“ Programmed cell death in plants: flower corolla as an example.”

According to the Nomenclature Committee on Cell Death (Kroemer et al., 2005), a generally accepted nomenclature for animal cell death (CD) has not established, because many historically used terms, like apoptosis, necrosis, autophagy etc are based mainly on morphological descriptions and frequently only of in vitro cultures. The Committee also observed that the term programmed cell death (PCD) implies the concept “genetically programmed” occurring during development and aging, opposed to “accidental cell death”. Sometimes it could be difficult to discriminate between programmed and accidental, thus, it is suggested to replace PCD by more detailed expressions describing the causative agent and the parameters that are measured. In plants, studies on CD are much less advanced and no attempts have been made to classify the types of cell death and frequently terms from animal literature are used. CD in plants can be induced by hormones, toxins, hypoxia, wounds, stresses, pathogens, but it can be a natural event of morphogenesis, tissue differentiation, sex determination and senescence, giving to the cell a functional specialization. The typical features of CD in plants are for some instances different from those in animals, for the presence of specific cell compartments and the absence of external phagocytosis, even though autophagocytosis can occur also in plants, as well as mummification and abscission of the entire organ. Developmental cell death in plants is accompanied by nuclear condensation, membrane blebbing, sometimes oligonucleotide DNA fragmentation and endonuclease and cysteine protease activity. Mitochondria could play a similar central role in both kingdoms. Chloroplasts and vacuoles may also play a role in the induction or execution of CD. Cell death is often a terminal stage of plant cell differentiation, to play specific functions or by contrast, cells die after having finished their function. In both cases, the term PCD appears to be appropriate. In flowers, some parts undergo CD and fall, like stamina and petals of the corolla. The petal senescence is a controlled event; a signal for the abscission of the petals is pollination in the long life flowers (beyond the 2 days) or an endogenous program in the short life flowers (1-2 day). Ethylene, cytokinins and abscisic acid seem to regulate petal senescence. Alifatic polyamines (PAs) have a role in the plants cellular division, growth and senescence. The corolla of flower of Nicotiana has been chosed as a model, whose life phases, described during its development, senescence and death, are defined by many parameters. Key transition events mark the no-return point, identified by changes in water, pigment and protein content, protease activity, DNA laddering and by the formation of a cell ring of reduced mechanical resistance. Under natural conditions the dead corolla remains on the thalamus to protect the developing ovary and only at a very late stage the corolla naturally falls entire, from the abscission ring. The senescence events show an acropetal gradient and the corolla has been divided into three parts (proximal, medial and distal) and studied during its life span also at sub-cellular levels. The target of this study was to delay the senescence of the corolla, by supplying spermine and to understand its mechanism of action. In Nicotiana PAs are free or bound to hydroxycinnamic acids and proteins; PAs are conjugated to endoglutamines of proteins by catalysis of transglutaminase (TGase), thus forming either intra- and inter-molecular bridges. In animals the post-translational modification of proteins by TGase appears a relevant factor in the CD. In Nicotiana this enzyme has been immuno detected both into the entire corolla and in all its three parts, with three antibodies raised either against TGase of animal or plant origin. All the antibodies recognised a main band of 58 kDa a...