Letting Go Is Never Easy – Abscission (Part 2)

The disclaimer: This is not meant to be an exhaustive review of the recent literature regarding abscission in plants. Instead, think of this as a selection of “recent highlights” (my opinion) in the study of abscission.

Let’s focus on three steps of abscission: (1) the development of the abscission zone (AZ), (2) the signals that activate the abscission process, and (3) some key abscission-related genes and what proteins they code for.

  • 1. Setting The Stage – It’s clear that the abscission zone is formed long before abscission actually takes place, often when the organ in question (leaf, flower petal, seed, etc.) is being formed. Unraveling the precise sequence of events that takes place in forming the abscission zone has been very difficult. The approach has generally been to identify developmental mutants showing abnormal organ shedding in Arabidopsis and tomato, for example, and to isolate the genes responsible. To date, around dozen genes have been shown to be involved abscission zone development, many of them coding for transcriptional factors. (Please see Ref. 2 below for a thorough list.) Although these mutants have revealed several aspects of abscission zone formation, the “…expected set of factors necessary for AZ differentiation have proved surprisingly elusive.” (from Ref. 4 below)
  • 2. Signaling to Go (or Stop) – Another confounding aspect of abscission in plants is what triggers it, specifically which plant hormones are involved, and how. Both auxin (IAA) and ethylene have been shown to be involved in the regulation of the timing of abscission. It’s generally accepted (at least in the textbooks I’ve read) that ethylene acts as a natural accelerator of abscission and that auxin usually functions as a brake. Other plant hormones have also been implicated, including the one called “abscisic acid” (a.k.a., ABA), which unfortunately turns out NOT to have a very important direct role in triggering abscission in most plants. (But that’s a story for another day.) The general consensus seems to be that factors such as stress, senescence, and ABA may actually stimulate abscission through increasing the production of ethylene. Ethylene, in turn, then stimulates the expression of key abscission-related genes in the cells of the AZ.

    But recent results (e.g., see Ref. 1 below) have somewhat complicated the story. The prevailing theory is “…that the auxin/ethylene balance ultimately dictates the triggering of abscission and the rate at which it proceeds. However, much of the evidence of an involvement of auxin in the regulation of abscission is correlative and is based on manipulating hormone concentrations by tissue excision and auxin application. In this study [Ref. 1 below], endogenous IAA activity and signaling, to our knowledge for the first time, have been specifically manipulated within the cells that constitute the AZ. The data reveal that auxin not only regulates the timing of organ shedding in planta but that there is also an absolute requirement for IAA signaling to be maintained for abscission to take place.” (from Ref. 1 below)

  • 3. Breaking Up is Hard To Do – To shed part of itself, a plant must selectively self-digest a small portion of its most fundamental structure, the plant cell wall. During abscission, the walls of the cells in the AZ are digested by enzymes such as cellulase and polygalacturonase, which are synthesized by the AZ cells and then secreted into the cell wall space. This causes the cell walls to become soft and weak, leading to the eventual breakage and separation from the main plant body. But this is sort of the end of the story.

    What happens between the triggering of the process of abscission in the AZ by ethylene (let’s say this is the beginning of the story, for now) and the end of the story (namely, cell wall digestion)? In other words, what’s happening in the AZ cell in between these two events?

    Well, much of what’s happening in this middle section of the story is signal transduction. That is, how does the initial signal – ethylene – cause the biochemical chain of events inside the AZ cells leading to abscission? This is also called the “abscission signaling cascade”, which has been the focus of much research in the past decade or so.

    Briefly, both protein kinases located on the plasma membrane of AZ cells and membrane vesicle trafficking appear to play key roles in disseminating and amplifying the initial signals involved in activating abscission and production and secretion of enzymes involved in cell wall degradation. (For detailed information about this, please see Ref. 3 below.)

    Despite the fact that dozens of genes have been identified, there are still many questions regarding their precise role in abscission.

    “Abscission signaling cascades…have been extensively elucidated, although our understanding of the many identified genes remains fragmentary and incomplete. Moreover, it remains to be determined whether a general regulatory mechanism for abscission may exist among different organs and whether the mechanism may be conserved in different plant species.” (from Ref. 5 below).

    Cellular signaling cascades are somewhat analogous to a cellular Rube Goldberg machine – for instance:

    In conclusion: “If letting go is never easy, neither is it easy to understand the reasons why. Moving forward, there are large gaps in our knowledge of abscission that remain to be filled.” (from Ref. 3 below)

    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)

    4. Liljegren, S. J. (2012) “Organ abscission: exit strategies require signals and moving traffic.”, Current Opinion in Plant Biology, Vol. 15, pp. 670–676. (Abstract)

    5. Nakano, T. and Y. Ito (2013) “Molecular mechanisms controlling plant organ abscission.”, Plant Biotechnology, Vol. 30, pp. 209–216. (PDF)

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