SnapShot: Plant Immune Response Pathways

Recognition of Microbe/Pathogen-Associated Molecular Patterns Plant innate immunity depends on the timely discrimination of self from nonself, which, as in animal cells, can be accomplished by membrane-anchored pattern recognition receptors (PRRs). These receptors monitor the exterior space for microbe/pathogen-associated molecular patterns or MAMPs (also called PAMPs) by binding to them directly or by associating with MAMP-binding proteins. Well-characterized plant PRRs comprise leucine-rich repeat (LRR) receptor-like kinases such as the flagellin receptor FLS2, its coreceptor BAK1, and the receptor for bacterial elongation factor EF-Tu called EFR. Membrane-bound proteins with peptidoglycan-binding LysM domains (for example, CEBiP and CERK1) represent PRRs or binding proteins for the fungal MAMP, chitin. Recognition of conserved MAMPs by PRRs triggers intracellular signaling via mitogen-activated protein kinase (MAPK) cascades (MAPKKK, MAPKK, MAPK). This results ultimately in transcriptional activation of defense genes by plant-specific transcriptional regulators including members of the WRKY superfamily. Execution of the immune response relies in part on the exocytosis pathway leading to vesicle-associated and SNARE protein-mediated focal secretion of defense-related proteins including PR-1. A " toxic load " of antimicrobial cargo such as that derived from the biochemical activation of nontoxic precursors can also be translocated to the extracellular space by members of the ATP-binding cassette (ABC) transporter family. Another early MAMP-triggered response is the extracellular generation of reactive oxygen species (O 2-and H 2 O 2) by membrane-localized NADPH oxidases (RbohD). In contrast, biosynthesis and deposition of the polyglucan callose (by the GSL5/PMR4 callose synthase) in the extracellular space is a comparatively late defense-associated response. Microbial effector proteins (such as AvrPto, AvrPtoB, HopM1) intercept innate immune responses at the level of MAMP signaling or by targeting the secretory defense execution machinery. For example, HopM1 binds to the ARF-GEF, MIN7, of Arabidopsis leading to ubiquitination-dependent proteolysis of MIN7. but also include a few sensors in the plasma membrane (CF-2, XA21). R proteins detect directly or indirectly isolate-specific pathogen effectors encoded by avirulence (Avr) genes such as AvrRpm1, AvrB, and AvrRpt2. Like PRR-triggered immune responses, R protein-conditioned immunity is linked to accumulation of reactive oxygen species and activation of defense genes but differs both quantitatively and kinetically from the former and typically leads to programmed host cell death at sites of attempted invasion. This " hypersensi-tive response " is thought to limit the spread of infection. Because PRR-and R protein-triggered output responses are similar, it is likely that their signaling pathways converge. A central nucleotide-binding (NB) …