| Ovule or seed abortion occurs when plants are stressed or grown under unfavorable conditions. Factors controlling ovule abortion are poorly understood and salt-tolerant crops have not been developed by traditional breeding methods. Consequently, a project was undertaken to study ovule abortion in Arabidopsis. Arabidopsis ovules undergo abortion when plants are stressed with 200 mM NaCl. Cells in aborting ovules vacuolized and DNA fragmented, both markers for programmed cell death (PCD). Salt stress also affected male gametophyte development by delaying filament elongation and causing defects in pollen grains. Under stress conditions, embryos formed from fertilized ovules usually senesced.; To discover what molecular responses occur during ovule abortion, gene expression was monitored using Affymetrix 24K genome arrays. Transcript levels in pistils that were stressed for 6, 12, 18, and 24 hours were compared with levels in controls. In microarray experiments, 535 genes were identified as salt-responsive genes. These genes encode proteins with a variety of functions. The expression of 65 transcription factors, some of which were known to be involved in stress responses, significantly changed during ovule abortion. Mineral analysis showed that Na+ ions accumulated inside flowers, while the concentration of other minerals significantly decreased these. The expression of ion transporters was induced, presumably to reestablish ion homeostasis after salt stress. Reactive oxygen species (ROS), a byproduct of normal metabolic reactions, first accumulated inside the embryo sac, then spread into the chalaza and integuments. The amount of ROS was proportional to stress duration. Expression of genes encoding ROS detoxification enzymes initially increased, but most of these were repressed once ovules were induced to abort.; One homeodomain transcriptional factor, induced six hours after plants were stressed, was further investigated. Plants that were transformed with constructs that increased the expression level of the HB-5 gene showed an early senescence phenotype. Analysis showed HB-5 transcripts were abundant in the senescing tissues. In addition, the expression of four senescence-associated genes increased in HB-5-overexpressing plants. HB-5 transcripts were present inside the mature embryo sac, vascular tissues, and ovule and stamen primordia. We propose that HB-5 plays a regulatory role in the onset of senescence, especially in the reproductive organs. |
