The PET1-CMS Mitochondrial Mutation in Sunflower Is Associated with Premature Programmed Cell Death and Cytochrome c Release

Abstract

In mammals, mitochondria have been shown to play a key intermediary role in apoptosis, a morphologically distinct form of programmed cell death (PCD), for example, through the release of cytochrome c , which activates a proteolytic enzyme cascade, resulting in specific nuclear DNA degradation and cell death. In plants, PCD is a feature of normal development, including the penultimate stage of anther development, leading to dehiscence and pollen release. However, there is little evidence that plant mitochondria are involved in PCD. In a wide range of plant species, anther and/or pollen development is disrupted in a class of mutants termed CMS (for cytoplasmic male sterility), which is associated with mutations in the mitochondrial genome. On the basis of the manifestation of a number of morphological and biochemical markers of apoptosis, we have shown that the PET1-CMS cytoplasm in sunflower causes premature PCD of the tapetal cells, which then extends to other anther tissues. These features included cell condensation, oligonucleosomal cleavage of nuclear DNA, separation of chromatin into delineated masses, and initial persistence of mitochondria. In addition, immunocytochemical analysis revealed that cytochrome c was released partially from the mitochondria into the cytosol of tapetal cells before the gross morphological changes associated with PCD. The decrease in cytochrome c content in mitochondria isolated from male sterile florets preceded a decrease in the integrity of the outer mitochondrial membrane and respiratory control ratio. Our data suggest that plant mitochondria, like mammalian mitochondria, play a key role in the induction of PCD. The tissue-specific nature of the CMS phenotype is discussed with regard to cellular respiratory demand and PCD during normal anther development.

Nitric oxide affects plant mitochondrial functionality in vivo

“…It is already recognised that the decrease of MMP is an important mechanism associated with apoptosis in animal systems [23] and that cytochrome c release serves as a signal in several forms of programmed cell death in both animals and plants [4,24]. In our system, NO triggers a set of alterations typically associated with PCD as shown by nuclear condensation presented in Fig.…”

Apoptotic‐like programmed cell death in plants

“…An ultrastructural analysis, by Papini et al (1999), of the morphological events involved in tapetal cell death in Lobivia rauschii (a dicot) and Tillandsia albida (a moncot) demonstrated that death was accompanied by shrinkage of the whole cell and nucleus, condensation of the chromatin at the periphery of the internal nuclear membrane, enlargement of the endoplasmic reticulum (ER) cisternae, and persistence of the mitochondria and microfilament bundles, until the final stages of tapetal cell death. Similar morphological features occur during the premature cell death of anther tissues during cytoplasmic male sterile (CMS)-associated cell death in sunflower (Balk & Leaver, 2001). These authors also showed that the cells and nuclei condensed, and that nuclear DNA fragmented and was cleaved into oligonucleosomal units, yet the mitochondria persisted until extremely late in the cell-death process.…”

“…Cytochrome c release has also been shown to occur during developmental cell death. In a study characterizing anther development in fertile and CMS sunflower, premature induction of PCD in the tapetum of CMS plants was found to be associated with release of cytochrome c from the mitochondria (Balk & Leaver, 2001). Release of cytochrome c was also found to be associated with death of poppy pollen tubes during self-incompatibility in Papaver pollen (Thomas & Franklin-Tong, 2004).…”

“…Following developmental PCD Nucleosomal cleavage has been observed in developing anthers (Wang et al, 1999), and oligonucleosomal cleavage has been observed in tapetal cell death in sunflower (Balk & Leaver, 2001) and during maize embryo formation (Giuliani et al, 2002). Laddering has been reported in a range of senescing plant tissues, such as pea carpels (Orzaez & Granell, 1997), rice coleoptiles (Kawai & Uchimiya, 2000), cucumber cotyledons and flowers (Delorme et al, 2000), and Alstroemeria and gladiolus petals Yamada et al, 2004).…”

Phytaspase, a relocalisable cell death promoting plant protease with caspase specificity

“…PCD in the plant and animal kingdoms share a number of morphological and biochemical features, including condensation and shrinkage of the nucleus and cytoplasm, DNA laddering and cytochrome c release from mitochondria (Danon et al, 2000;Balk and Leaver, 2001;Lam et al, 2001;Hoeberichts and Woltering, 2003). However, direct structural homologues of animal caspases with an analogous cleavage specificity and function have not been shown to exist in plants (reviewed by Bonneau et al, 2008).…”