Abstract
In plant-microbe interactions, a pathogenic microbe initially has to overcome preformed and subsequently induced plant defenses. One of the initial host-induced defense responses is microbe-associated molecular pattern (MAMP)-triggered immunity (MTI). Successful pathogens attenuate MTI by delivering various effectors that result in effector-triggered susceptibility and disease. However, some host plants developed mechanisms to detect effectors and can trigger effector-triggered immunity (ETI), thereby abrogating pathogen infection and propagation. Despite the wide acceptance of the above concepts, more and more accumulating evidence suggests that the distinction between MAMPs and effectors and MTI and ETI is often not given. This review discusses the complexity of MTI and ETI signaling networks and elaborates the current state of the art of defining MAMPs versus effectors and MTI versus ETI, but also discusses new findings that challenge the current dichotomy of these concepts.
Highlights
Plants are constantly exposed to a wide variety of adverse environmental conditions that can be broadly classified as biotic or abiotic stresses
Plants are able to detect damage-associated molecular patterns (DAMPs) which are plant degradation products resulting from the action of invading pathogens, or endogenous peptides, constitutively present or newly synthesized, that are released by plants following a pathogen attack (Boller and Felix, 2009)
Besides the very rapid auto- and trans-phosphorylation reactions at the level of the receptor complexes, other protein kinases get activated in a matter of minutes and most of these belong to the CDPK and mitogen-activated protein kinases (MAPKs) protein kinases and are key elements in regulating defense at the level of the transcriptional and metabolic responses (Boudsocq et al, 2010; Frei dit Frey et al, 2014; Lassowskat et al, 2014)
Summary
Plants are constantly exposed to a wide variety of adverse environmental conditions that can be broadly classified as biotic (bacteria, viruses, fungi, parasites, etc.) or abiotic stresses (drought, extreme temperature, chemicals, salinity, etc.). The cuticle is a hydrophobic layer present on the external surface of the aerial epidermis of all land plants and is mainly composed of cutin and waxes (Yeats and Rose, 2013) Does it play a role in defense but it acts as a barrier to transpirational water loss and as a protection against UV radiation. While fungal pathogens are equipped with cuticle and cell wall degrading enzymes to penetrate the epidermis, bacterial pathogens on the other hand do not typically enter plant tissues by directly penetrating the cuticle and cell wall As a result they evolved strategies to enter the plant through a number of natural surface openings, such as stomata and through surface wounds caused by various environmental factors (Melotto et al, 2008).
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