Arabidopsis cell wall beta-galactosidase gene family: Expression, catalytic activities and biological function in galactose dynamics

Alterations in cell wall assembly and remodeling through hydrolysis of galactosides and galactans in the cell wall accompany several plant developmental events. Exo-galactanases/beta-galactosidases are responsible for the hydrolysis of cell-wall galactose-containing polymers. Based on a catalytic consensus sequence, seventeen sequences of Arabidopsis are predicted to encode beta-galactosidase proteins. Twelve are predicted to be targeted to the cell exterior, suggesting their potential role in cell wall modification. Based on AtBGAL-promoter-GUS (beta-glucuronidase protein) reporter gene expression and quantitative real-time PCR, four of these genes were highly expressed in the root elongation and maturation zones, leaves, stem and flowers. Expression of AtBGALs was absent from the root tips and floral style. AtBGAL1 was highly expressed in the root and leaf tissues relative to other genes. AtBGAL8 was consistently expressed at higher levels in the floral stems and flowers compared to other members. A T-DNA insertional mutant of AtBGAL1 was selected from the Salk Institute collection. This atbgal1 mutant was impaired in its ability to hydrolyze galactose during early stages of leaf development. Elevated Gal levels were particularly evident in young leaves, and cell wall fractionation studies and methylation analysis showed a specific enrichment of the (1 → 4)-beta-galactan in the pectic component of the cell wall. FTIR spectroscopy revealed other changes in pectin content and RG I branching coincident with elevated galactan levels. In conclusion, AtBGAL1 is broadly expressed and encodes an exo-beta-galactanase that functions during cell expansion.