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“..And all the leaves on the trees are falling
To the sound of the breezes that blow…”

– Van Morrison

How Can I Miss You, If You Won’t Go Away?

Ah, November in the Pacific Northwest, when the autumn leaves are being scoured from the trees by blustery winds and driving rain,… and a plant physiologist’s thoughts turn to….abscission. Yes, abscission.

You probably already knew that abscission refers to the process by which a plant sheds one or more of its parts, such as a leaf, fruit, flower, or seed. (“Abscission”, from the Latin “ab“, meaning “away“, and “scindere“, meaning “to cut“.)

The shedding of autumn leaves, the dropping of petals from an old rose flower, and the dispersal of dandelion seeds in the wind are all the result of abscission in plants.

Why do plants do this?

It’s because abscission “…can limit the spread of systemic invasion by pathogens, provide a mechanism to remove damaged or inefficiently functioning tissues, remove competition for pollinators from fertilized flowers, and contribute to seed dispersal in dry and fleshy fruits.” (from Ref. 1 below)

And why should anybody care about this?

The timing of flower and fruit abscission is a process of substantial interest to the horticultural and agricultural industries, as it can affect both the quantity and quality of yield.” (from Ref. 1 below)

Some basic information about abscission was explored in a previous post.

Basically, the story goes like this: During plant development, the plant will form a so-called abscission zone (AZ) typically in a region at the base of a leaf, flower, fruit, or other plant part. Later on (often months later), the activation of the plant cells in the AZ by certain signals (primarily the plant hormone ethylene) ultimately results in the separation of that part from the plant body.

There are many questions about the cellular mechanisms of abscission that remain unanswered. Such as: How does the plant “pre-determine” which of its parts to shed? (Why shed leaves, for example?); What triggers abscission?; What genes are directly involved the separation process?

Even from the simplification of abscission depicted in the illustration below, it appears likely that multiple pathways and processes must somehow be integrated to bring about abscission in plants.

The basic story of abscission (by the way, the little green circles are plant cells) – a simplified version of Fig. 2 from Ref. 2 below.

1 s2 0 S0168945212002191 gr2

Plant scientists have used different approaches – physiological and genetic – to discover various parts of the story. The challenge, of course, is fitting all the pieces together in order to reveal the “big-picture” of abscission’s “clockwork”. When I last investigated this a few years ago, many parts of the story were missing, and, also, it was unclear how some of the parts actually fit into the story.

Several reviews have been published in the past couple of years regarding recent (past decade or so) research into the mechanisms of abscission in plants. Two that are openly accessible online are Refs. 2 and 3 below. (Many thanks to the publishers of these journals, by the way, for allowing such open access.)

I read these and a couple of other such reviews (so that you don’t have to). What I found out was a bit encouraging, but also a bit disappointing.

Next Time: What’s new about abscission. (The good, the bad , and the ugly.)

References

1. Basu, M. M., Z. H. González-Carranza, S. Azam-Ali, S. Tang, A. A. Shahid and J. A. Roberts (2013) “The manipulation of auxin in the abscission zone cells of Arabidopsis flowers reveals that indoleacetic acid signaling is a prerequisite for organ shedding.” Plant Physiology, Vol. 162, pp. 96-106. (Full Text)

2. Estornell, L. H., J. Agustí, P. Merelo, M. Talón, and F. R. Tadeo (2013) “Elucidating mechanisms underlying organ abscission.”, Plant Science, Vols. 199–200, pp. 48–60. (Full Text)

3. Chad E. Niederhuth, Sung Ki Cho, Kati Seitz, and John C. Walker (2013) “Letting go is never easy: Abscission and receptor-like protein kinases.” Journal of Integrative Plant Biology, Vol. 55, pp. 1251–1263. (Full Text)

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