Chapter 5 Cell Death in Plant Development and Defense

CHAPTER

5

CELLDEATHINPLANT

DEVELOPMENTANDDEFENSE

RONMITTLERANDALICEY. CHEUNG

Programmed cell death (PCD) is emerging as a fundamental concept in

modern biology. It plays a vital role in almost all known organisms from uni-

cellular bacteria to complex multicellular organisms such as animals. Within

these two evolutionary extremes, plants may serve as an excellent example

of how a relatively simple multicellular organism uses PCD for many of its

functions. The study of PCD in plants may therefore provide a link between

PCD in unicellular organism and PCD in more complex multicellular organ-

isms from different kingdoms.

Unlike animal cells, plant cells contain rigid cell walls composed mainly

of cellulose, a large vacuole that participates in many chemical and bio-

chemical processes, and a specialized organelle conducting photosynthesis,

that is, the chloroplast. Plants do not have an immune system similar to

animals, but are capable of mounting a large array of defenses, including

PCD, when attacked by pathogens. In addition, the development of plants

is solely determined by cell division and not by a combination of cell divi-

sion and cell migration, as in animals. Despite these differences, plants, much

like animals, require PCD for their proper development and response to dif-

ferent environmental stimuli (Greenberg, 1996; Dangl et al., 1996; Lam et al.,

2000; Beers and McDowell, 2001).

When considering PCD in plants, it is important to remember that

although PCD in plants is similar in its conceptual and functional definitionsto PCD in animals, in many characteristic and mechanistic aspects PCD in

When Cells Die II,Edited by Richard A. Lockshin and Zahra Zakeri.ISBN 0-471-21947-9© 2004 John Wiley & Sons, Inc.

99

plants is different from apoptosis in animals (Mittler, 1998). For example, the

lack of an immune system eliminates the need for the formation of apoptotic

bodies and the “clean” removal of cells, because an inflammatory response

is not likely to occur when the content of a plant cell is spilled. In addition,

in most examples of PCD in plants the cell wall remains intact, making the

trafficking of apoptotic bodies from one cell to the other, or the engulfment

of a dying cell by neighboring cells, unlikely. Despite these differences, the

genome of plants appears to encode for a large number of PCD pathways

activated during development, differentiation, and the defense of plants

against different biotic and abiotic insults.

PCDDURINGDEVELOPMENT

From embryogenesis and seed germination to fertilization, the life cycle of

a plant is marked by dramatic morphological evolution on the organ and

whole plant levels. The biochemical, molecular, and cellular events that

orchestrate the ontogeny of new cells at the meristems which give rise to

morphologically distinct organs, and those that activate distinct biochemi-

cal pathways to support specialized functions of different tissue and cell

types have been extensively studied. Similarly important but often less com-

manding of our attention are cell dedifferentiation, degeneration, and death

processes that permit the plasticity which is associated with plant develop-

ment. Every stage of plant development is marked by PCD events.

Examples include selective cell death during embryogenesis, xylem (water-

conducting system) differentiation, leaf and flower petal senescence. Devel-

opment of the reproductive organs and the reproductive processes of

pollination and fertilization are a progression of PCD processes in meris-

tematic cells to terminally differentiated cell types within the male and

female organs as well as in the gametophytes (pollen and embryo sac)

themselves (Fig. 1).

LEAFSENESCENCE

In vegetative development, leaf senescence provides a most vivid example

of the consequence of PCD in plants (Quirino et al., 2000). Leaf senescence

is apparently a genetically regulated process involving active gene activity

(Oh et al., 1997; Nam, 1997). It is also regulated by both intracellular signals

such as the phytohormone cytokinins and ethylene and environmental

signals such as light (Park et al., 1998). Visible loss of chlorophyll in senesc-

ing leaves is actually preceded by structural and biochemical deterioration

of the chloroplasts. Increased cysteine proteases, nucleases, stress-related

enzymes, and defense-related proteins are associated with leaf senescence100II.BIOLOGICALROLEOFCELLDEATHINDEVELOPMENTANDHOMEOSTASIS