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Zhang, Xu

Botany and Agricultural Biochemistry

2005

PhD

Plant structural cell wall proteins are thought to model the structure of extracellular matrix of specific cell types in a developmentally regulated manner. Proline-rich proteins (PRPs) are one major class of plant structural cell wall proteins. Previous studies have identified four genes encoding PRPs from Arabidopsis thaliana (AtPRPs). Two of them, ATPRP1 and ATPRP3, share high sequence identity and have similar domain organization. AtPRP3 has been shown to be expressed exclusively in root hair-bearing cells. To characterize the expression pattern of AtPRP1, transgenic lines expressing AtPRP1 promoter -glucuronidase gene (GUS) construct were analyzed by histochemical assays. These studies demonstrate that AtPRP1 expression is associated with root hair formation in arabidopsis roots and that AtPRP1 is under the transcriptional controls of effectors involved in root hair development in a manner similar to that of AtPRP3. To identify ATPRP1 and its subcellular localization, epitope-tagged versions of ATPRP1 were expressed under the control of either the CaMV 35S promoter or AtPRP1 endogenous promoter. Antibodies against the epitope recognize two protein species of ATPRP1 only in the cell wall fractions of transgenic seedlings. Immunohistochemical analysis of the AtPRP1::AtPRP1-HA or AtPRP1-cmyc transgenic lines demonstrates that ATPRP1 accumulates at the basal portion of root hair-bearing cells, in contrasts to ATPRP3, which accumulates extensively at the growing root hair tip and becomes crosslinked within cell wall in mature root hairs. These results suggest different roles of these proteins in modeling cell wall structure during root hair development, which is further supported by the genetic studies of atprp1 and atprp3 mutant alleles. The atprp1 mutants have no detectable abnormality in root hair shape when grown on solid media, while the atprp3 mutant seedlings display dramatic root hair deformations, implying a disturbed tip growth. Moreover, atprp3 seedlings show several morphological changes including decreased cotyledon opening and longer hypocotyls, suggesting altered overall growth responses resulted from their defect root hairs. The atprp1 and atprp3 mutants have distinct phenotypes when grown in soil. Interestingly, plants lackinng both ATPRP1 and ATPRP3 are more similar to atprp1 mutants in respect with root hair morphology and whole plant phenotypes, implicating interactions of different signaling pathways induced by these mutants. Nutrient uptakes, especially nitrate uptake, are affected in both atprp1 and atprp3 plants, with atprp1/atprp3 double mutant plants showing the most significant growth deficiency. These results demonstrate the contribution of structural cell wall proteins to cell shape and function of specific cells, presumably through modeling their cell wall structures. I also characterized the roles of PRP domains and unique domains in the process of ATPRP1 and ATPRP3 protein localization in root hair cell wall. These studies establish a role for the unique domain in the localization of these proteins, and demonstrate that the PRP domain is important for appropriate protein function within root hair cell wall.