Howard Sid Thomas | Chlorophyll in senescence

Chlorophyll

Why are leaves green and not black? Black is a better colour for a solar panel because it uses the whole of the visible spectrum. Spectrum-stealers of early evolution

Some people think the earliest forms of life that evolved to use light energy had pigments that absorbed light from most of the middle range of the spectrum. This is why chlorophyll (which evolved later) has to work with wavelengths at the extreme ends (red and blue), and reflects the green colour we see.

DasSarma has suggested the earliest photosynthetic organisms were like modern purple bacteria. Purple and green membrane absorbance spectra It’s very striking the way the absorbance profile of the purple pigment retinal neatly sits in the gap between the red and blue-absorbing peaks of chlorophyll.

Anyway, it’s clear that chlorophyll, and chlorophyll metabolism, have been around from early in evolution, and originated in the primeval ocean.

Chlorophyll synthesis usually requires light and occurs by a highly conserved biochemical pathway. Most unicellular and multicellular plants except the angiosperms (flowering plants) are also capable of making chlorophyll in the dark.

Actually, some people (us included) think angiosperms have some capacity to make chlorophyll without light, but this must be by yet another pathway, different from the one in algae and cryptogams (non-flowering terrestrial plants).

Chlorophyll synthesis in the dark

Helen Ougham and Caron James did this experiment in which grass leaves were kept in the dark for (left to right) 0, 2, 4, 6 and 8 days. The lower set of leaves were treated with the plant hormone ABA and have clearly become greener towards the end of the experiment. Nothing is known about the mechanism of this controversial behaviour.

As for the evolution of chlorophyll breakdown, it may be that there was no particular biochemical mechanism at first. Evidently a lot of prehistoric chlorophyll simply fell to the bottom of the ocean and became fossilised.

We know of a few biochemical routes by which chlorophyll is partially broken down in aquatic environments. One occurs in phosphorescent dinoflagellates and euphausids.

Of more direct relevance to chlorophyll loss during senescence is the transformation of pigments in certain unicellular green algae under light and nutrient stress.

Chlorophyll degradation and red pigment production by Chlorella When Chlorella protothecoides is starved of nitrogen and put in the dark, it turns yellow – rather like a leaf does when treated the same way.

Removing the yellow cells from the medium by centrifugation reveals a red pigment. The chemical structure of the red product shows it to be derived from chlorophyll.

When the pathway of chlorophyll catabolism (breakdown) in leaves was worked out, a red intermediate almost identical to the Chlorella product was found to be a transient intermediate.

Unlike Chlorella, the multicellular tissues of a land plant can’t get rid of the products of chlorophyll catabolism by squirting them out into the growth medium.

It turns out that terrestrial plants deal with the red chlorophyll catabolite (RCC) by using a biochemical mechanism evolved to make toxic chemicals harmless.

RCC (and the green catabolites upstream of it) can be harmful to plant tissues in the light, just as pigments can be phototoxic to animals and humans under some circumstances.

We can show this by knocking out one or other of the enzymes that metabolise RCC and its precursors. For example the Arabidopsis mutants acd1 and acd2 are deficient in, respectively phaeophorbide a oxygenase (the enzyme that makes RCC) and RCC reductase (the enzyme that destroys the phototoxic red colour of RCC).

lls1 mutant of maize

This picture is of a leaf of the maize lls1 mutant which, like Arabidopsis acd1, lacks PaO, accumulates phototoxic chlorophyll catabolites and develops cell death lesions in the light. From the very interesting article by Gurmukh Johal.

The detoxification pathway in senescing cells typically involves conjugation of the harmful chemical with, for example, a sugar molecule and active transfer across the tonoplast membrane into the vacuole.

The vacuoles in a leaf cell expressing a fluorescent protein specifically targetted to the organelle The vacuole acts as a kind of dustbin, accumulating all manner of terminal metabolites that are discarded when the leaf is shed.

By taking a bomb-disposal approach to dealing with chlorophyll during mesophyll senescence, the viability of the cell is protected and nutrient mobilisation proceeds without threat from photodamage.

We may speculate that forging the metabolic link between the RCC-production part of the catabolic pathway found in green algae and the detoxification route leading to sequestration in the vacuole was one of the prerequisites for evolution of multicellular land plants.