Mitosis

Quiescence

Quiescence

Figure 1.1 Illustration of a cell's life history consisting of both mitotic and postmitotic processes. When the cell stops dividing, it is called mitotic senescence or replicative senescence or proliferative senescence. The active degenerative and attrition process of the cell that can no longer undergo cell division is postmitotic senescence. If a cell stops dividing due to, for example, adverse conditions, but will resume division, the status of the cell is called cell quiescence.

Postmitosis

Postmitosis

Figure 1.1 Illustration of a cell's life history consisting of both mitotic and postmitotic processes. When the cell stops dividing, it is called mitotic senescence or replicative senescence or proliferative senescence. The active degenerative and attrition process of the cell that can no longer undergo cell division is postmitotic senescence. If a cell stops dividing due to, for example, adverse conditions, but will resume division, the status of the cell is called cell quiescence.

more daughter cells, the temporarily undividing or resting status or process is called cell quiescence (Stuart and Brown, 2006). Although a mitotically senescent cell is not dead it may undergo degenerative process leading to death. If the degeneration is solely a function of age, 'aging' is the right word to describe it. In animal literature, the term 'cell(ular) aging' or 'postmitotic aging', or 'postmitotic senescence' is used for this process. If the degeneration is an active yet quick process, it is a form of 'apoptosis' or 'programmed cell death'. It however should be noted that mitotically senescent mammalian cells in culture are resistant to 'apoptosis'. Most of the postmitotic cells are somatic in nature (e.g. brain, neuron, and muscle cells); the term somatic senescence is therefore also used in literature concerning animals. As will be discussed below, postmitotic senescence also occurs in plant somatic tissues such as leaves, flowers and fruits. Compared with postmitotic senescence in animals, leaf/flower/fruit senescence (that involves an active but slow degenerative process) and hypersensitive response (involving an active yet very quick degenerative process) are typical postmitotic senescence processes in plants.

At the organismal level, when an organism's ability to respond to stress declines, its homeostasis becomes increasingly imbalanced, and its risk of disease increases with age, which leads to the ultimate death of the whole organism. This is the aging of the whole organism, and is often referred to as organismal senescence. Although cellular senescence may contribute to organismal senescence (Ben-Porath and Weinberg, 2005), the latter is much more inclusive, for example many age-related diseases, such as Alzheimer's disease, are parts of organismal aging. In literature concerning plants, organismal senescence is senescence of the whole plant. Among the most studied whole plant senescence processes is monocarpic senescence. Annuals (e.g. Arabidopsis), biennials (e.g. wheat) and some perennials (e.g. bamboo) possess a monocarpic life pattern, which is characterized by only a single reproductive event in the life cycle. After flowering (and setting seeds or fruits), the whole plant will senesce and die. Monocarpic senescence includes three coordinated processes: (a) senescence of somatic organs and tissues such as leaves (a form of postmitotic senescence, see below), (b) arrest of shoot apical meristems (SAM), a form of mitotic senescence or proliferative senescence (see below), and (c) permanent suppression of axillary buds to prevent formation of new shoots/branches. This third aspect of whole plant senescence has not received enough attention in the senescence research community.

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