Submitted to: Physiological and Molecular Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/2001
Publication Date: N/A
Citation: Interpretive Summary: An important means of plant tissue defense against infection by pathogens involves programmed cell death, apoptosis, of plant cells around the invading disease organism. The dead cells created by this process act as a barrier to retard further advancement of the invading pathogen. The signals initiating the various processes responsible for this means of disease resistance are of great agricultural importance especially in fleshy fruits and vegetables such as tomato and potato. Research was conducted, using cultured tomato cells as a model system, to investigate the nature of signaling molecules that induce programed cell death. The results show that certain fatty acids liberated from plant and fungal material are oxidized by the lipoxygenase system of plant enzymes. Specific fatty acid oxidation products produced by this process act as potent signals to induce programed cell death. The results also demonstrate that another subset of these fatty acid oxidation products induce changes in DNA that are associated with apoptosis. The results provide important insight into the signaling processes responsible for this means of resistance and illustrate a parallel with lipid peroxides that induce apoptosis in mammalian cells.
Technical Abstract: Arachidonic acid (AA) is a fatty acid elicitor abundant in the glycerolipids of the late blight pathogen Phytophthora infestans and related Oomycete species. Lipoxygenases (LOX), which catalyze the addition of molecular oxygen to the 1 or 5 position of a cis, cis1,4-Z,Z-pentadiene system in polyunsaturated fatty acids, is induced in host plants such as tomato and potato during infection by P. infestans. Here it is reported that AA, the LOX metabolites of AA, 5- and 15-hydroperoxyeicosatetraenoic acid (5- and 15-HpETE), and the LOX metabolite of linoleic acid, 9- hydroperoxyoctadecadienoic acid (9-HpODE), are potent inducers of programmed cell death (PCD) in tomato protoplasts. 5- and 15-HpETE increased DNA fragmentation as detected by terminal deoxynucleotidyl transferase-mediated dUTP-X nick end labeling (TUNEL) and increased DNA laddering as visualized by ligation-mediated PCR. Background levels of DNA laddering were decreased in intensity by Zn2+ and increased by Ca2+, effects that are consistent with the reported action of these cations on PCD-associated endonucleases in other systems. H2O2, methyl jasmonate, and linoleic and linolenic acids were not toxic to tomato protoplasts at concentrations up to 350 uM, and lipid peroxides (LD100=80uM) were far more potent inducers of death than free AA within this same concentration range. These results indicate the potential of fatty acid peroxides and LOX- related metabolism to engage an apoptotic type of PCD in higher plant cells.