The Scope of PCD in Plants

Although the widespread occurrence of PCD in plants was recognized in earlier literature (Barlow, 1982; Nooden, 1988), the importance has not been recognized broadly in biology. Ameisen (1998) has stated, "the existence of PCD in plants has long remained neglected", and he offered that evidence for its existence has not yet revealed much about the extent of its role, the nature of the mechanisms involved, or the nature of its genetic regulation. Thus, there is a real need to emphasize the scope and significance of PCD in plants.

It is now well documented that PCD plays important roles throughout plant development (Table 2-1). Not only does it occur throughout the plant life cycle, but in many parts of the plant and even whole plants. This point is now illustrated within single species

Table 2-1. Programmed Cell Death in the Life History of Plants

Stage/description

Type of regulation0

References

Vegetative development

Seed

aleurone cells

1

Wang etal., 1996c; Kuo etal, 1996

cotyledons

1

Barlow, 1982

scutellum

1

Lindholm et al., 2000

Shoots

holes in leaves

1

Kaplan, 1984

shoot aerenchyma

1,2

Drew, 1997

leaf aerenchyma

1

Barlow, 1982

xylogenesis

1,2

Fukuda, 1996; Groover and Jones, 1999

xylem ray cells

1

Barlow, 1982

secretary ducts

1

Barlow, 1982

pith cell death

1

Pappelis and Katsanos, 1969

trichomes

1

Greenberg, 1996

senescence

1,2

Noodén, 1988

abscission zone cells

1,2

González-Carranza et al., 1998

cork/bark/wound healing

1,2

Barlow, 1982

Roots

root hairs

1

Noodén, 1988

xylogenesis

1,2

Fukuda, 1996

root cap cells

1

Wang etal., 1996a

cortical parenchyma

1

Barlow, 1982

aerenchyma

1,2

Drew, 1997

lateral roots

2

Barlow, 1982

salt stress

2

Katsuhara, 1997

Reproductive development

Floral organ abortion

1

Grant et al., 1994a;

Dellaporta and Calderon-Urrea, 1993

Megagametogenesis

megaspore abortion

1

Bell, 1996; Christensen etal., 1998

nucellus

1

Barlow, 1982

synergid cell

1

Barlow, 1982

Microgametogenesis

1

Goldberg etal., 1993

Embryogenesis

1

Olson and Cass, 1981

antipodal cells

1

Young etal., 1997

endosperm cells

1

Yeung and Meinke, 1993;

suspensor

Schwartz etal., 1997

Somatic embryogenesis

1

Havel and Durzan, 1996

Continued

Continued

Table 2-1. Continued

Stage/description

Type of regulation0 References

Apomictic embryogenesis loculus wall cells filament cells tapetum cells

Pollen incompatibility

Fertilization (stigma, style and pollen cells)

Flower petal senescence

Post reproductive development

Seed development endosperm cells integument, palisade and epidermal cells pod, carpel senescence

Monocarpic senescence

Barlow, 1982

Goldberg etal., 1993; Wang etal, 1999 Goldberg etal., 1993; Wang etal., 1999 Goldberg etal., 1993; Wang etal., 1999

Bell, 1995

Barlow, 1982; Wang etal., 1996b Chapter 21

Knowles and Phillips, 1988; Young etal., 1997 Barlow, 1982; Greenberg, 1996 Orâez and Granell, 1997

21, developmental regulation; 2, environmental or stress regulation.

(maize, Buckner et al, 1998; arabidopsis, Gray and Johal, 1998). While participating in development, PCD also can be environmentally induced thereby contributing both to form and acclimation. PCD occurs in annuals and perennials. PCD is known in all families of angiosperms, and it also occurs in many species of green algae (Hay, 1997), thus spanning the range from simple to complex plants. Studies of PCD usually focus on the cell or cells that die, but the process can involve intricate cell to cell interactions such as incompatibility during host/pathogen interactions and pollination/fertilization processes. The scale of PCD can vary from the death of one or a few cells in some reproductive processes (Barlow, 1982) to the massive death of organs or whole plants during autumnal leaf senescence or monocarpic senescence (Chapter 1).

The survey of PCD in Table 1 illustrates its scope based on evidence ranging from "predictable cell death at specific places and times" (Barlow, 1982) to more definitive biochemical and molecular evidence. The following sections will deal with some of the better studied examples.

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