NITROGEN REMOBILIZATION AND THE NUCLEOSIDE PHOSPHORYLASE-LIKE VEGETATIVE STORAGE PROTEIN FAMILY IN POPULUS: CHARACTERIZATION, REGULATION AND TRANSGENES

Abstract

Nutrient remobilization and storage allow plants to direct resources toward growth, maintenance and reproduction and redirect nutrients in response to environmental conditions or stresses. Particularly for perennial plants, these capabilities are critical to surviving periods of unfavorable growth such as winter and nutrient limited environments. In <italic>Populus</italic>, bark storage proteins (BSPs) have a dominant role in seasonal storage, and proteins related to BSPs, known as nucleoside phosphorylase-like (NP-like) proteins, can also participate in short-term storage. This research presents a comprehensive examination of the NP-like gene family by characterizing their expression, exploring evolutionary relationships within the plant kingdom and investigating metabolic regulation. I also developed and tested a set of qPCR reference genes to use for data normalization in two <italic>Populus</italic> species and four tissue-types. Lastly, transgenic trees were created to investigate the developmental or physiological functions of altered levels of BSP.

Experiments characterizing the spatial and temporal expression of NP-like genes implicated a functional role for all members. Those results also support the phylogenetic analyses demonstrating the expansion of the gene family, which may have occurred through subfunctionalization. I also examined the regulation of carbon (C) and nitrogen (N) metabolites on the NP-like gene family expression and observed that amino acids, N compounds and gamma-aminobutyric acid (GABA) treatments modulate expression and likely have a role in regulatory pathways. By investigating transgenic trees with altered BSP levels, I present preliminary evidence that BSPs may have a role in nutrient signaling capable of modulating photosynthesis in young leaves.

The results of this work deepen our understanding of nutrient remobilization and storage in <italic>Populus</italic> on regulatory, evolutionary and functional levels. Practically, the results can advance efforts to increase N use efficiency for sustainable biomass increases in <italic>Populus</italic> for use in agro-forestry, as biofuel feedstock, in phytoremediation and for carbon sequestration.