Marker and readout genes for priming in the Arabidopsis - Pseudomonas cannabina pv. alisalensis interaction

After a local infection, the entire plant foliage becomes primed for the superinduction of defense responses after rechallenge. The identity of genes expressed during priming (priming-marker genes) or with hyperexpression in primed plants after rechallenge (priming-readout genes) remained largely unknown. We show in Arabidopsis thaliana that genes AT1G76960 (with unknown function), AT3G51860 (encoding vacuolar Ca2+/H+ antiporter CAX3), and AT3G45860 (cysteine-rich receptor-like protein kinase CRK4) are strongly expressed during Pseudomonas cannabina pv. alisalensis-induced priming but not, or much less, in uninfected or challenged plants, or in primed plants after rechallenge. Expression of these genes thus solely marks the primed state for enhanced defense. In contrast to these genes, expression of AT2G14610 (pathogenesis-related protein PR1), AT2G32680 (receptor-like protein RLP23), AT3G25010 (RLP41) and some less expressed loci are activated, about equally, in primed plants before and after rechallenge. They may also serve as marker genes for priming. In further contrast, genes AT2G39530 (encoding Casparian strip domain-like protein 4D1), AT2G19190 (flg22-induced receptor-like kinase FRK1), AT3G28510 (P loop-containing nucleoside triphosphate hydrolases superfamily protein) and some less strongly expressed genes are not, or only faintly activated in uninfected plants, during priming and upon challenge. However, their expression is very strong in primed plants after rechallenge. They are, therefore, specific readout genes for the primed state. Remarkably, mutation in solely priming-readout gene AT5G36970 (encoding NDR1/HIN1-like protein 25) impaired both infection-induced defense priming and systemic acquired resistance suggesting a previously unknown critical role of this gene in the systemic plant immune response.