The project goals for this dissertation were to manipulate Populus trichocarpa source-sink relationships to optimize this woody crop species for specific agricultural traits such as increased growth rate, stress tolerance and/or improvements in overall biomass accumulation. We targeted specific tissues such as xylem, where alterations in the relationship of source and sink tissues can lead to the control of xylem cell deposition or of various wood properties. This led to the characterization of 165 protein-protein interactions and 20 protein-DNA interaction which constitute numerous woody tissue related subnetworks. One such network, centered on the DIVARACATA and RADIALIS INTERACTING FACTOR (PtrDRIF), identified PtrWOX13c as an interacting protein. Characterization of PtrWOX13c shows that it displays the ability to control promoters related to lignin biosynthesis genes and overexpression phenotypes show alterations in axillary branch activity. Genes which control the differentiation and specialization of cells such as members of the WOX family are also highly responsive to abiotic stress which can force major changes in plant metabolism and nutrient mobilization. ABA, a prominent plant phytohormone with known roles in the adaptation to stress has shown novel connections in the regulation of growth promoting complexes such as TOR through antagonistic regulatory actions of the SnRK2 protein kinase in Arabidopsis. Characterization of the core ABA signaling in P. trichocarpa has identified a regulatory clade A protein phosphatase which interacts with numerous PtrSnRK2 proteins and when overexpressed in hybrid poplar results in increased height and node production potentially by indirect control of growth promoting complexes like TOR through SnRK2 inhibition. This work has also demonstrated that in addition to the involvement of phytohormones in the regulation of plant development, sugar phosphates such as T6P can exert significant control of plant architecture. Together, these studies comprise the discovery and subsequent characterization of novel wood associated networks, hormone pathways and sugar signaling in the manipulation of P. trichocarpa source-sink relationships for the promotion of biomass accumulation. / PHD / Detailed analyses of gene activity in different tissues or under the influence of various environmental conditions have identified numerous genes that control desirable traits and plant characteristics. However, the activities and functions of the proteins produced from these genes is less understood. One of the ways proteins work is through the formation of complexes with other proteins. Using the commercially valuable tree Populus trichocarpa (poplar) as our research model, we have identified novel complexes of interacting proteins with the potential to sense and respond to the environment and to promote plant growth. We tested the function of some of the members of these newly discovered protein complexes using transgenic poplar. As a result, we revealed previously unknown functions for two poplar proteins: PtrWOX13c promoted increased branching and PtrHAB2 promoted an increase in tree height. Independent of these functional analyses of poplar proteins, we also tested the ability of a sugar phosphate, trehalose6-phosphate, known from previous work to regulate plant growth, for its ability to promote poplar growth. We found that reducing levels of trehalose-6-phosphate resulted in increased branch growth, similar to the impact of the PtrWOX13c protein. In summary, identification of new protein complexes is a valuable strategy for the discovery of proteins that can increase tree growth. Additionally, combining targeted changes in both proteins and regulatory sugars may be a promising path toward future crop improvement and tree domestication.