| I examined effects of gypsy moth herbivory and jasmonates, a class of defense hormone, on spatial and chemical partitioning of carbon in Populus, using in-vivo imaging of carbon-11 radioisotope tracer. I found a novel plant response to jasmonic acid and herbivory, faster export of carbon from leaves, and increased partitioning of carbon to stems and roots. I tested whether this increased carbon partitioning would result in increased concentrations of storage compounds in stems and roots, by applying chronic low intensity herbivory or methyl jasmonate treatments to Populus plants. Jasmonate treatment increased starch concentrations in roots, but herbivory decreased root starch concentrations. There was little effect of either herbivory or jasmonate on growth of Populus plants. In fact, there were hints that both herbivory and jasmonate stimulated branch growth. A negative correlation between branch mass and mature leaf starch suggests leaf storage, rather than root or stem storage, may be important to stimulating compensatory growth. To better understand the differences between jasmonates and herbivory, global patterns of gene expression in Populus in response to herbivory or jasmonic acid were studied using cDNA microarrays. Greater than 500 genes were upregulated by herbivory or jasmonic acid. There was a large amount of overlap between genes upregulated by herbivory and jasmonic acid, but there were many more genes uniquely upregulated by each treatment. Differences between herbivory and jasmonic acid included genes related to signaling and defense, as well as genes related to primary metabolism. Phenolic biosynthesis was more highly induced by herbivory than by jasmonic acid. The cost of increased phenolic biosynthesis may explain why the chronic low intensity herbivory decreased root starch. Further studies will be required to understand how plants tolerate herbivory. New technologies, including microarrays and in-vivo isotope imaging promise the level of detailed analysis that will be required to reveal how defenses are induced, and how functions in different organs are coordinated to generate the whole plant response. |
