Graduate Thesis Or Dissertation
 

Structure and expression of two Populus trichocarpa homologs of the floral homeotic gene AGAMOUS

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  • Because of their small genomes, facile clonal propagation, fast growth, and susceptibility to Agrobacterium transformation, poplars (genus Populus) are widely considered model systems for the application of molecular genetics and biotechnology in forestry. However, a major concern over commercial use of genetically engineered trees is the release of transgenes into wild populations. The goal of this study was to characterize two genes that are expected to be critical for female and male reproductive development in poplars, and thus, could be used for genetic engineering of reproductive sterility. This trait would mitigate ecological risks associated with commercial deployment of transgenic trees by preventing the spread of transgenes via pollen and seed. A secondary goal was to use these genes as probes to help understand the reproductive biology of poplars, whose two-whorled, unisexual flowers are distinct from those of any previously studied plant species. We isolated and characterized two closely related genes from P. trichocarpa (black cottonwood), a native tree of the Pacific Northwest. These genes are homologous to AGAMOUS (AG), a gene controlling reproductive development in the model herbaceous plant Arabidopsis. The proteins encoded by PTAG1 and PTAG2 are 89% identical, and phylogenetic analysis suggests that they are most closely related to genes which specify the identity of both stamens and carpels in herbaceous plants. Gene structure is conserved between PTAG1, PTAG2, AG, and the Antirrhinum AG ortholog, PLENA. The floral RNA expression patterns of the PTAG genes are also very similar to those of AG and PLENA. In situ hybridization studies revealed that PTAG1 and PTAG2 are expressed in the center of both female and male floral meristems before reproductive organ primordia have initiated, and in developing stamen and carpels. Unlike AG, PLENA, and other close AG homologs, PTAG transcripts are detected in vegetative tissue. These results suggest that PTAG1 and PTAG2 may function in a largely redundant manner to specify reproductive organs in Populus. Therefore, inhibiting the endogenous genes or proteins is likely to be an effective way to genetically engineer reproductive sterility. However, weak vegetative expression may preclude use of their promoters to ablate floral tissues.
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