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A membrane is a layer of material that functions as a barrier that separates the inside from the
outside. In cells these membranes consist of two layers of lipids and several imbedded
proteins. Early thoughts concerning the cellmembrane were that these lipids only functioned
as a barrier and gave rigidity to the cell, whereas the proteins were responsible for all other
tasks.
Recent discoveries, however, show that certain membrane lipids seem to play a prominent
role in all sorts of signaling cascades. Lipid signaling refers to any biological signaling event
involving a lipid messenger that binds to a target protein and thereby influencing the proteins
activity. Increasing amounts of lipids will bind more protein, leading to increased or
decreased activity. One of these signaling lipids is phosphatidic acid (PA), whereas PA levels
rise upon various kinds of induced stresses. A signaling pathway in which PA is putatively
involved is the ethylene signaling pathway in Arabidopsis thaliana.
Ethylene is a gaseous plant hormone, which is known to be involved in a variety of plant
processes such as for instance the stimulation of shoot and root growth, differentiation, fruit
ripening, and flower and leaf senescence. The ethylene signaling pathway has been well
studied over the past years and is shown in figure 1.
Figure 1: Major players within the ethylene response pathway. Their activity has been given in both presence
and absence of ethylene with the green boxes indicating an active status of the protein and the red boxes
indicating an inactive status.
One of the major players in the pathway is constitutive triple response 1 (CTR1); which is
usually active when no ethylene is present, but upon addition of ethylene becomes inactive.
There is evidence that PA binds and inhibits the activity of CTR1. When PA levels rise upon
ethylene addition, it is possible for PA to bind CTR1 in greater amounts.
PA can be formed through two different pathways as shown in figure 2.
a.
b.
Figure 2: Two pathways in which PA can be produced.
a) Phosphatidylinositol 4,5-bisphosphate (PIP2) is converted into diacylglycerol (DAG) and inositol 1,4,5triphosphate (IP3) by phospholipase C (PLC), DAG is further converted into PA by diacylglycerol kinase
(DGK). Functional proteins are shown in purple, whereas substrates are shown in orange.
b) Both phosphatidylcholine (PC) and phosphatidylethanolamine (PE) can be converted into PA by
phospholipase D (PLD). Functional proteins are shown in purple, whereas substrates are shown in orange.
To establish whether PA indeed binds to CTR1 and thereby inhibits CTR1s function upon
ethylene addition, several mutants were tested being defective in either one of the signaling
pathways. Although there are nine different PLC genes, seven different DGK isozymes, and
twelve different PLD genes found within Arabidopsis thaliana, mutants in either one of them
might show that the mutant protein tested is specifically involved in production of PA for
ethylene signaling.
Mutants were screened for differences in ethylene sensitivity. Here, three mutants were
interesting: plc2-1, pldβ2-1, and pldγ1-3. Of two of these mutants, plc2-1 and pldβ2-1, it is
confirmed that they have altered ethylene sensitivity. Since abscisic acid (ABA, another plant
hormone that is among other things involved in seed dormancy) and ethylene have
intertwined pathways and thereby one of the major players of ABA signaling also is shown to
interact with PA, ABA sensitivity was tested. Two out of the three mutants indeed seem to
have different ABA sensitiveness. Although attempts to establish the identity of the mutation
have been made here, it hasn’t been confirmed yet and therefore still has to be done in the
future.