Author(s) : Jean-Pierre BLEIN, UMR 692, Laboratoire de phytopharmacie et de biochimie des interactions cellulaires, Inra, BP 86510, 21065 Dijon Cedex, France.
Summary : Increasing concerns about the environmental impact of modern agricultural have prompted research for alternate practices to pesticide treatments, notably using plant defense mechanisms. Thus, isolation and characterization of plant defense elicitors have been the main step of studies in many groups. Moreover, in the global concept of interactions between organisms and their environment, a major concern is to discriminate recognition between exogenous and endogenous signals, notably during pathogenic or allergenic interactions involving small proteins, such as elicitins or lipid transfer proteins (LTPs). Elicitins and lipid transfer proteins (LTP) are both able to load and transfer lipidic molecules and share some structural and functional properties. While elicitins are known as elicitors of plant defense mechanisms, the biological function of LTPs is still an enigma. They are ubiquitous plant proteins able to load and transfer hydrophobic molecules such as fatty acids or phospholipids. Among them, LTPs1 (type 1 lipid transfer proteins) constitute a multigenic family of secreted plant lipid binding proteins that are constitutively expressed in specific tissues and/or induced in response to biotic and abiotic stress (for reviews [1-4]). Their biological function is still unknown, even if some data provide arguments for a role of these proteins in the assembly of extracellular hydrophobic polymers (i.e., cutin and suberin) [2, 4] and/or in plant defense against fungal pathogens [1, 3]. Beside their involvement in plant defense, LTPs1 are also known to be pan-allergens of plant-derived foods [5].
Finally, the discovery of the sterol carrier-properties of elicitins has opened new perspectives dealing with the relationship between this function and the elicitor activity of these small cystein-rich proteins. Nevertheless, this elicitor activity is restrained to few plant species, and thus does not appear in accordance with a universal lipid transfer function. These considerations required a reassessment of the precise role of elicitins for both fungi and plants [6].
Pictures
Elicitin is represented by the green ellipse. From the left, the different
plasmalemma proteins involved are: the putative receptor (2 subunits,
a 160kDa and a 50kDa protein), a calcium channel, a chloride channel,
the H+-ATPase (inhibited) and the NADPH oxidase. The signs
+ and - indicate the transmembrane potential. The protein phosphorylation
steps are indicated by the blue "P". Orange arrows show the systems that
create the changes in pH. The blue arrows indicate the positive feedback
effects of the extracellular medium alkalization, and the numbers 1-11
indicate the events in their chronological order.
Figure 1. Hypothetical signalling scheme that summarizes the pathways
involved in the early responses of tobacco cells treated with elicitin
(9 steps).
From PDB coordinates of native and ergosterol-complexed cryptogein.
Figure 2. Involvement of Y87
and Y47 during sterol intake in cryptogein.
The receptor of elicitins, located on the plant plasma membranes, is presumed
to be a calcium channel, constituted from four basal subunits (a 160kDa
and a 50kDa protein), each of them able to specifically bind an elicitin
molecule. The first elicitin-channel interaction needs an elicitin loaded
from plant plasmamembrane sterols and triggers a conformational change of
the channel, probably associated with the phosphorylation of the subunit
bound to elicitin. This new state triggers the biological response and allows
the conformational change of the other subunits (cooperative effect), which
then binds either a loaded (upper scheme) or an unloaded (lower scheme)
elicitin. This explains why all elicitins are able to saturate the receptor
subunits, and why only loaded elicitins will trigger a new set of biological
responses.
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