“Everything is leaf” wrote Goethe, a conceptual insight into plant form that helps us to understand its evolutionary origins.

Stated simply, Goethe’s idea is that each of the different organs of a plant is either a leaf or a modified leaf.

It’s easy to imagine a petal could have evolved from a leaf.  You can even find species in which the evolutionary transition from one to the other seems to have been caught in the act – for example in the “flowers” of Bougainvillea.

Rather more extreme evolutionary remodelling has been necessary to turn leaves into anthers or fruit parts, but land plants have been around for almost half a billion years, which is plenty of time to acquire the necessary developmental moves.

Senescence turns chloroplasts into gerontoplasts; a similar cellular event can be recognised during the lifespan of many of the plant parts that have evolved from leaves.

For example, as fleshy fruits  such as tomatoes and bananas ripen, they start off green and turn yellow.  Their plastids change from chloro- to geronto-, just like those of leaves do.

Ripening is related to (we might say evolved from) senescence, sharing a lot of the same cellular, biochemical and genetic mechanisms.

In many plants, such as maples and other deciduous species, senescing leaves do not simply turn yellow but instead acquire the beautiful fiery pigmentations we associate with autumn in temperate regions.

The chemistry of the golden, orange, red and purple pigments is a study in its own right, and there are several different opinions about why plants should indulge in making them.

Very broadly speaking, the biochemistry that makes the autumnal pigments of leaves is the same as that responsible for the bright colours of petals and fruits.

So we can speculate that flowers and fruits, in which colour became so important for attracting pollinating and seed-dispersing animals, have their evolutionary origins not just in leaves, but in senescing leaves.

As a tomato, or a bell pepper, ripens and turns from yellow to orange to red, the new pigment (a zigzag-shaped molecular structure called lycopene) accumulates in the gerontoplasts, turning them into a different class of plastid called chromoplasts.

This leads to the notion that the gerontoplast is the evolutionary predecessor of the chromoplast.

What about senescence in plants that pre-date the flowering plants in evolution?  This hasn’t been studied in much detail but there are indications that senescence-like behaviour is an ancient trait, recognisable even in single-celled algae.

In particular, the biochemistry of chlorophyll and protein breakdown in chloroplasts seems to have been broadly conserved across the whole plant kingdom.

The change from unicellular to multicellular organisation, and from aquatic to terrestrial existence, required the plastid transition machinery to be retrofitted with mechanisms to make the toxic products of chlorophyll breakdown harmless and to relocate the nitrogen from protein recycling.

In this sense many of the critical events in plant evolution are written in the genes that molecular analysis of leaf senescence allows us to read.

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