AtPGL3 is an Arabidopsis BURP domain protein that is localized to the cell wall and promotes cell enlargement

The BURP domain is a plant-specific domain that has been identified in secretory proteins, and some of these are involved in cell wall modification. The tomato polygalacturonase I complex involved in pectin degradation in ripening fruits has a non-catalytic subunit that has a BURP domain. This protein is called polygalacturonase 1 beta (PG1β) and the Arabidopsis genome encodes three proteins that exhibit strong amino acid similarities with PG1β? We generated Arabidopsis lines in which expression levels of AtPGLs are altered in order to investigate the biological roles of the Arabidopsis PG1β-like proteins (AtPGLs). Among the three AtPGLs ( AtPGL1-3), AtPGL3 exhibited the highest transcriptional activity throughout all developmental stages. AtPGL triple mutant plants have smaller rosette leaves than those of wild type plants because the leaf cells are smaller in the mutant plants. Interestingly, when we overexpressed AtPGL3 using a 35S promoter, leaf cells in transgenic plants grew larger than those of the wild type. A C-terminal GFP fusion protein of AtPGL3 complemented phenotypes of the triple mutant plants and it localized to the cell wall. A truncated AtPGL3-GFP fusion protein lacking the BURP domain failed to rescue the mutant phenotypes even though the GFP protein was targeted to the cell wall, indicating that the BURP domain is required for the protein's effect on cell expansion. Quantitative RT-PCR and immunoblot analyses indicated that the α-expansin 6 gene is up-regulated in the overexpressor plants. Taken together, these results indicate that AtPGL3 is an apoplastic BURP domain protein playing a role in cell expansion.