Cell types

Leaves and floral organs are not uniform structures. Typically, only about half of the cells in a mature leaf contain chloroplasts (Pyke 1994), but research on the molecular basis of senescence has almost always treated leaf tissues as if they are made up exclusively of mesophyll cells. Although senescence-associated genes are assumed to be expressed and active in senescing green tissues, in few cases has this been verified. One such gene for which there is information about tissue distribution of expression is that encoding the cytosolic nitrogen mobilisation enzyme glutamine synthetase I. It has been located not in senescing mesophyll but in the vascular tissue (Kamachi et al. 1992).

Is mesophyll senescence cell-autonomous? Anecdotal evidence suggests that it is, but definitive studies are lacking. The transdifferentiation of Zinnia mesophyll cells into tracheids begins with senescence-like changes in plastids (Fukuda 1996). It would be interesting to compare these events (rather than the later processes of xylogenesis and cell death, which have been the focus of attention to date) with what happens in senescing Zinnia leaves. Something similar happens during culture of mechanically isolated green cells of asparagus (Guinel and Bown 1951), but in this case there are clear differences from normal senescence of mesophyll cells in leaf tissue (Harikrishna et al. 1992). A promising single-cell model for mesophyll senescence is Chlorella. Degreening in Chlorella can be induced by changing culture conditions and there is good evidence that the enzymic step in chlorophyll degradation that is activated by this treatment is the same one that is stimulated in leaves at the initiation of senescence. Protein mobilisation also takes place, and some of the up-regulated genes are similar to those found in senescing terrestrial plant tissues (Hörtensteiner et al. 2000). Degreening in Chlorella is also reversible (Aoki et al. 1965). The evidence, soft and indirect as it is, points towards a capacity for senescence within each individual mesophyll cell, just as cell autonomy is a characteristic of PCD.

Flower cell death is equally complex; in the few detailed anatomical studies made it is clear that mesophyll cell degradation occurs before outward symptoms of senescence are visible to the casual observer. In Alstroemeria (Wagstaff et al. 2003), for example, the mesophyll cells at the petal margins are completely degraded by the time the flower opens. A similar situation occurs in other short-lived flowers such as Hemerocallis (Stead and van Doorn 1994) and Iris (Bailly et al. 2001). In Sandersonia, however, a similar collapse of the mesophyll cells occurs but not until the corolla wilts, and it is not clear that the collapse of mesophyll cells near the petal margins occurs earlier than that of other mesophyll cells (O'Donoghue et al. 2002). This again illustrates the dangers of considering whole organs as being composed of a collection of homogeneous cells undergoing development and senescence in synchrony with one another, and of assuming that senescence of a given tissue proceeds in an identical manner in different organisms.

References

• Aoki SM, Matsuka M, Hase E (1965) De- and re-generation of chloroplasts in the cells of Chlorella protothecoides. V. Degeneration of chloroplasts induced by different carbon sources, and effects of some antimetabolites upon the process induced by glucose. Plant Cell Physiology 6: 487-497.
• Bailly C, Corbineau F, van Doorn WG (2001) Free radical scavenging and senescence in Iris petals. Plant Physiology and Biochemistry 39: 649-656.
• Fukuda H (1996) Xylogenesis: initiation, progression, and cell death. Annual Review of Plant Physiology and Plant Molecular Biology 47: 299-325.
• Guinel FC; Bown AW (1951) Mechanically isolated photosynthetic cells from asparagus cladophylls originate from two distinct tissue locations. Canadian Journal of Botany 72: 1051 1056.
• Harikrishna K, Darby R, Draper J (1992) Chloroplast dedifferentiation in mechanically isolated asparagus cells during culture initiation. Plant Physiology 100: 1177-1183.
• Hörtensteiner S, Chinner J, Matile P, Thomas H, Donnison IS (2000) Chlorophyll breakdown in Chlorella protothecoides: characterization of degreening and cloning of degreening-related genes. Plant Molecular Biology 42: 439-450.
• Kamachi K, Yamaya T, Hayakawa T, Mae T, Ojima K (1992) Vascular bundle-specific localization of cytosolic glutamine-synthetase in rice leaves. Plant Physiology 99: 1481-1486.
• O'Donoghue EM, Somerfield SD, Heyes JA (2002) Organization of cell walls in Sandersonia aurantiaca floral tissue. Journal of Experimental Botany 53: 513-523.
• Pyke K (1994) Arabidopsis - its use in the genetic and molecular analysis of plant morphogenesis. New Phytologist 128: 19-37.
• Stead AD, van Doorn W (1994) Strategies of flower senescence – a review. In: Scott RJ, Stead AD, eds. SEB Seminar Series 55: Molecular and Cellular Aspects of Plant Reproduction. Cambridge: University Press, 215-237.
• Wagstaff C, Malcolm P, Rafiq A, Leverentz M, Griffiths G, Thomas B, Stead AD, Rogers HJ (2003) Programmed cell death (PCD) processes begin extremely early in Alstroemeria petal senescence. New Phytologist 160: 49-59.