Progress 12/01/03 to 12/01/04

Outputs
During seed formation, there are multiple points where environmental stress can terminate plant reproduction. In Arabidopsis thaliana, reductions in plant fertility resulted from gametophyte abortion and embryo senescence. In our studies, three-fourths of the gametophytes aborted prior to fertilization, while those gametophytes that survived were fertilized and formed embryos. Most of these embryos senesced and did not mature into viable seeds. Cells in the gametophyte produced reactive oxygen species (ROS), which are known to signal programmed cell death. After these signaling molecules were synthesized in aborting gametophytes, concentric rings of endoplasmic reticulum and autophagic bodies formed. These structures have been found in stressed plant and animal cells. Autophagic bodies degrade and recycle cellular contents. Based on the observed alterations in anatomy, we propose that some of the resources present in ovules were degraded and exported prior to abortion. Arabidopsis oligonucleotide microarrays were used to identify changes in transcript abundance that arise during ovule abortion. These Affymetrix microarrays measure transcript abundance throughout the entire Arabidopsis genome. Following environmental stress that leads to over 90% ovule abortion, a total of 535 transcripts changed significantly. These transcripts encode proteins that regulate carbon metabolism, cell division, ion homeostasis, signaling components, hormone biosynthesis, and transcription. Most importantly, transcripts were identified in pathways that are known to regulate ROS biosynthesis, ROS detoxification, and programmed cell death.

Impacts
Environmental stress reduces crop yields of seed crops, such as corn, wheat, soybean, and barley, four- to ten-fold below their optimum. These reductions are caused, in part, by embryo senescence and ovule abortion. Molecular tools and microarrays were used from a model plant to identify candidate genes that regulate these two processes. A number of these genes, which are expressed in stressed ovules, are known to cause cell death. The expression of these genes can now be evaluated in the above crop species to determine if they are responsible for ovule abortion and, as a result, reduced seed set. For those genes that regulate ovule abortion, traditional breeding or molecular techniques can be used to develop cultivars that express these genes at low levels, thereby increasing crop productivity.

Publications

• Bernard A. Hauser and Kelian Sun (2005). Environmental stress induces the formation of autophagic bodies and concentric rings of endoplasmic reticulum in Arabidopsis thaliana gametophytes. Submitted.
• Kelian Sun and Bernard A. Hauser (2005). Environmental stress causes changes in gene expression and induces ovule abortion. In preparation.
• Sung Ok Park, Zhengui Zheng, David G. Oppenheimer, and Bernard A. Hauser (2005). The PRETTY FEW SEEDS2 locus encodes an Arabidopsis homeodomain protein that regulates ovule development. Submitted.
• Kelian Sun, Kimberly Hunt, and Bernard A. Hauser (2004). Ovule abortion in Arabidopsis thaliana triggered by stress. Plant Physiol. 135: 2358-2367.
• Sung Ok Park, Soon Hwang, and Bernard A. Hauser (2004). The phenotype of Arabidopsis ovule mutants mimics an ancestral reproductive structure: telomes surrounding a sporangium. Proc. R. Soc. Lond. B 271: 311-316.