Analysis of inducible anti-herbivore defenses and signals in Populus

The genus Populus, which includes both aspens and poplars, is an important tree for forestry but often undergoes severe defoliation by herbivores. Outbreaks of forest tent caterpillar (Malacosoma disstria , FTC) and the subsequent massive defoliation of its natural host, trembling aspen (Populus tremuloides Michx.), are known to periodically occur in North America. Within aspen populations, however, individual clones show variation in susceptibility to FTC, and this suggests the importance of innate defenses of aspen. Although it has been known that aspen leaves contain phenolic phytochemicals as defensive compounds, the involvement of defensive proteins was not known when this work began. Therefore, one aim of this study was to investigate protein-based induced defenses in trembling aspen, using a molecular approach.; In order to initiate investigation of protein-based induced defenses in trembling aspen, genes for polyphenol oxidase (PPO) and trypsin inhibitor (TI), were isolated and characterized. In time course analyses, it was demonstrated that PPO and TI mRNAs accumulated within several hours in both wounded leaves and unwounded leaves of the same plant. This was consistent with the wound response previously reported from other plant species including hybrid poplar (Populus trichocarpa x P. deltoides), and is indicative of the presence of signaling mechanisms for systemic induction of defense proteins in trembling aspen.; To further obtain insight into mechanisms for inducible defenses, signal molecules for induction of defenses were investigated using a model system, poplar suspension cultures, based on the observation that plant cell cultures often show rapid alkalinization of the medium in response to defense-related signal molecules. Based on the expression profile of rapid alkalinization factors (RALF) genes, it was predicted that RALF may be involved in general cellular signaling such as growth and development rather than defense signaling.; A second alkalinization peptide causing slower alkalinization, slow alkalinization factor (SALF), was also isolated and partially sequenced by Edman degradation. (Abstract shortened by UMI.)...