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.
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)
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)
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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