Plant Cell Death Caused by Fungal, Bacterial, and Viral Elicitors: Protective Effect of Mitochondria-Targeted Quinones

The strategy of innate immunity in humans and animals is as follows: cell receptors recognize the conservative molecular structures of large groups of pathogens, i.e. pathogen-associated molecular patterns (PAMPs) 1322 Abbreviations: DCF, 2′,7′-dichlorofluorescein; DCFH-DA, 2′,7′-dichlorofluorescin diacetate; EC, epidermal cells; ETI, effectortriggered immunity; GC, guard cells; HR, hypersensitive response; LPS, lipopolysaccharide; PAMP, pathogen-associated molecular patterns; PCD, programmed cell death; PG, peptidoglycan; PI, propidium iodide; PTI, PAMP-triggered immunity; ROS, reactive oxygen species; SkQ1, 10-(6′-plastoquinonyl)decyl triphenylphosphonium; SkQ3, 10-(6′-methylplastoquinonyl)decyl triphenylphosphonium; TMV, tobacco mosaic virus. * To whom correspondence should be addressed. Abstract-Chitosan (partially deacetylated chitin), a component of fungal cell walls, caused epidermal cell (EC) death in the leaves of pea (Pisum sativum L.) and tobacco Nicotiana tabacum or Nicotiana benthamiana detected by destruction of cell nuclei. The mitochondria-targeted quinone SkQ1 prevented the destruction of EC nuclei induced by chitosan. Chitosan increased and SkQ1 suppressed the activity of protein kinases in N. benthamiana and P. sativum and eliminated the effect of chitosan. Chitosan induced the generation of reactive oxygen species (ROS) in the guard cells (GC) of pea plants. Treatment with chitosan or H 2 O 2 did not cause destruction of GC nuclei; however, it resulted in disruption of the permeability barrier of the plasma membrane detected by propidium iodide fluorescence. Treatment with bacterial lipopolysaccharide but not peptidoglycan caused destruction of pea EC nuclei, which was prevented by SkQ1. Leaves of tobacco plants containing the N gene responsible for resistance to tobacco mosaic virus (TMV) were infiltrated with Agrobacterium tumefaciens cells. These cells contained a genetic construct with the gene of the helicase domain of TMV replicase (p50); its protein product p50 is a target for the N-gene product. As a result, the hypersensitive response (HR) was initiated. The HR manifested itself in the death of leaves and was suppressed by SkQ3. Treatment of tobacco epidermal peels with the A. tumefaciens cells for the p50 gene expression stimulated the destruction of EC nuclei, which was inhibited by SkQ1 or SkQ3. The p50-lacking A. tumefaciens cells did not induce the destruction of EC nuclei. The protective effect of mitochondria-targeted antioxidants SkQ1 and SkQ3 demonstrates the involvement of mitochondria and their ROS in programmed cell death caused by pathogen elicitors. Plant Cell Deat