Induced resistance protects plants against a wide spectrum of\ud diseases; however, it can also entail costs due to the allocation of\ud resources or toxicity of defensive products. The cellular defense\ud responses involved in induced resistance are either activated\ud directly or primed for augmented expression upon pathogen\ud attack. Priming for defense may combine the advantages of enhanced\ud disease protection and low costs. In this study, we have\ud compared the costs and benefits of priming to those of induced\ud direct defense in Arabidopsis. In the absence of pathogen infection,\ud chemical priming by low doses of ß-aminobutyric acid caused\ud minor reductions in relative growth rate and had no effect on seed\ud production, whereas induction of direct defense by high doses of\ud ß-aminobutyric acid or benzothiadiazole strongly affected both\ud fitness parameters. These costs were defense-related, because the\ud salicylic acid-insensitive defense mutant npr1-1 remained unaffected\ud by these treatments. Furthermore, the constitutive priming\ud mutant edr1-1 displayed only slightly lower levels of fitness than\ud wild-type plants and performed considerably better than the\ud constitutively activated defense mutant cpr1-1. Hence, priming\ud involves less fitness costs than induced direct defense. Upon\ud infection by Pseudomonas syringae or Hyaloperonospora parasitica,\ud priming conferred levels of disease protection that almost\ud equaled the protection in benzothiadiazole-treated wild-type\ud plants and cpr1 plants. Under these conditions, primed plants\ud displayed significantly higher levels of fitness than noninduced\ud plants and plants expressing chemically or cpr1-induced direct\ud defense. Collectively, our results indicate that the benefits of\ud priming-mediated resistance outweigh the costs in environments\ud in which disease occurs
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