Abstract
In Nicotiana benthamiana, the expression of the Xanthomonas effector XANTHOMONAS OUTER PROTEIN Q (XopQ) triggers RECOGNITION OF XOPQ1 (ROQ1)-dependent effector-triggered immunity (ETI) responses accompanied by the accumulation of plastids around the nucleus and the formation of stromules. Both plastid clustering and stromules were proposed to contribute to ETI-related hypersensitive cell death and thereby to plant immunity. Whether these reactions are directly connected to ETI signaling events has not been tested. Here, we utilized transient expression experiments to determine whether XopQ-triggered plastid reactions are a result of XopQ perception by the immune receptor ROQ1 or a consequence of XopQ virulence activity. We found that N. benthamiana mutants lacking ROQ1, ENHANCED DISEASE SUSCEPTIBILITY 1, or the helper NUCLEOTIDE-BINDING LEUCINE-RICH REPEAT IMMUNE RECEPTORS (NLRs) N-REQUIRED GENE 1 (NRG1) and ACTIVATED DISEASE RESISTANCE GENE 1 (ADR1), fail to elicit XopQ-dependent host cell death and stromule formation. Mutants lacking only NRG1 lost XopQ-dependent cell death but retained some stromule induction that was abolished in the nrg1_adr1 double mutant. This analysis aligns XopQ-triggered stromules with the ETI signaling cascade but not to host programmed cell death. Furthermore, data reveal that XopQ-triggered plastid clustering is not strictly linked to stromule formation during ETI. Our data suggest that stromule formation, in contrast to chloroplast perinuclear dynamics, is an integral part of the N. benthamiana ETI response and that both NRG1 and ADR1 hNLRs play a role in this ETI response.