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. 2008 Aug;147(4):1575-83.
doi: 10.1104/pp.108.121566.

Secretory pathways in plant immune responses

Chian Kwon  1 , Pawel Bednarek, Paul Schulze-Lefert
Affiliations

Secretory pathways in plant immune responses

Chian Kwon et al. Plant Physiol. 2008 Aug.
No abstract available

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Figures

Figure 1.
Figure 1.
Dual function of secretory pathways in plant immune responses and plant development. Leaf cross section showing epidermal and mesophyll cell layers. Single plant cells target immune responses to contact sites of fungal or bacterial pathogens, leading to papilla formation (dark yellow semicircle) in the paramural space. The indicated flagellate bacteria (red) enter the leaf interior through stomata and remain in the apoplastic space for multiplication. The indicated fungal parasite (blue) attempts to penetrate the plant cell wall to access nutrients from an epidermal cell. Stomatal closure (denoted by short arrows) is one resistance mechanism against bacterial ingress (Melotto et al., 2006). Known vesicle-associated and SNARE protein-mediated exocytosis pathways drive focal (long straight arrows) as well as nondirectional (semicircles with arrowheads) secretion of antimicrobial cargo into the apoplastic space. A second function of these secretory pathways in plant development might involve constitutive nondirectional secretion of cell wall building blocks and cell wall modifying enzymes during cell growth.
Figure 2.
Figure 2.
Targeted vesicle-associated and SNARE protein-mediated secretion at microbial contact sites. The indicated vesicles are targeted to the plant plasma membrane beneath a bacterial (right) or fungal (left) contact site along polarized actin cables (not shown). Vesicles are thought to be loaded with cargo derived from the ER/Golgi protein secretory pathway. Vesicles contain constitutive molecules required for the maintenance of plasma membrane and cell wall functions (blue squares) and/or transport additional pathogen-inducible antimicrobial molecules (red squares), leading to the formation of a papillary cell wall scaffold containing toxic cocktails (dark yellow semicircle). At contact sites with powdery mildew fungi (left), vesicle cargo is discharged subsequent to ternary SNARE complex formation involving plasma membrane-resident PEN1 syntaxin (red line), SNAP33 (blue line), and endomembrane-resident VAMP721/722 (black line). At contact sites with bacterial pathogens (right), resistance responses require plasma membrane-resident SYP132 syntaxin (red line), and possibly SNAP33 (blue line) as well as yet unknown VAMPs (black line). During immune responses to bacteria, expansion of the protein-folding machinery including BIP2 and DAD1 is believed to reflect an increased cellular requirement to build up secreted PR proteins in the ER (green). One defense molecule secreted in a SYP132-dependent manner is the pathogen-inducible PR-1 protein. Fungi and bacteria can interfere with the host secretory pathway by inhibiting ARF-GEFs, which regulate vesicle formation, using effector molecules such as brefeldin A (BFA) or HopM1, respectively.
Figure 3.
Figure 3.
Pathogen-triggered and ABC transporter-driven efflux of small molecules into the apoplast. Plants secret a wide range of secondary metabolites in response to pathogen challenge. In tobacco, the PDR1 ABC transporter is required for the translocation of sclareolide, a toxic phytochemical (left). In Arabidopsis, the PEN3 ABC transporter is required for preinvasive resistance to a broad range of fungal parasites (right). Genetic and biochemical data suggest that peroxisome-associated PEN2 glycosyl hydrolase generates toxic products from glucosinolates that are translocated into the apoplast by PEN3. Due to the self-cytotoxicity of the secreted secondary metabolites, it is likely that their generation and release may occur in direct proximity to microbial contact sites.
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