Overexpression of Cicer arietinum βIII-Gal but not βIV-Gal in arabidopsis causes a reduction of cell wall β-(1,4)-galactan compensated by an increase in homogalacturonan

In Cicer arietinum, as in several plant species, the β-galactosidases are encoded by multigene families, although the role of the different proteins is not completely elucidated. Here, we focus in 2 members of this family, βIII-Gal and βIV-Gal, with high degree of amino acid sequence identity (81%), but involved in different developmental processes according to previous studies. Our objective is to deepen in the function of these proteins by establishing their substrate specificity and the possible alterations caused in the cell wall polysaccharides when they are overproduced in Arabidopsis thaliana by constructing the 35S::βIII-Gal and 35S::βIV-Gal transgenic plants. βIII-Gal does cause visible alterations of the morphology of the transgenic plant, all related to a decrease in growth at different stages of development. FTIR spectroscopy and immunological studies showed that βIII-Gal causes changes in the structure of the arabidopsis cell wall polysaccharides, mainly a reduction of the galactan side chains which is compensated by a marked increase in homogalacturonan, which allows us to attribute to galactan a role in the control of the architecture of the cell wall, and therefore in the processes of growth. The 35S::βIV-Gal plants do not present any phenotypic changes, neither in their morphology nor in their cell walls. In spite of the high sequence homology, our results show different specificity of substrate for these proteins, maybe due to other dissimilar characteristics, such as isoelectric points or the number of N-glycosylation sites, which could determine their enzymatic properties and their distinct action in the cell walls.