Department of Plant Pathology and the NSF Center for Engineering Plants for Resistance against Pathogens (CEPRAP). University of California, Davis, CA 95616, USA

Background and objectives
Programmed cell death occurs in plants during development of tracheary elements, floral organs and roots, during senescence, in genetic lesions (lesion mimics) and in disease where death is expressed in susceptibility (lesions) and resistance (hypersensitive response). Yet existence of a genetic template and conserved signal transduction pathways directed to the selective elimination of certain cells to achieve cell homoeostasis is a very recent revelation in modern biological research. Today, apoptosis as a form of programmed cell death (PCD) is accepted among animal biologists as a genetically controlled process that directs cells to die following transduction of signals through multiple converging pathways when cells have completed an essential but transient role, when they are in surplus, or develop improperly or have the potential to be deleterious to the organism. Although apoptosis was initially described as a process like senescence of plant leaves, linkage of death morphology and signalling molecules to PCD, as described in animals, has emerged only recently in plants [1, 2]. Characterization and manipulation of gene expression-dependent PCD processes in plants to assess functional roles of PCD in disease will require linkage to specific genes and gene products that respond to a diverse set of signals transduced by the range of stimuli.

Materials and methods
We are using various triggers of cell death, including mycotoxins, apoptosis-related genes from animal systems, lesion mimic mutants and plant pathogens, to study cellular and organism rises of both plant and animal cells to the induction of PCD at both the genetic and signalling Ievels. These studies involve transgenic plants, subpopulations of plant cells and lipid-based signalling systems.

Results and conclusions
Results to be described indicate considerable conservation at the cellular level of both the morphological and signalling features of apoptosis in plants. Both ceramide-linked signalling and transgenes from animal systems alter the response of plants to inducers of apoptotic-like PCD and to cellular pathogens of plants. The existence and role of ceramide-based signalling in plant cells is virtually unknown and has not been studied in relation to plant responses to pathogens. Our parallel studies with animal and plant cells responding to diverse apoptotic stimuli suggest functional linkages between cell cycle regulation, apoptosis and lipid-based signalling pathways in plants during disease [2].

The extent of conservation of the animal-resolved apoptotic programme in plant cells is unknown, even though the final morphological characteristics of apoptosis can occur in plants [1]. Regardless of the extent of similarity, it is likely that the genes and signal molecules that define programmed cell death in plants will become important targets for both understanding and manipulating this process in development and disease of plants [2].

1. Wang H et al., 1996. Plant Cell 8, 375-391.
2. Gilchrist DG, 1997. Cell Death and Differentiation 4, 689-698.