Received March 10, 2009
Accepted May 14, 2009
AtBXL1 encodes a bifunctional 
-D-xylosidase/
-L-arabinofuranosidase required for pectic arabinan modification in Arabidopsis thaliana mucilage secretory cells
Andrej A. Arsovski , Theodore M. Popma , George W. Haughn , Nicholas C. Carpita , Maureen C. McCann , and Tamara L. Western *
Biology Department, McGill University, 1205 ave. Docteur Penfield, Montreal, QC, H3A 1B1, Canada; Botany Department, University of British Columbia, 6270 University Blvd., Vancouver, BC, V6T 1Z4, Canada; Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907; Department of Biological Sciences, Purdue University, West Lafayette, IN 47907
* Corresponding author; email: tamara.western{at}mcgill.ca.
Following pollination, the epidermal cells of the Arabidopsis thaliana ovule undergo a complex differentiation process that includes the synthesis and polar secretion of pectinaceous mucilage followed by the production of a secondary cell wall. Wetting of mature seeds leads to the rapid bursting of these mucilage secretory cells to release a hydrophilic gel that surrounds the seed and is believed to aid in seed hydration and germination. A novel mutant is identified where mucilage release is both patchy and slow, and whose seeds display delayed germination. While developmental analysis of mutant seeds reveals no change in mucilage secretory cell morphology, changes in monosaccharide quantities are detected, suggesting the mucilage release defect results from altered mucilage composition. Plasmid rescue and cloning of the mutant locus revealed a T-DNA insertion in AtBXL1, which encodes a putative bifunctional 
-D-xylosidase/
-L-arabinofuranosidase that has been implicated as a -D-xylosidase acting during vascular development. Chemical and immunological analyses of mucilage extracted from bxl1 mutant seeds, and antibody staining of developing seed coats reveal an increase in (1
5)-linked arabinans suggesting that BXL1 is acting as an -L-arabinofuranosidase in the seed coat. This implication is supported by the ability to rescue mucilage release through treatment of bxl1 seeds with exogenous -L-arabinofuranosidases. Together, these results suggest that trimming of rhamnogalacturonan I arabinan side chains is required for correct mucilage release, and reveal a new role for BXL1 as an -L-arabinofuranosidase acting in seed coat development.
