Systemic acquired resistance is a powerful mechanism, based on the salicylic acid (SA) signaling pathway, which allows plants to resist to a wide range of pathogens. High SA, moreover, plays a key role in plant tolerance to abiotic stress. It seems, therefore, desirable to supply analogs of SA or stimulate the production of endogenous SA. Unfortunately, the chemical substances or physical means used for this effect often display a variable efficacy. After providing a review of them, we defend three major ideas: (i) plant resistance inducers (PRIs) must be combined for higher efficacy, notably for exploiting synergic effects between the SA and other signaling pathways, (ii) disease pressure can be reduced by exploiting the fungicidal properties displayed by some PRIs, (iii) biostimulants and crop management techniques should be used to ensure that plants have the resources they need to synthesize the compounds and structures required for efficient and lasting resistance. Some PRIs could also be used for their biostimulant effects in stress conditions. It could be concluded that holistic approaches which jointly address the issues of defense and tolerance stimulation, disease pressure and resource availability in plants are the ones that will allow for substantial reduction in fungicide use without sacrificing crop performance.
Light is an important regulator of plant morphogenesis and plant-pathogen interactions via specific photoreceptors and signaling pathways. Besides visible light, other electromagnetic radiations may play roles, notably ultraviolet (UV) light. The UV part of the electromagnetic spectrum includes UV-A (315 nm - 400 nm), UV-B (280 nm - 315 nm) and UV-C radiations (200 nm - 280 nm). UV-B and UV-C have been reported to increase plant resistance to plant pathogens after the UV perception and signaling stages. The perception of UV-B radiation is achieved by the dimer protein UVR8 (UV RESISTANCE LOCUS 8). Even though the action spectrum of this photoreceptor overlaps in the UV-C domain, it has never been formally demonstrated that UVR8 could also act as a photoreceptor of UV-C light. We provide here original observations showing that UVR8 can indeed perceive UV-C light provided that the latter is in the form of flashes (1s) and not continuous illuminations (the 60s). Our observations also show that the response of UVR8 to flashes of UV-C light is dose-dependent. They could explain why flashes of UV-C light are more effective for stimulating plant defenses than continuous illuminations for the same amount of energy delivered to plants (J/m2). Eventually, our observations support ongoing trials that aim at using UV-C light as an environmental-friendly plant resistance inducer in field conditions.
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