The expression of a pectinmethylesterase (PME) gene in root tips of pea and its impact on border cell separation and plant-microbe interactions in the rhizosphere

Abstract

Plant exudates have been implicated as a driving force for rhizosphere interactions, the molecular mechanisms of root exudation and release of plant signal molecules remain unknown. Molecular dissection of the process of root exudation may eventually lead to the genetic engineering of plants to manage rhizosphere interactions. In this study, the release of microbial gene inducers was examined by manipulating border cell separation. Specifically, the hypothesis to be tested is whether border cell separation contributes to the release of nodulation (nod) gene inducers. The model system was pea and Rhizobium leguminosarum bv viciae. The experimental approach was to identify gene that play a role in border cell separation, which can be used as a tool to manipulate the process experimentally. Molecular cloning and genetic manipulation by antisense mutagenesis of rcpme1 was carried out to test whether expression of rcpme1 in root tips of pea is required for border cell separation. The cDNA and genomic copy of rcpme1 were cloned and characterized. The rcpme1 promoter was isolated and analyzed by expression of a GUS reporter genes fused to the promoter. Using Agrobacterium rhizogenes-mediated transgenic hairy roots, the effect of PME on border cell separation was examined by expressing antisense rcpme1 mRNA driven by its own promoter. Transgenic hairy roots of pea expressing antisense rcpme1 mRNA showed various phenotypes including incomplete separation of root border cells, decreased border cell number, abnormal root tip morphology, and stunted hairy root development. To test the hypothesis that the process of border cell separation plays a role in root-microbe interactions, pea and its symbiotic partner Rhizobium leguminosarum bv viciae were used. Results from this study indicated that transgenic hairy roots with reduced border cell separation resulted in reduced nod gene induction, while physiological treatments that increase border cell separation activity resulted in enhanced nod gene induction. Increased nod gene induction was correlated with increased nodulation on pea roots. These results are consistent with the hypothesis that the process of border cell separation from root tips of pea is important in border cell separation and consequent release of root exudate-derived nod inducers.