Comparative expression analysis of genes induced during development of bacterial rot and induction of hypersensitive cell death in lettuce

Stressed to Death: The Role of Transcription Factors in Plant Programmed Cell Death Induced by Abiotic and Biotic Stimuli

Programmed cell death (PCD) is a genetically controlled pathway that plants can use to selectively eliminate redundant or damaged cells. In addition to its fundamental role in plant development, PCD can often be activated as an essential defense response when dealing with biotic and abiotic stresses. For example, localized, tightly controlled PCD can promote plant survival by restricting pathogen growth, driving the development of morphological traits for stress tolerance such as aerenchyma, or triggering systemic pro-survival responses. Relatively little is known about the molecular control of this essential process in plants, especially in comparison to well-described cell death models in animals. However, the networks orchestrating transcriptional regulation of plant PCD are emerging. Transcription factors (TFs) regulate the clusters of stimuli inducible genes and play a fundamental role in plant responses, such as PCD, to abiotic and biotic stresses. Here, we discuss the roles of different classes of transcription factors, including members of NAC, ERF and WRKY families, in cell fate regulation in response to environmental stresses. The role of TFs in stress-induced mitochondrial retrograde signaling is also reviewed in the context of life-and-death decisions of the plant cell and future research directions for further elucidation of TF-mediated control of stress-induced PCD events are proposed. An increased understanding of these complex signaling networks will inform and facilitate future breeding strategies to increase crop tolerance to disease and/or abiotic stresses.

Alternative oxidase in resistance to biotic stresses: Nicotiana attenuata AOX contributes to resistance to a pathogen and a piercing-sucking insect but not Manduca sexta larvae

The role of the alternative respiratory pathway in the protection of plants against biotic stress was examined in transgenic tobacco (Nicotiana attenuata) plants (irAOX) silenced in the expression of ALTERNATIVE OXIDASE (AOX) gene. Wild-type and irAOX plants were independently challenged with (1) chewing herbivores (Manduca sexta), (2) piercing-sucking insects (Empoasca spp.), and (3) bacterial pathogens (Pseudomonas syringae pv tomato DC3000), showing that all these treatments can strongly elicit accumulation of AOX gene transcripts in wild-type plants. When N. attenuata chemical defenses and resistance were examined, irAOX plants showed wild-type levels of defense-related phytohormones, secondary metabolites, and resistance to M. sexta. In contrast, piercing-sucking leafhoppers (Empoasca spp.) caused more leaf damage and induced significantly higher salicylic acid levels in irAOX compared with wild-type plants in the field and/or glasshouse. Subsequently, irAOX plants accumulated lower levels of defense metabolites, 17-hydroxygeranyllinalool diterpene glycosides, caffeoylputrescine, and nicotine compared with wild-type plants under prolonged attack of Empoasca spp. in the glasshouse. Finally, an accelerated cell death phenotype was observed in irAOX plants infected with P. syringae, which correlated with higher levels of salicylic acid and hydrogen peroxide levels in pathogen-infected irAOX compared with wild-type leaves. Overall, the AOX-associated changes in phytohormone and/or redox levels appear to support the resistance of N. attenuata plants against cell piercing-sucking insects and modulate the progression of cell death in pathogen-infected tissues but are not effective against rapidly feeding specialist herbivore M. sexta.

Coordination of a mitochondrial superoxide burst during the hypersensitive response to bacterial pathogen in Nicotiana tabacum

We characterized responses of Nicotiana tabacum to pathovars of the bacterial pathogen Pseudomonas syringae. These included a compatible response associated with necrotic cell death (pv. tabaci), an incompatible response that included hypersensitive response (HR) cell death (pv. maculicola) and an incompatible response that induced defences but lacked the HR (pv. phaseolicola). Signalling molecules (salicylic acid, nitric oxide, H(2)O(2)) known to induce the stress responsive tobacco Aox1a gene [that encodes the mitochondrial electron transport chain (ETC) component alternative oxidase (AOX)] accumulated preferentially during the HR, but this did not elevate Aox1a transcript or AOX protein, while the transcript and protein were strongly elevated during the defence response to pv. phaseolicola. In addition, matrix manganese superoxide dismutase (MnSOD) activity declined during the HR, unlike its response to the other pathovars, and unlike the response of other superoxide dismutase (SOD) enzymes. Finally, the HR (but not the response to pv. phaseolicola or pv. tabaci) was accompanied by an early and persistent mitochondrial superoxide (O(2)(-)) burst prior to cell death. We propose that a coordinated response of the major ETC mechanism to avoid O(2)(-) generation (AOX) and the sole enzymatic means to scavenge mitochondrial O(2)(-) (MnSOD) is important in the determination of cell fate during responses to pathogen.

The expression, function and regulation of mitochondrial alternative oxidase under biotic stresses

To survive, plants possess elaborate defence mechanisms to protect themselves against virus or pathogen invasion. Recent studies have suggested that plant mitochondria may play an important role in host defence responses to biotic stresses. In contrast with animal mitochondria, plant mitochondria possess a unique respiratory pathway, the cyanide-insensitive alternative pathway, which is catalysed by the alternative oxidase (AOX). Much work has revealed that the genes encoding AOX, AOX protein and the alternative respiratory pathway are frequently induced during plant-pathogen (or virus) interaction. This raises the possibility that AOX is involved in host defence responses to biotic stresses. Thus, a key to the understanding of the role of mitochondrial respiration under biotic stresses is to learn the function and regulation of AOX. In this article, we focus on the theoretical and experimental progress made in the current understanding of the function and regulation of AOX under biotic stresses. We also address some speculative aspects to aid further research in this area.