Organelles

Leaf and fruit senescence are characterized by dramatic changes in the major organelles, particularly the plastids of parenchymatous (mesophyll) cells. Far from deteriorating in structure and function, the chloroplasts of leaf mesophyll cells and green immature fruits respectively redifferentiate into gerontoplasts and chromoplasts (Sitte 1977, Parthier 1988, Cheung et al. 1993, Camara et al. 1995).

At the same time there may be visible changes in cell vacuoles, as occurs in the intensely coloured autumn leaves of many temperate trees and shrubs (Wagner 1979, Ougham et al. 2005). Subcellular reorganisation during senescence is not confined to pigmented tissues. During the senescence of storage cotyledons and the endosperm of many dicot seeds, and in the aleurone layer of germinating cereal grains, vacuoles change from protein storage organelles to large lytic vacuoles and retain the integrity of the tonoplast (Jiang and Rogers 2001). The enzyme complement of the vacuole has been compared with that of the lysosome of animal cells and, with respect to the vacuole in senescing plant cells, there is some functional similarity with lysosomal processes (Parish 1975, Klionsky and Emr 2000, Nakatogawa et al. 2007). The plant vacuole, however, has many other roles, including osmoregulation, stress responses and the storage of metabolites, macromolecules and inorganic nutrients. The vacuole is intimately associated with the endomembrane system and senescence is a time of active trafficking and sorting of the products of specific up-regulated genes (Bassham and Raikhel 2000, Otegui et al. 2005).

Cotyledons, endosperm tissue and also mesophyll of dicots and monocots may also accumulate oleosomes (Lersten et al. 2006), specialised lipid-storage organelles, which are lost during senescence. Lipid metabolism in storage tissues is associated with the formation of a new organelle, the glyoxysome, which plays an important role in gluconeogensis. In senescing photosynthetic tissues, peroxisomes are converted into glyoxysomes (Nishimura et al. 1993). As green tissues senesce their metabolism becomes increasingly heterotrophic in character and mitochondrial respiration assumes a more active role in energy generation (Keech et al. 2007). In some tissues, notably ethylene-sensitive ripening fruits, senescence is associated with a burst of respiration, the climacteric (Plaxton and Podestá 2006).

To proceed normally, senescence requires a coherent gene expression system and this is reflected in the maintenance of the structural and functional integrity of the nucleus until the terminal stages when cell death takes over. This is accompanied by a virtually unchanging number of organellar genome copies per cell (Li et al. 2006). During the ripening of some fruits that soften, the cell wall undergoes major compositional modification as a result of the secretion of enzymes into the extracellular matrix (Brummell 2006). Otherwise cell wall changes in senescence are more subtle, often taking the form of increasing cross-linking (Passardi et al. 2004) and localisation of aggressive enzymes with possible functions in defending against biotic stresses.

References

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