Microbial pathogens infect host cells by delivering virulence factors (effectors) that interfere with defenses. In plants, intracellular nucleotide-binding/leucine-rich repeat receptors (NLRs) detect specific effector interference and trigger immunity by an unknown mechanism. The Arabidopsis-interacting NLR pair, RRS1-R with RPS4, confers resistance to different pathogens, including Ralstonia solanacearum bacteria expressing the acetyltransferase effector PopP2. We show that PopP2 directly acetylates a key lysine within an additional C-terminal WRKY transcription factor domain of RRS1-R that binds DNA. This disrupts RRS1-R DNA association and activates RPS4-dependent immunity. PopP2 uses the same lysine acetylation strategy to target multiple defense-promoting WRKY transcription factors, causing loss of WRKY-DNA binding and transactivating functions needed for defense gene expression and disease resistance. Thus, RRS1-R integrates an effector target with an NLR complex at the DNA to switch a potent bacterial virulence activity into defense gene activation.

Although ions play important roles in the cell and chloroplast metabolism, little is known about ion transport across the chloroplast envelope. Using a proteomic approach specifically targeted to the Arabidopsis chloroplast envelope, we have identified HMA1, which belongs to the metal-transporting P 1B -type ATPases family. HMA1 is mainly expressed in green tissues, and we validated its chloroplast envelope localization. Yeast expression experiments demonstrated that HMA1 is involved in copper homeostasis and that deletion of its N-terminal His-domain partially affects the metal transport. Characterization of hma1 Arabidopsis mutants revealed a lower chloroplast copper content and a diminution of the total chloroplast superoxide dismutase activity. No effect was observed on the plastocyanin content in these lines. The hma1 insertional mutants grew like WT plants in standard condition but presented a photosensitivity phenotype under high light. Finally, direct biochemical ATPase assays performed on purified chloroplast envelope membranes showed that the ATPase activity of HMA1 is specifically stimulated by copper. Our results demonstrate that HMA1 offers an additional way to the previously characterized chloroplast envelope Cu-ATPase PAA1 to import copper in the chloroplast.Chloroplasts contain a large variety of ions among which metal ions such as copper, iron, manganese, and zinc that are essential for their development and function. Copper is an essential redox cofactor required for a wide variety of processes, including photosynthetic electron transfer reactions (plastocyanin) and detoxification of superoxide radicals (Cu/Zn-superoxide dismutase, SOD) 4 (1). Other metal ions are cofactors for several enzymatic reactions: zinc is associated with chloroplast SOD, methionine synthase, carbonic anhydrase; manganese is required for oxygen evolution in photosynthesis; whereas iron is a cofactor of iron SOD and is found in iron-sulfur clusters of cytochrome b 6 f complex, ferredoxin, photosystem I (PSI), and photosystem II (PSII) (2). All these metals are essential micronutrients but are toxic when present in excess (3). To maintain the concentration of metals within physiological limits, cells possess mechanisms that control the uptake, accumulation, trafficking, and also detoxification of metal ions. Little is known about metal transport into chloroplasts. Until now, the sole chloroplast proteins demonstrated as being involved in metal ions transport are PAA1 (4) and very recently PAA2 (5), two P 1B -type ATPases. PAA1, localized into the chloroplast envelope, supplies copper to the chloroplast, whereas PAA2, localized into the thylakoid membrane, delivers copper to the thylakoid lumen. One important result derived from this work is the fact that the disruption of the PAA1 gene does not fully abolish the import of copper into the chloroplast. From this recent observation, Abdel-Ghany and coworkers (5) concluded that an as yet unidentified and additional way to PAA1 must exist to import copper into the chloroplast....