Plant programmed cell death and disease/stress resistance.

摘要

Programmed cell death (PCD) is genetically controlled cell suicide. PCD is involved in numerous physiological and pathological processes including the selective elimination of unwanted, unneeded or damaged cells during development and in response to environmental stresses both in plants and in animals. The morphological and biochemical similarities between animal apoptosis (a morphologically described PCD) and plant PCD suggest that some of the steps in this process may be conserved. In contrast to the well-defined cell death pathway in animals, mechanisms and regulators of plant PCD are poorly understood. In our laboratory, we generated transgenic plants harboring animal anti-apoptotic genes, some of which comprise the core regulators for animal apoptosis. Previous studies suggested conserved biological functions of some of these genes, including Bcl-2 Bcl-xl, Ced-9 and IAP, when expressed in plants that have been subjected to pathogen challenge or environmental stresses. In my studies, human Bcl-2 and insect SfIAP were stably introduced into plants. Functional examination of these transgenic plants was carried out. Bcl-2 containing tobacco conferred tolerance to heat, cold, hydrogen peroxide and menadione. SfIAP expressing plants were resistant to heat, salt, fumonisin B1 and the fungal pathogen Alternaria alternata. When these stresses were imposed on wild type plants, hallmark features associated with apoptosis including cell shrinkage, chromatin condensation, DNA fragmentation (TUNEL positively reacting cells), DNA laddering and formation of apoptotic bodies were observed. Transgenic plants did not show any of these characteristics and were resistant. In addition, a consistent delay of tomato fruit ripening was observed in SfIAP expressing plants. Although there is no sequence homolog between anti-apoptotic genes and genes in plant database, we predict that structural/functional homologs exist that regulate plant PCD. I found that SfIAP is an E3 ubiquitin ligase, the activity of which is necessary for cell protection. I found that ethylene signaling was altered and was necessary for plant resistance to A. alternata, and delayed tomato fruit ripening. Similarly, the E3 ubiquitin ligase activity of SfIAP also regulated tolerance to salt stress. These findings indicated that the anti-apoptotic SfIAP protein confers transkingdom protection via the ubiquitin/proteasome pathway.