Apoptosis: A Functional Paradigm for Programmed Plant Cell Death Induced by a Host-Selective Phytotoxin and Invoked during Development

Abstract

The host-selective AAL toxins secreted by Alternaria alternata f sp lycopersici are primary chemical determinants in the Alternaria stem canker disease of tomato. The AAL toxins are members of a new class of sphinganine analog mycotoxins that cause cell death in both animals and plants. Here, we report detection of stereotypic hallmarks of apoptosis during cell death induced by these toxins in tomato. DNA ladders were observed during cell death in toxin-treated tomato protoplasts and leaflets. The intensity of the DNA ladders was enhanced by Ca2+ and inhibited by Zn2+. The progressive delineation of fragmented DNA into distinct bodies, coincident with the appearance of DNA ladders, also was observed during death of toxin-treated tomato protoplasts. In situ analysis of cells dying during development in both onion root caps and tomato leaf tracheary elements revealed DNA fragmentation localized to the dying cells as well as the additional formation of apoptotic-like bodies in sloughing root cap cells. We conclude that the fundamental elements of apoptosis, as characterized in animals, are conserved in plants. The apoptotic process may be expressed during some developmental transitions and is the functional process by which symptomatic lesions are formed in the Alternaria stem canker disease of tomato. Sphinganine analog mycotoxins may be used to characterize further signaling pathways leading to apoptosis in plants.

Abrogation of disease development in plants expressing animal antiapoptotic genes

“…Lesion size was reported to be slightly larger in the transgenic plants (37), although the overall role of these transgenes with respect to plant disease remained an open question. Previous studies showed that tomato protoplasts treated with the mycotoxin fumonisin B1 underwent PCD-like response as indicated by DNA ladders and terminal deoxynucleotidyltransferasemediated UTP end labeling-positive reacting cells (11). Recently, Ausubel and colleagues (38,39) have shown that fumonisin B1 treatment of Arabidopsis protoplasts results in terminal deoxynucleotidyltransferase-mediated UTP end labelingpositive reacting cells and also that toxin infiltration of intact plants resulted in cell death, generation of reactive oxygen intermediates, and other biochemical markers associated with plant defense responses.…”

“…Similar indicators of PCD can appear in the diseased state (susceptible or compatible reaction). For example, in plants that are sensitive to toxin-producing fungi, e.g., Fusarium moniliforme, Alternaria alternata, and Cochliobolus victoriae, certain features of apoptosis are observed (11,12). In addition, cucumber mosaic virus D satellite RNA has been shown to induce morphological features of PCD in infected tomato (13).…”

Do Plant Caspases Exist?

“…Furthermore, cell death in response to pathogen attack, and in response to a variety of abiotic factors such as ozone and UV radiation also fall within the definition of PCD. A number of morphological similarities were found between animal cells undergoing apoptosis and dying plant cells, including compaction and shrinkage of the cytoplasm and nucleus, DNA and nuclear fragmentation, and the formation of DNA-containing (apoptotic-like) bodies (Wang et al, 1996;De Jong et al, 2000). Although such typical apoptotic hallmarks have not been established in all the cases of plant PCD, the observations do suggest the existence of an apoptotic machinery in plant cells.…”

Coordinate involvement of cysteine protease and nuclease in the executive phase of plant apoptosis

“…Several apoptogenic effectors, such as caspases (cysteine proteases) [9][10][11][12], CAD/DFF40 (caspase-activated DNase/40 kDa DNA fragmentation factor) [9][10][11][12], endoG (mitochondrial endonuclease G) [13], DNase I [14], DNase II [15], AIF (apoptosis-inducing factor) [16] and Acinus (apoptosis chromatin condensation inducer in the nucleus) [17], have been implicated in mediating these nuclear disintegration processes using a cell-free system. In plants, chromatin condensation and DNA laddering are induced during PCD in response to biotic/abiotic stresses and developmental factors [4,6,[18][19][20][21][22]. However, only limited information is available about plant effectors, especially those involved in the executive phase of plant PCD.…”

“…The defense reaction often includes the rapid and localized collapse of the challenged cells, known as the hypersensitive response (HR) [1]. The HR displays several biochemical and morphological features of animal apoptosis [2][3][4][5][6], which is a process of programmed cell death (PCD) essential in development, tissue homeostasis and response to environmental stresses [7].…”