FIGURE 1 | Examples of programmed cell death in plants.

a | An example of the hypersensitive response: a tobacco leaf is infiltrated with two different strains of Pseudomonas syringae. Zones 1 and 2 are infiltrated with strain NPS3121 and zones 3 and 4 with strain NPS4000. The latter strain is isogenic to NPS3121 except for a mutation in the HRP locus, which renders it incapable of inducing the hypersensitive response in tobacco. Visible cell death of the inoculated regions can be observed in zones 1 and 2 one day post-inoculation, whereas no significant morphological changes were observed in zones 3 and 4. Enlarged views with back lighting show cleared cells with little chlorophyll remaining in zones 1 and 2, in contrast to zones 3 and 4. b | An example of developmental death that has a critical role in the housekeeping function of mechanical support and long-distance transport is the formation of the xylem in vascular plants. The top panel shows a cross-section of a tobacco leaf at the mid-rib region, with the boxed region showing the central files of tracheary elements that are critical for transport of solutes and water. These cells have already undergone programmed cell death and the remaining corpses have reinforced secondary cell walls that are highly autofluorescent (shown by arrowheads in enlarged views). Top enlarged view: transmission light microscopy image. Bottom enlarged view: epifluorescence of the same viewing field with excitation filter at 436 nm and emission filter at 480 nm.

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