When fall comes and plants begin to prepare for the long winter, the chlorophyll in their leaves is broken down. This is probably the most dramatic example of the general process known as senescence, a form of programmed cell death that deciduous plants use to divert nutrients to areas of active growth or to store them in anticipation of a resource-intensive task like seed production. (Programmed cell death is also a normal component of human development, where it's called apoptosis). Now, researchers have found that it may be possible to track the process of senescence by taking advantage of the fact that, in some species, one chlorophyll breakdown product is fluorescent.

In fruits, the release of ethylene, a gaseous hormone derived from the amino acid methionine, triggers the process of aging, or, as it’s more commonly referred to, ripening. As in senescent leaves, the chlorophylls in ripe fruits are degraded into colorless end products, known as nonfluorescent chlorophyll catabolites (NCCs), through a series of reactions. At the same time, enzymes in the fruits begin to hydrolyze starch and pectin, making them softer and sweeter and, thus, more appealing to hungry animals (like humans).

For a while, scientists believed that chlorophyll catabolism followed a similar trajectory in most plants and fruits. Recently, however, a team of researchers led by Simone Moser of the University of Innsbruck in Austria discovered a unique class of catabolites in banana peels, called fluorescent chlorophyll catabolites (FCCs), which glowed blue when exposed to ultraviolet (UV) light. In follow-up work, they have now demonstrated that FCCs act as in vivo markers for sections of the peel that are undergoing rapid senescence, allowing the researchers to closely track the ripening process. More importantly, the discovery of these catabolites suggests that there may be more than one path of chlorophyll breakdown in fruits and, perhaps, in some plants. Their findings are detailed in PNAS.

To identify the FCCs, the group analyzed extracts of peels taken from the regions surrounding ripe, dark spots using high performance liquid chromatography (HPLC), which separates the constituents of a mixture. The major component, Mc-FCC-49 (a hypermodified, polar molecule), was highly abundant in the luminescent areas around the spots while two less polar compounds, Mc-FCC-56 and Mc-FCC-53, were found in large amounts in extracts of whole banana peels.

In general, as the dark spots on the peel began to accumulate, Mc-FCC-49 becomes more common while Mc-FCC-53, the main FCC of ripening bananas, becomes scarcer. The appearance of Mc-FCC-49 thus marked the transition of "ripe" bananas to "rotten" ones, according to the authors.

An analysis of the fluorescence spectra of the glowing rings showed a maximum around 445 nm, the blue region of the visible spectrum, which was the result of high concentrations of Mc-FCC-49 collecting in the areas surrounding the spots.

They then tracked the spread of a dark spot with a 24-hour fluorescence analysis to determine how the intensity of the luminescence fluctuated over time. At first, the intensity of the luminescence in the ring increased sharply as that from within the dark spot continuously decreased. By the end of the 24-hour period, however, the ring had lost most of its luminescence.



Image credit: Enlarged images of areas of a dark spot and its surroundings showing hypermodified FCCsImage credit: PNAS

Further analyses by scanning electron microscopy (SEM) revealed that the stomata, tiny pores found on the surface of plants that enable gas exchange, were still active in the intact peels of the ripening bananas. Since the dark spots first began to appear in areas surrounding the stomata, the authors were able to study the luminescent rings at a cellular level. While the cells within the spots were dead and shrunken, those found within the rings were still alive, though close to death.

FCCs, which appear in many forms in a variety of plant tissues, could play a similar role in indicating the progression of senescence elsewhere. For instance, a polar FCC with an almost identical structure to one found in extracts from banana leaves was recently identified in the Peace Lily, a distant relative of the banana, where it is thought to be the main chlorophyll catabolite.

This study expands significantly on the researchers' previous work by showing that the intensity of the blue luminescence produced by the FCCs could be used to track senescence in vivo in a variety of plants. The discovery and further study of FCCs in other plants could shed light on aging and other mysterious processes related to cell death. On a more practical level, FCCs could act as molecular trackers for ripening, making it easier to determine whether a piece of fruit is just right or too ripe.

PNAS, 2009. DOI: 10.1073/pnas.0908060106

Listing image by PNAS