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When No Really Means No (In Plants)

When it comes to sexual reproduction, nearly half of all flowering plant species are self-incompatible.

That is, the pollen (male parts of flowers) produced by an individual plant is somehow recognized by the plant and rendered ineffectual in self-fertilizing the ovules (female parts of flowers).

This prevention of self-fertilization promotes outcrossing and allogamy and, thus, increased genetic diversity. Self-incompatibility (SI) “…is one of the most important means of preventing inbreeding and promoting the generation of new genotypes in plants, and it is considered as one of the causes for the spread and success of angiosperms on the earth. (from Wikipedia)

Considering the above, I suppose you might say that SI is not really an example of the “selfish” plant… just the opposite.

But, in order for this sexual self-incompatibility to work, the plant must be able to somehow discern its own pollen from that of other plants. In other words, here we have yet another example of plants being able to recognize “self” from “non-self”.

In this case of SI, however, the plant rejects “self” pollen, but not “non-self” pollen.

When I was teaching this subject in class at university (in a former life), the students (who were still awake) would typically have three questions: (1) How do SI plants tell “self” from “non-self” pollen? (2) How do they “reject” the “self” pollen?, and (3) How did SI evolve in flowering plants?

Well, I’ll try to briefly answer these questions as follows:

  • (1) Pollen grains from SI plants have very specific protein “keys” on their surfaces that precisely fit only into “locks” on the surface of the stigmas of flowers from the same plant. Thus, when a “self” pollen grain lands on the stigma’s surface, the rejection mechanism is “unlocked” (activated). A “non-self” pollen grain does not have a matching “key” on its surface, so the rejection mechanism remains “locked” (inactive).
  • (2) In general, the rejection mechanism usually involves the biochemical inhibition of pollen germination by the stigma.
  • (3) “…different molecular mechanisms for avoiding self-breeding have evolved at least 35 times in angiosperm history.” (from Ref. 1 below) Because of this, the evolution of SI in flowering plants is currently unclear, although some (e.g., see Ref. 2 below) think that SI may be related to non-self rejection mechanisms (see previous post).
  • Please Note: The nature, mechanisms, and evolution of self-incompatibility (SI) in plants are quite complex. Rather than delve deeper into these subjects here, I’ll refer you to two of the best summaries I’ve found online, namely, Wikipedia and here.

    Since SI may be considered as a sort of plant kin recognition, in this case to discourage inbreeding, are there other examples of kin recognition in plants?

    Plant Nepotism?

    It’s well-know that kin recognition exists in animals, often resulting in both increased competition toward strangers and reduced interference (increased cooperation) toward kin.

    In 2007, a paper (Ref. 3 below) was published reporting that “…plants grown alongside unrelated neighbours are more competitive than those growing with their siblings….” (from Nature News), which (seems to me at least) sort of got the subject of plant kin recognition taken more seriously among plant scientists.

    Of course, one of the main questions regarding kin recognition in plants is how exactly the plants are able to discern “kin” from “non-kin”.

    The most popular theory appears to involve specific compounds in root exudates, but other theories have emerged including root electrical signals and also the quality of reflected light (Ref. 4 below) from adjacent plants above ground.

    Please see Ref. 5 below for a good review of this interesting subject.


    1. Fujii, S., K-I. Kubo and S. Takayama (2016) “Non-self- and self-recognition models in plant
    self-incompatibility.” Nature Plants, 6;2(9):16130. doi: 10.1038/nplants.2016.130. (Abstract)

    2. Kear, P. J. and B. McClure (2012) “How did flowering plants learn to avoid blind date mistakes? Self-incompatibility in plants and comparisons with nonself rejection in the immune response.” Advances in Experimental Medicine and Biology, Vol.738, pp.108-123. doi: 10.1007/978-1-4614-1680-7_7. (Abstract)

    3. Dudley, S. A. and A. L. File (2007) “Kin recognition in an annual plant.” Biology Letters, Vol. 3, pp. 435-438; doi: 10.1098/rsbl.2007.0232. (Full Text)

    4. Crepy, M. A. And J. J. Casal (2015) “Photoreceptor-mediated kin recognition in plants.” New Phytologist, Vol. 205, pp. 329-338. (Full Text – PDF)

    5. Depuydt, S. (2014) “Arguments for and against self and non-self root recognition in plants.” Frontiers in Plant Science, 5:614. doi: 10.3389/fpls.2014.00614 (Full Text)

    For Next time:
    The Self-Aware Plant?

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