Investigations Are Blossoming On Programmed Cell Death In Plants

SIDEBAR : Plant Cell Death Resources A FIELD IN BLOOM: Micheèle Heath observes that plant cell death research is starting to take off. The study of programmed cell death in plants can be likened to a rapidly growing sapling, still in its infancy but ripe with the promise of eventually bearing fruit. In the past few years, several research articles have begun to reveal a complex picture of cell death in plants, much like the initial boom in the study of animal cell death, especially apoptosis-a type of programmed cell death-in the early 1990s (R. Lewis, The Scientist, Feb. 6, 1995, page 15). Just as in animals, programmed cell death plays an important role in plant development. Researchers generally recognize two times at which plants invoke cellular suicide. First, the process functions to eliminate cells that are no longer needed at certain points in a plant's life history, such as the germination of a seed into a seedling. Second, programmed cell death may also be triggered during attacks by pathogens such as viruses, bacteria, and fungi to sequester and kill invaded cells, a phenomenon called the hypersensitive response. Investigators are already anticipating applying knowledge gained from these basic studies to increase the seed quality of cereal crops, engineer disease-resistant plants, and mediate disease susceptibility. GRIM REAPER: Thale cress in the grip of cell death: dying plants, left; healthy plants, right. The field is just starting to take root, investigators agree, although the phenomenon has been recognized for decades. "This area is really in its infancy," remarks Michèle Heath, a professor of plant pathology at the University of Toronto. "We don't know yet that we're seeing a uniform phenomenon. We still need to know if there is any basic similarity between all of the different forms of programmed cell death in plants." The myriad types of plant cell death were a topic of lively discussion at a Gordon Research Conference held in July at Plymouth State College in New Hampshire. Many key players in the world of plant-and animal-cell death research were in attendance. SPANNING KINGDOMS: Similarities exist between animals and plants, according to Barbara Osborne. "My impression from the Gordon Conference is that the plant cell death people have lots of examples of cell death, and they're just beginning to realize that this is the same sort of cell death that animal people are looking at," remarks Barbara Osborne, a professor of molecular and cellular biology and veterinary and animal sciences at the University of Massachusetts, Amherst. Osborne, who studies programmed cell death in mammals, maintains: "I think that in that sense they're farther behind us. But that said, I think that they have an awful lot of examples that are very interesting and may very well mimic what we're looking at in animal cells." An outgrowth of the conference is a collection of six minireviews on plant cell death planned for an early 1997 issue of the journal Cell Death and Differentiation. One of the major debates within the field is how much plant cell death is like cell death in animals, especially apoptotic cell death. Many involved, though, question the merits of even asking such a question. American Society for Plant Physiology 15501 Monona Dr., Rockville, Md. 2085 251-0560 -- Fax: (301) 279-2996 E-mail: aspp@.org -- Web page: http://aspp.org 5,000 members President: Donald Ort Executive Director: Kenneth M. Beam Journals: The Plant Cell, Plant Physiology American Phytopathology Society 3340 Pilot Knob Rd., St. Paul, Minn. 55121 (612) 454-7250 -- Fax: (612) 454-0766 E-mail: aps@scisoc.org -- Web page: http://www.scisoc.org 5,000 members President: Larry Madden Executive Director: Steven C. Nelson Journals: Phytopathology, Plant Disease International Society for Molecular Plant-Microbe Interactions 3340 Pilot Knob Rd., St. Paul, Minn. 55121 (612) 454-7250 -- Fax: (612) 454-0766 E-mail: ismpmi@scisoc.org -- Web page: http://www.scisoc.org/ismpmi 5,000 members President: Barry Rolfs Executive Director: Steven C. Nelson Journal: Molecular Plant-Microbe Interactions Other Sources Cell Death and Differentiation Gerry Melino, scientific director E-mail: melino@tovvx1.ccd.utovrm.it Jeffrey Dangl's home page: http://www.unc.edu/depts/biology/dangl.html "I personally think that some people are jumping on bandwagons and are trying too hard to make this connection between plant programmed cell death and animal apoptosis," asserts Heath. "If we look at natural cell death in plants, we see a variety of phenomena. Some seem to resemble apoptosis in animal cells and some do not." The hallmarks of apoptotic cell death in animals are distinctive and well-studied: The cell membrane tightens, forming bulges called blebs; the nuclear membrane splits open; enzymes then snip chromosomes into fragments that resemble the rungs of an ever-widening ladder on electrophoresis plates; and finally the DNA segments are packaged into units called apoptotic bodies. "The demonstrated similarities between plant programmed