Abstract
Some fungal endophytes of forest trees are recognized as beneficial symbionts against stresses. In previous works, two elm endophytes from the classes Cystobasidiomycetes and Eurotiomycetes promoted host resistance to abiotic stress, and another elm endophyte from Dothideomycetes enhanced host resistance to Dutch elm disease (DED). Here, we hypothesize that the combined effect of these endophytes activate the plant immune and/or antioxidant system, leading to a defense priming and/or increased oxidative protection when exposed to the DED pathogen Ophiostoma novo-ulmi. To test this hypothesis, the short-term defense gene activation and antioxidant response were evaluated in DED-susceptible (MDV1) and DED-resistant (VAD2 and MDV2.3) Ulmus minor genotypes inoculated with O. novo-ulmi, as well as two weeks earlier with a mixture of the above-mentioned endophytes. Endophyte inoculation induced a generalized transient defense activation mediated primarily by salicylic acid (SA). Subsequent pathogen inoculation resulted in a primed defense response of variable intensity among genotypes. Genotypes MDV1 and VAD2 displayed a defense priming driven by SA, jasmonic acid (JA), and ethylene (ET), causing a reduced pathogen spread in MDV1. Meanwhile, the genotype MDV2.3 showed lower defense priming but a stronger and earlier antioxidant response. The defense priming stimulated by elm fungal endophytes broadens our current knowledge of the ecological functions of endophytic fungi in forest trees and opens new prospects for their use in the biocontrol of plant diseases.
Highlights
Browning of vascular tissues was only detected as a reaction to O. novo-ulmi inoculation, but not in E−O− or endophyte inoculum suspension (E+)O− plants (Figure 2A)
Our study provides new evidence of the role of fungal endophytes on the priming of plant defense responses
Endophytic recognition by U. minor upon inoculation and the resulting transient defense stimulation mediated primarily by salicylic acid (SA) and secondarily by jasmonic acid (JA) are possibly key processes in the modulation of the priming response observed when plants are exposed to O. novo-ulmi
Summary
When a pathogen attack is initiated, the plant recognizes pathogen-released molecules (pathogen or microbe-associated molecular patterns; PAMPs or MAMPs) through plant receptors (pattern-recognition receptors, PRR), inducing the so-called PAMP or MAMP-triggered immunity (PTI or MTI) [5]. This PTI together with the recognition of effector molecules released by the pathogen (effector triggered immunity, ETI) leads to the local accumulation of signaling molecules such as phytohormones salicylic acid (SA), jasmonic acid (JA), or ethylene (ET) that trigger the plant immune signaling network [6]. Increasing evidence demonstrates that the host RNA interference machinery is involved in the modulation of the plant immunity, including pathogen perception, ROS production, signal transduction, or downstream immune response [11]. The trafficking of smallRNA between the plant and the pathogen has been recently discovered to modulate the virulence of the pathogen and the expression of plant genes, respectively [12]
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