Mitochondrial Markers of Programmed Cell Death in Arabidopsis thaliana.

In plants, apoptosis-like programmed cell death (AL-PCD) is readily distinguished from other forms of programmed cell death (PCD) through a distinct morphology. Detection of cytochrome c release from mitochondria and changes in mitochondrial morphology are the earliest markers for detection of this form of PCD in plants. In this chapter we provide detailed technical methods for the visualization of both of these mitochondrial markers of AL-PCD in Arabidopsis.

The fusarium mycotoxin deoxynivalenol can inhibit plant apoptosis-like programmed cell death.

The Fusarium genus of fungi is responsible for commercially devastating crop diseases and the contamination of cereals with harmful mycotoxins. Fusarium mycotoxins aid infection, establishment, and spread of the fungus within the host plant. We investigated the effects of the Fusarium mycotoxin deoxynivalenol (DON) on the viability of Arabidopsis cells. Although it is known to trigger apoptosis in animal cells, DON treatment at low concentrations surprisingly did not kill these cells. On the contrary, we found that DON inhibited apoptosis-like programmed cell death (PCD) in Arabidopsis cells subjected to abiotic stress treatment in a manner independent of mitochondrial cytochrome c release. This suggested that Fusarium may utilise mycotoxins to suppress plant apoptosis-like PCD. To test this, we infected Arabidopsis cells with a wild type and a DON-minus mutant strain of F. graminearum and found that only the DON producing strain could inhibit death induced by heat treatment. These results indicate that mycotoxins may be capable of disarming plant apoptosis-like PCD and thereby suggest a novel way that some fungi can influence plant cell fate.

Commentary: the cellular condensation of dying plant cells: programmed retraction or necrotic collapse?

In this commentary we argue that the recent renaming of all types of plant programmed cell death (PCD) into two categories of vacuolar cell death and necrosis is premature and does not fully take into account different forms of cell death that may operate in plant cells. Specifically, we believe that the condensed protoplast morphology associated with many instances of PCD may come about due to an active cell death-associated cellular retraction rather than simple rupture of the plasma membrane. We argue that it is important to be able to distinguish between cells that have died having undergone this protoplast retraction and those which have died without protoplast retraction. In our opinion this differentiation is essential as the control of these two types of death may differ in several respects.

Sphingolipid long chain base phosphates can regulate apoptotic-like programmed cell death in plants.

Sphingolipids are ubiquitous components of eukaryotic cells and sphingolipid metabolites, such as the long chain base phosphate (LCB-P), sphingosine 1 phosphate (S1P) and ceramide (Cer) are important regulators of apoptosis in animal cells. This study evaluated the role of LCB-Ps in regulating apoptotic-like programmed cell death (AL-PCD) in plant cells using commercially available S1P as a tool. Arabidopsis cell cultures were exposed to a diverse array of cell death-inducing treatments (including Cer) in the presence of S1P. Rates of AL-PCD and cell survival were recorded using vital stains and morphological markers of AL-PCD. Internal LCB-P levels were altered in suspension cultured cells using inhibitors of sphingosine kinase and changes in rates of death in response to heat stress were evaluated. S1P reduced AL-PCD and promoted cell survival in cells subjected to a range of stresses. Treatments with inhibitors of sphingosine kinase lowered the temperature which induced maximal AL-PCD in cell cultures. The data supports the existence of a sphingolipid rheostat involved in controlling cell fate in Arabidopsis cells and that sphingolipid regulation of cell death may be a shared feature of both animal apoptosis and plant AL-PCD.

Apoptotic-like regulation of programmed cell death in plants.

In plants, apoptotic-like programmed cell death (PCD) can be distinguished from other forms of plant cell death by protoplast condensation that results in a morphologically distinct cell corpse. In addition, there is a central regulatory role for the mitochondria and the degradation of the cell and its contents by PCD associated proteases. These distinguishing features are shared with animal apoptosis as it is probable that plant and animal cell death programmes arose in a shared unicellular ancestor. However, animal and plant cell death pathways are not completely conserved. The cell death programmes may have been further modified after the divergence of plant and animal lineages leading to converged, or indeed unique, features of their respective cell death programmes. In this review we will examine the features of apoptotic-like PCD in plants and examine the probable conserved components such as mitochondrial regulation through the release of apoptogenic proteins from the mitochondrial intermembrane space, the possible conserved or converged features such as "caspase-like" molecules which drive cellular destruction and the emerging unique features of plant PCD such as chloroplast involvement in cell death regulation.

Apoptotic-like programmed cell death in plants.

Programmed cell death (PCD) is now accepted as a fundamental cellular process in plants. It is involved in defence, development and response to stress, and our understanding of these processes would be greatly improved through a greater knowledge of the regulation of plant PCD. However, there may be several types of PCD that operate in plants, and PCD research findings can be confusing if they are not assigned to a specific type of PCD. The various cell-death mechanisms need therefore to be carefully described and defined. This review describes one of these plant cell death processes, namely the apoptotic-like PCD (AL-PCD). We begin by examining the hallmark 'apoptotic-like' features (protoplast condensation, DNA degradation) of the cell's destruction that are characteristic of AL-PCD, and include examples of AL-PCD during the plant life cycle. The review explores the possible cellular 'executioners' (caspase-like molecules; mitochondria; de novo protein synthesis) that are responsible for the hallmark features of the cellular destruction. Finally, senescence is used as a case study to show that a rigorous definition of cell-death processes in plant cells can help to resolve arguments that occur in the scientific literature regarding the timing and control of plant cell death.

Programmed cell death in plants: distinguishing between different modes.

Programmed cell death (PCD) in plants is a crucial component of development and defence mechanisms. In animals, different types of cell death (apoptosis, autophagy, and necrosis) have been distinguished morphologically and discussed in these morphological terms. PCD is largely used to describe the processes of apoptosis and autophagy (although some use PCD and apoptosis interchangeably) while necrosis is generally described as a chaotic and uncontrolled mode of death. In plants, the term PCD is widely used to describe most instances of death observed. At present, there is a vast array of plant cell culture models and developmental systems being studied by different research groups and it is clear from what is described in this mass of literature that, as with animals, there does not appear to be just one type of PCD in plants. It is fundamentally important to be able to distinguish between different types of cell death for several reasons. For example, it is clear that, in cell culture systems, the window of time in which 'PCD' is studied by different groups varies hugely and this can have profound effects on the interpretation of data and complicates attempts to compare different researcher's data. In addition, different types of PCD will probably have different regulators and modes of death. For this reason, in plant cell cultures an apoptotic-like PCD (AL-PCD) has been identified that is fairly rapid and results in a distinct corpse morphology which is visible 4-6 h after release of cytochrome c and other apoptogenic proteins. This type of morphology, distinct from autophagy and from necrosis, has also been observed in examples of plant development. In this review, our model system and how it is used to distinguish specifically between AL-PCD and necrosis will be discussed. The different types of PCD observed in plants will also be discussed and the importance of distinguishing between different forms of cell death will be highlighted.