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 protein domain that has been identified in secretory proteins, and some of these are involved in cell wall remodeling. Among Arabidopsis BURP domain proteins, three proteins exhibit strong amino acid similarities with the tomato polygalacturonase 1 beta (PG1β) protein that interacts with a pectin-digesting enzyme. To investigate biological roles of the Arabidopsis PG1β-like proteins (AtPGLs), we generated Arabidopsis lines in which expression of AtPGLs is altered. Among the three AtPGLs, AtPGL3 exhibited highest transcriptional activity throughout all developmental stages. When tissue-specific expression pattern of AtPGL3 was examined, the gene was observed to be active in epidermal cell layers of rosette leaves and in the trichomes. AtPGL triple mutant plants were smaller than wild type plants because cells were smaller in the mutant plants. Interestingly, when we overexpressed AtPGL3 using a 35S promoter, cells in transgenic plants grew larger than those of the wild type, suggesting that AtPGL3 plays a role in cell expansion. A C-terminal GFP fusion protein of AtPGL3 complemented phenotypes of the triple mutant plants and localized to the cell wall. A truncated AtPGL3-GFP fusion protein that lacks 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 its effect on cell expansion. Quantitative RT-PCR and immunoblot analyses indicated that 2 α-expansin genes are down-regulated and up-regulated in the triple mutant and overexpressor lines, respectively. Taken together, AtPGL3 is a cell wall protein required for normal cell expansion and the coexpression results suggest that AtPGLs regulate cell wall loosening, in conjunction with α-expansins, to promote cell growth.