elongation and signal transduction pathways. AGPs function not only as markers of cellular identity but also as signaling molecules, which might initiate signal transduction. Aluminum promotes the elongation of tea (Camellia sinensis L.) roots. Although some mechanisms by which Al induced root elongation in tea have been reported, there is no direct evidence on the signaling molecules affected by aluminum and triggering signaling.  In this study the possible involvement of AGP in regulation of auxin, gibberellin, and callose contents after exposure of tea seedlings to 400 μM Al was evaluated.  The results show that the Al-induced elongation of tea roots was accompanied by significant increment of the AGP. The maximum content of endogenous GA3 in the tea root was found after 6 h of the Al treatment. The content of IAA, rapidly (6 h of the treatment) and remarkably declined due to Al to 50% of the control. In addition Al exposure for 6 and 24h decreased the callose content of tea seedlings by 20% and 50% of that of the control cells, respectively. However, Al treatment for 6h increased the gene expression (167% of the control), and the activity of β-1,3-glucanase (150% of the control). The findings suggest a cross-talk between AGP, callose, and the two main growth hormones auxin and gibberellin in tea seedling which was started by Al exposure and resulted in elongation of root.