Biotechnology in Forestry
David E. Harry, Jeffrey O. Dawson, and Robert M. Skirvin
Alder seedlings sprayed with water containing Frankia collected from different places. Later, roots will be examined to determine the number of nitrogen-fixing nodules formed by the different Frankia isolates. Molecular fingerprinting may provide a means to directly identify the particular Frankia isolate present in the module.
Scientists at the Department of Forestry are conducting biotechnology research to better understand some of the diverse organisms that occupy forests, as well as to enhance methods for genetically manipulating these organisms.
Gene structure and function. Trees must often cope with environmental stress to survive. Anaerobic stress, caused by a lack of oxygen, occurs whenever demand for oxygen exceeds supply. Although a plant may experience anaerobic stress after a flood, it may also occur within metabolically active tissues of a nonflooded plant. Anaerobic stress activates a small set of genes and alters biochemical processes that generate cellular energy.
Among the genes activated by anaerobic stress, those encoding alcohol dehydrogenase (ADH) are understood the best. ADH is the enzyme that catalyzes the interconversion of acetaldehyde and ethanol. This enzyme is essential for plants to survive short-term exposures to flooding, but its physiological function is not clearly understood.
Patterns of ADH expression in woody plants are unusual when compared to nonwoody plants, probably because anaerobic stress commonly occurs within metabolically active tissues of tree stems, such as the cambium. Using classical genetic tools and recombinant DNA methods, we are studying the gene (or genes) responsible for ADH expression in different tissues of pines and cottonwoods to understand the molecular mechanisms responsible for switching these genes on and off. These studies will enable us to better understand how anaerobic stress affects tree growth and wood production.
Tissue culture and plant regeneration. Moving genes from test tubes into actual trees requires three methods: one method to shuttle genes into plant cells, one to aseptically grow plant tissues, and one to regenerate trees from isolated tissues. Our research addresses several of these areas.
One project regenerates plants from selected clones of the eastern cottonwood (Populus deltoides) and from immature Populus embryos. Because they grow so rapidly, Populus species are prime candidates for windbreaks and for producing wood or fiber. Four- to ten-year-old Populus trees can be harvested for woody biomass (to be burned directly or converted to chemical fuels) or for pulpwood.
We can dramatically alter plant development in culture by changing the type of sugar or hormones contained in growth media. We can now regenerate plants from isolated tissues of mature individuals as well as from immature embryos. Because hybrid embryos can be rescued before they abort, the latter method allows sexually incompatible Populus species to be crossed.
In collaboration with researchers from the Department of Plant Pathology, another project transfers genes into trees and shrubs that form root nodules containing Frankia, a nitrogen-fixing bacterium with a diverse range of plant hosts. Initial experiments have infected plant tissues from red alder (Alnus rubra) and Russian olive (Elaeagnus angustifolia) with the bacterium Agrobacterium rhizogenes.
By infecting these and other plant species, Agrobacterium transfers a few bacterial genes into the cells of the host plants. This natural process provides a mechanism to shuttle selected genes into plants. From infected plants, we have observed morphologically and biochemically distinct shoots and roots we believe to contain transferred genes. These methods will enable genetic manipulation of nitrogen fixation, which may reduce the need for nitrogen fertilizers.
Although plants need nitrogen to grow but cannot use atmospheric nitrogen, nitrogen fixation is a process that converts atmospheric nitrogen to forms that the plants can use. Legumes such as soybeans fix nitrogen using Rhizobium, a different bacterium.
Many kinds of adaptations allow these baldcypress trees to tolerate flooding. Although their stems are above water and bathed in air, rapidly growing tissues beneath the bark may suffer a shortage of oxygen. Alcohol dehydrogenase enzymes may allow such tissues to maintain rapid growth despite shortages of oxygen.
Microbial biology of Frankia. Despite the economic and environmental importance of nitrogen fixation, we know relatively little about Frankia. Recent technological advances allow "fingerprinting" of Frankia strains based on the nucleotide sequence of RNA contained in ribosomes or cellular organelles that facilitate protein synthesis. We have found that ribosomal RNA varies considerably among Frankia strains, so we can now characterize Frankia collected from different nodules on the same plant, from plants growing in different areas, or even from different soil samples. These methods will help to characterize the population dynamics of these poorly known nitrogen-fixing microorganisms.
David E. Harry, assistant professor of forestry; Jeffrey O. Dawson, professor of forestry; and Robert M. Skirvin, professor of horticulture