cell death and animal apoptosis are still fairly limited, including DNA laddering in some cases and the activation of proteases, the enzymes that break down the cells' contents," says Heath. She and Douglas Ryerson, one of her Ph.D. students, were among the first to show laddering, in their case, in a fungal infected cowpea plant (D.E. Ryerson, M.C. Heath, Plant Cell, 8:393-402, 1996). David Gilchrist, associate director of the University of California, Davis, Center for Engineering Plants for Resistance Against Pathogens, and colleagues have also observed laddering and compartmentalization of fragmented DNA in tomato cells treated with mycotoxins (H. Wang et al., Plant Cell, 8:375-91, 1996). But, as Heath points out, others have looked at plant cell death caused by viruses and bacteria and have not detected laddering (for example, A. Levine et al., Current Biology, 6:427-37, 1996). "The tricky thing about seeing the so-called stereotypical hallmarks of apoptosis in plants is that, to a great extent, the search is conditioned by the asynchrony in the system in which the observations are being made," warns Gilchrist. Once triggered, the key events of individual cell death occur in small windows of time-in as little as a few minutes-and not all cells die simultaneously. "It's a matter of sampling in the right place at the right time." Chris Lamb, a professor of plant biology at the Salk Institute for Biological Studies in La Jolla, Calif., has seen apoptotic like cell corpses and evidence of DNA processing in plants displaying the hypersensitive response. "What we don't yet know is whether apoptosis is mechanistically conserved between the plant and animal kingdoms," declares Lamb. "I think the key question is whether there is an involvement of ICE [interleukin-1b converting enzyme]-like proteases in plants. I'm not aware of that yet, but I wouldn't rule it out." ICE-like proteases were intensely discussed at the Gordon Conference, notes co-organizer Alan Bennett, a professor of plant science at Davis who studies fruit ripening. "The proteases induced during cell death that have been found in plants haven't been shown to play an analogous role as the ICE like proteases," he adds. Enzymes that break down nucleic acids called endonucleases are also enlisted in cell death. Eric Lam, an associate professor of plant science at Rutgers University, and colleagues have identified such enzyme activities in tobacco plants infected with the tobacco mosaic virus (R. Mittler, E. Lam, Plant Cell, 7:1951-62, 1995). He says the induction of these types of molecules during programmed cell death are "one of the similarities you'd expect to find between plants and animals." "We have some analogies of plant cell death in animal cells perhaps apoptosis induced by viral oncogenes in animal cells and the hypersensitive response in plants, for example-but I don't want to equate the two," says Osborne. "It's hard to say that they're apoptotic because a plant cell is constrained by a cell wall, so you don't have membrane blebbing like you would in an animal cell. There is some chromosome condensation, although there's a lot of argument among the people in the plant world [about] whether they actually have an apoptotic cell death." Investigators within the plant research community echo Osborne's assertions. "We know that certain features of what an apoptotic animal cell does can be found in certain cell deaths in plants," says Jeffrey Dangl, an associate professor of biology at the University of North Carolina, Chapel Hill. "But some features don't make a whole definition. If there are some shared mechanistic features, we will find them, but it needn't all be just like animals." Osborne argues that all of the discussion of apoptosis in plants is merely an exercise in semantics: "The cells are dying in a developmental or programmed fashion whether it's apoptotic or not." She also sees parallels in the progress of studies on plant and animal cell death. "I think what's happening to the plant people is the same thing that happened to us a few years ago." She recalls that paper and grant reviewers would "ask us to prove six ways to Sunday whether this was apoptotic death, and we'd think: 'Why does it matter?' The most important thing, I think, is to describe the molecular events that lead to this death." One of the most active areas of research in plant cell suicide is trying to elucidate the molecular genetics of the death knell. "The recognition that very rapid cell death occurs as a response to pathogen attack has been known since 1914," says Dangl. "It's the realization that there's intrinsic genetic control of cell death in plants that's relatively new." GREEN THUMB: Robert Dietrich is part of a team that studies the molecular genetics of plant cell suicide. Dangl, along with Robert Dietrich and Thorsten Jabs, postdocs in his lab, published a paper last month describing the role of the signaling molecule superoxide in triggering cell death in mutant plant cells (T. Jabs et al., Science, 273:1853-6, 1996). Using a mutant lacking the lsd-1 gene, they showed that when the