Pectic galactan affects cell wall architecture during secondary cell wall deposition

Main conclusion beta-(1,4)-galactan determines the interactions between different matrix polysaccharides and cellulose during the cessation of cell elongation. Despite recent advances regarding the role of pectic beta-(1,4)-galactan neutral side chains in primary cell wall remodelling during growth and cell elongation, little is known about the specific function of this polymer in other developmental processes. We have used transgenic Arabidopsis plants overproducing chickpea beta I-Gal beta-galactosidase under the 35S CaMV promoter (35S::beta I-Gal) with reduced galactan levels in the basal non-elongating floral stem internodes to gain insight into the role of beta-(1,4)-galactan in cell wall architecture during the cessation of elongation and the beginning of secondary growth. The loss of galactan mediated by beta I-Gal in 35S::beta I-Gal plants is accompanied by a reduction in the levels of KOH-extracted xyloglucan and an increase in the levels of xyloglucan released by a cellulose-specific endoglucanase. These variations in cellulose-xyloglucan interactions cause an altered xylan and mannan deposition in the cell wall that in turn results in a deficient lignin deposition. Considering these results, we can state that beta-(1,4)-galactan plays a key structural role in the correct organization of the different domains of the cell wall during the cessation of growth and the early events of secondary cell wall development. These findings reinforce the notion that there is a mutual dependence between the different polysaccharides and lignin polymers to form an organized and functional cell wall.