Light conditions influence specific defence responses in incompatible plant?pathogen interactions: uncoupling systemic resistance from salicylic acid and PR-1 accumulation

Abstract

In incompatible plant-pathogen interactions, disease resistance is generated by rapid activation of a multitude of plant defence reactions. Little is known about the dependency of these resistance responses on external factors. The plasticity of plant defence mechanisms in terms of light conditions is studied here. Interaction of Arabidopsis thaliana (L.) Heynh. with an avirulent strain of Pseudomonas syringae pv. maculicola in the dark resulted in increased apoplastic bacterial growth and therefore reduced local resistance as compared to an infection process in the presence of light. Several characteristic defence reactions, including activation of phenylalanine ammonia-lyase, accumulation of salicylic acid (SA), expression of the pathogenesis-related protein PR-1 and the development of a microscopically defined hypersensitive response, proved to be light dependent. In contrast, the extent of the oxidative burst, as estimated by induction of the protectant gene glutathione- S-transferase, was not weakened by the absence of light. Moreover, pathogen-induced accumulation of jasmonic acid, production of the phytoalexin camalexin and transcriptional induction of a pathogen-inducible myrosinase were even more pronounced in the dark. Apart from affecting local defence responses, light also influenced the establishment of systemic acquired resistance (SAR). SAR development in response to infection by avirulent bacteria was completely lost when the primary infection process occurred in the absence of light. SAR developed both under medium (70 micromol photons m(-2) s(-1)) and strong (500 micromol photons m(-2) s(-1)) light conditions but was in the latter case not associated with an accumulation of SA and PR-1 in systemic leaves, demonstrating that SAR can be executed independently from these molecular SAR markers. Our results are consistent with the notion that SA accumulation in infected primary leaves is necessary for induction of systemic resistance and indicate that defence mechanisms different from SA signalling and PR-protein action exist in systemic tissue to confer resistance during SAR.

Singlet Oxygen Is the Major Reactive Oxygen Species Involved in Photooxidative Damage to Plants

“…Our data suggest that 1 O 2 production prevails over FR production and hence 1 O 2 appears as the most relevant ROS with respect to the execution of cell death in leaf tissues. An example of ROSdependent plant cell death is also the hypersensitive response (Torres et al, 2006) during which light seems to be required both to initiate a cell death program, early on in the infection, as well as at later stages of cell death execution (Zeier et al, 2004;Liu et al, 2007). Interestingly, 1 O 2 HOTE markers significantly increased in Arabidopsis leaves challenged with the avirulent strain DC3000 of Pseudomonas syringae (Grun et al, 2007).…”

Light Regulation and Daytime Dependency of Inducible Plant Defenses in Arabidopsis: Phytochrome Signaling Controls Systemic Acquired Resistance Rather Than Local Defense

“…Light is the major external factor influencing plant growth and development, and an appropriate light environment is also required for the establishment of a complete set of resistance responses in several plant-pathogen interactions (Roberts and Paul, 2006). In tobacco (Nicotiana tabacum), rice (Oryza sativa), and Arabidopsis (Arabidopsis thaliana), HR-associated programmed cell death triggered by bacterial and viral pathogens is light dependent (Lozano and Sequeira, 1970;Guo et al, 1993;Genoud et al, 2002;Zeier et al, 2004;Chandra-Shekara et al, 2006). Similarly, the constitutive cell death phenotype of Arabidopsis acd11 and lsd1 mutants is only evident when light of a certain quantity or duration is present (Brodersen et al, 2002;Mateo et al, 2004).…”

“…Deposition of lignin-like polymers in Xanthomonas oryza-treated rice leaves decrease when light is absent during the first hours after inoculation (Guo et al, 1993). Moreover, Arabidopsis plants inoculated in darkness with an avirulent strain of Pseudomonas syringae are not able to substantially accumulate the phenolic metabolite SA and fail to induce expression of the key phenylpropanoid pathway enzyme Phe ammonia lyase (PAL; Zeier et al, 2004). Light is not only required for SA biosynthesis, but also controls SA perception, because treatment of Arabidopsis leaves with exogenous SA in dim light or in the dark results in strongly reduced expression of the SA-induced defense gene PR-1 (Genoud et al, 2002).…”

“…Light is not only required for SA biosynthesis, but also controls SA perception, because treatment of Arabidopsis leaves with exogenous SA in dim light or in the dark results in strongly reduced expression of the SA-induced defense gene PR-1 (Genoud et al, 2002). Both impaired production and perception of SA therefore account for the observation that PR-1 expression in P. syringae-treated Arabidopsis leaves is completely suppressed in darksituated plants (Zeier et al, 2004).…”

Lesion mimic mutants

“…15,16 Light seems to be required for HR formation at least for some incompatible interactions. 57 Boccara et al, 58 recently showed that after treatment with the elicitor harpin excess excitation energy (EEE) is being formed in chloroplasts leading to overexcited chlorophyll metabolites. They will react with 3 O 2 and form singlet oxygen ( 1 O 2 ).…”