gene is not functioning properly, the programmed death of thale cress leaf cells initiated by superoxide does not stop. "This mutation is in a gene that would normally tell those cells: 'Don't die,'" explains Dangl. As a result of the genetic mistake, cells just keep on dying until the whole leaf goes. "We don't know what controls this balance between killing a plant cell and telling a cell to stay alive," he says. "We think lsd-1 is a central regulator of that process." If this turned out to be true, he speculates, "we could manipulate when and where in the plant that disease resistance could be turned on." Salk Institute's Lamb notes that his lab has shown that at least in some systems the oxidative burst plays an important role in triggering cell death in plants (A. Levine et al., Cell, 79:583-93, 1994; Hot Papers, The Scientist, Sept. 2, 1996, page 14). "The involvement of active oxygen species as triggers of programmed cell death and the conservation of the oxidant-generating NADPH oxidase between [animal] neutrophils and plants already indicates intriguing parallels between plants and animals," he says. What's more, Lamb adds, in soybean cells, "hydrogen peroxide triggers a big calcium influx [to the cell's interior], which trips the cell death program. We see apoptotic-like features such as shrunken cell corpses, plasma membrane blebbing, nuclear condensation, and DNA processing akin to some animal systems." A plant doesn't have to be under attack by a pathogen to undergo programmed cell death. Intrinsically controlled cell death is part of a normal plant's healthy course of development. UC-Riverside biochemist Daniel Gallie's interest in seed quality led him to his current work with developmental related programmed cell death in maize endosperm. Starch and protein-rich endosperm tissue provides seeds with the nutrients required for seedling growth. "We found that maize endosperm did undergo programmed cell death," he reports. "We're now looking to see if this is specific to maize, or a trait of cereals in general." Seeds aren't the only place where plants need to orchestrate finely tuned life-and-death cues. After pollination, the reproductive parts within the flower undergo many physiological changes, which include programmed cell death. Working with tobacco plants, Alice Y. Cheung, an associate professor of biology at Yale University, and colleagues found that the controlled death of transmitting tissue cells allows the pollen tube to elongate so that it can eventually reach the ovary within the optimal time for fertilization (H. Wang et al., Plant Journal, 9:715-27, 1996). In this study, cell death was not apoptotic-like; the characteristics of this death included vacuolation, loss of cytoplasm, and nuclear disintegration. Gallie believes that practical applications of his line of research could have big payoffs. "If one looks at maize seed which feeds a substantial fraction of the people living on Earth-part of its endosperm does not contain stored reserves," he says. "There is considerable effort going on by seed companies to increase the quality of the seed by improving the quantity of starch or protein, or the quality of protein by increasing the lysine content in endosperm." Seed companies want to improve the seed quality of such cereals as rice, barley, wheat, and maize for human and animal consumption. Gallie's scientific goal is to understand developmentally related programmed cell death in the endosperm, so agricultural scientists can delay its onset. "This should allow the endosperm more time to produce additional starch and protein." Plant scientists say the future of their emerging field holds many topics that are ripe for the picking. For Gallie, the big issue in understanding developmental programmed cell death is to find out what's regulating it. "In terms of pathogeninduced cell death, it's clear that it has to somehow involve the pathogen, but in development-induced cell death, there has to be other kinds of internal triggers. What determines the signal transduction pathway? What are the hormones involved and the end products of the pathway?" Lamb comments that "in addition to the obvious questions about mechanism, there may be some interesting new biology at the organismal level. For example, in disease resistance, since plant cells cannot migrate, specialized killer cells equivalent to mammalian T cells cannot operate, so it will be interesting to see if plants have long-distance cell death signals." And for Gilchrist, determining the genes regulating plant cell death is a major unresolved question. He says that in a conceptual sense, researchers who study plant cell death are in many ways heading down the same path as their colleagues studying the phenomenon in animals: "This is a very fundamental process, one that is clearly involved in managing cell and tissue stability and that can be co-opted during disease." Manipulation of these genes, plant scientists agree, should yield many scientific plums.

Investigations Are Blossoming On Programmed Cell Death In Plants

Interested in reading more?

Become a Member of