| The phloem tissue of higher plants has been shown to contain a diverse population of signaling molecules, implicating the phloem as a long-distance information delivery system. In addition to hormones and proteins, the phloem has recently been shown to contain a heterogeneous population of RNA molecules, suggesting a novel mechanism by which plants use the phloem to transport RNA in order to regulate developmental processes. To investigate this hypothesis, we chose to examine the developmental effects of two highly related RNA molecules previously found in the phloem of pumpkin. C&barbelow;ucurbita m&barbelow;axima GAI p&barbelow;hloem (CmGAIP) and CmGAIP-B were chosen based on their homology to the putative transcription factors, G&barbelow;ibberellic A&barbelow;cid I&barbelow;nsensitive (GAI) and R&barbelow;epressor of g&barbelow;a1-3 (RGA) from Arabidopsis thaliana. Grafting experiments demonstrate that, whereas CmGAIP and CmGAIP-B transcripts are translocated in the phloem, transcripts from the Arabidopsis GAI and RGA homologs are not, suggesting species-specific regulation of long-distance RNA translocation. However, as the tissues used for grafting differ in developmental stages (i.e., vegetative stem tissue used for cucurbit grafts vs. reproductive inflorescence tissue used for Arabidopsis grafts), the possibility also exists that RNA translocation in the phloem is developmentally regulated. Grafting experiments between vegetative wild-type tomato scions and transgenic tomato stock plants expressing the Arabidopsis gai gene further support this hypothesis, as RNA from the gai transgene was phloem mobile. Finally, growth conditions were observed to affect RNA translocation in the phloem, suggesting environmental factors may also influence macromolecular transport in the phloem. This dissertation describes experiments aimed at testing the above hypotheses in order to gain insight into the functional role of phloem-mediated RNA transport. |
