Masters Theses

Date of Award


Degree Type


Degree Name

Master of Science


Plant Sciences

Major Professor

Zong-Ming Cheng

Committee Members

Robert N. Trigiano, Gerald A. Tuscan, Timothy J. Tschaplinski, Timothy G. Rials


Populus is one of the most important tree species for pulp, paper, wood products, and more recently for biomass energy. The increasing need for wood and decreasing land area for forest trees demand the development of fast-growing trees with desirable quality. This experiment was conducted to alter poplar growth and development by manipulating endogenous auxin and ethylene levels through genetic transformation.

Since auxin stimulates vascular differentiation and wood formation, indole-3-acetic acid (IAA) biosynthetic gene,iaaM, driven by a vascular specific promoter, glycine-rich protein promoter (GRP), was inserted into a hybrid aspen (P. canescens x P. grandidentata) to increase the endogenous auxin level. However, because elevated auxin can trigger overproduction of ethylene, which can inhibit plant growth, an ethylene inhibition gene,1-acetocyclopropane-1-carboxylic acid deaminase (ACC deaminase) under the control of cauliflower mosaic virus 35S (CaMV35S) promoter conjugated with GRP-iaaMgene was also inserted into the aspen via the Agrobacterium tumefaciensTi plasmid vectors. Fourteen transgenic plants having GRP-iaaM and thirteen transgenic plants having GRP-iaaM-35S-ACC deaminasegenes were confirmed by polymerase chain reaction (PCR) and Southern blot. Transgenic plants were propagated by cuttings and their heights and diameters were measured to determine the effect of the inserted genes on aspen. The amounts of IAA and ethylene were also measured to investigate the expression of the inserted genes.

Two lines in 2003 and four lines in 2004 of the GRP-iaaM-35S-ACC deaminase transgenic plants were significantly taller than non-transgenic plants while the GRP-iaaM transgenic plants were shorter or similar height with non-transgenic plants. No significant differences in the growth of height and diameter of the GRP-iaaM transgenic plants may be due to the use of a weak promoter because the levels of IAA showed slightly higher than non-transgenic plants, but the differences were not statistically significant. The GRP-iaaM-35S-ACC deaminase transgenic plants showed no significant differences in IAA levels, but had less or similar levels of ethylene compared to nontransgenic control while the GRP-iaaM transgenic plants had elevated ethylene amount. This indicates that the iaaM gene expressed at a low level increased the level of ethylene, but the ACC deaminase gene appeared to reduce the elevated ethylene in the GRP-iaaM-35S-ACC deaminase transgenic plants.

The GRP-iaaM-35S-ACC deaminase transgenic plants having modified IAA and ethylene levels showed more growth in height and volume than the GRP-iaaM transgenic in the linear contrast, and a negative correlation between ethylene amounts and heights was shown. This result indicates that an increased auxin level might have a negative effect in the growth of trees resulted from elevated ethylene level triggered by elevated auxin, but a decreased ethylene level in addition to an increased auxin level might have a positive effect on the growth of trees. Although further detailed analyses are needed, this research suggests that it is possible to manipulate plant hormones, especially ethylene, to change plant growth.

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