Abstract
Exposure of plants to biotic stress results in an effective induction of numerous defense mechanisms that involve a vast redistribution within both primary and secondary metabolisms. For instance, an alteration of tricarboxylic acid (TCA) levels can accompany the increase of plant resistance stimulated by various synthetic and natural inducers. Moreover, components of the TCA flux may play a role during the set-up of plant defenses. In this study, we show that citrate and fumarate, two major components of the TCA cycle, are able to induce priming in Arabidopsis against the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Both citrate and fumarate show no direct antimicrobial effect and therefore enhanced bacterial resistance found in planta is solely based on the induction of the plant defense system. During the priming phase, both TCA intermediates did not induce any changes in transcript abundances of a set of defense genes, and in phytohormones and camalexin levels. However, at early time points of bacterial challenge, citrate induced a stronger salicylic acid and camalexin accumulation followed later by a boost of the jasmonic acid pathway. On the other hand, adaptations of hormonal pathways in fumarate-treated plants were more complex. While jasmonic acid was not induced, mutants impaired in jasmonic acid perception failed to mount a proper priming response induced by fumarate. Our results suggest that changes in carboxylic acid abundances can enhance Arabidopsis defense through complex signaling pathways. This highlights a promising feature of TCAs as novel defense priming agents and calls for further exploration in other pathosystems and stress situations.
Highlights
The continuous exposure of plants to biotic stressors and environmental changes forces them to constantly remodel their defense strategies as well as their metabolism (Stael et al, 2011; Nomura et al, 2012; Zhang et al, 2014b; Van Aken et al, 2016)
During the priming phase induced by β-aminobutyric acid (BABA), a series of metabolic changes occur which are generally characterized by a massive boost of the primary metabolism through a specific accumulation of tricarboxylic acids (TCAs) such as citrate, fumarate, (S)-malate and 2-oxoglutarate (Pastor et al, 2014)
The treatment of Arabidopsis plants with 5 and 10 mM of citrate resulted in an induction of resistance against PstDC3000 compared to water- and HCltreated plants (Figure 1A)
Summary
The continuous exposure of plants to biotic stressors and environmental changes forces them to constantly remodel their defense strategies as well as their metabolism (Stael et al, 2011; Nomura et al, 2012; Zhang et al, 2014b; Van Aken et al, 2016). There is a massive reprogramming of the plant cell in order to activate and deploy an efficient immune reaction in response to different kinds of stress In this context, the phenomenon of priming has been well described to be part of the resulting intricate networks of inducible defenses. The phenomenon of priming has been well described to be part of the resulting intricate networks of inducible defenses In this regard, priming is defined as an induced state whereby basal and further layers of defense are potentiated to react more rapidly and TCAs Prime for Antibacterial Defense more efficiently to a stress (Conrath et al, 2002; Pastor et al, 2013). It is possible that BABA-induced priming functions through the potentiation of the TCA flux and that the primary metabolism plays an important role in BABA-induced priming
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