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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

AtBXL1 Encodes a Bifunctional β-D-Xylosidase/-L-Arabinofuranosidase Required for Pectic Arabinan Modification in Arabidopsis Mucilage Secretory Cells^{1,[C],[W],[OA]}

Biology Department, McGill University, Montreal, Quebec, Canada H3A 1B1 (A.A.A., T.L.W.); Botany Department, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 (T.M.P., G.W.H.); and Department of Botany and Plant Pathology (N.C.C.) and Department of Biological Sciences (M.C.M.), Purdue University, West Lafayette, Indiana 47907

Following pollination, the epidermal cells of the Arabidopsis (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 β-D-xylosidase/-L-arabinofuranosidase that has been implicated as a β-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 (15)-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.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Tamara L. Western (tamara.western{at}mcgill.ca).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.138388

^* Corresponding author; e-mail tamara.western{at}mcgill.ca.

Received March 10, 2009; accepted May 14, 2009; published May 20, 2009.