Plasmodesmata: the battleground against intruders Jung-Youn Lee, Hua Lu  Trends in Plant Science  Volume 16, Issue 4, Pages 201-210 (April 2011) DOI: 10.1016/j.tplants.2011.01.004 Copyright © 2011 Elsevier Ltd Terms and Conditions

Figure 1 The role of plasmodesmata in the symplastic pathway in plants. (a) A schematic diagram of a plasmodesma illustrating the ultrastructure and cell-to-cell trafficking of diffusible signaling molecules [28]. Orange-yellow spheres and short rods represent hypothetical proteinaceous and filamentous components observed within plasmodesma. (b) Plasmodesmal-mediated signaling among symplastically connected cells. Some signals (red arrows) move only into cells adjacent to the original cell that generated them for local communication, whereas systemic signals (black arrows) move farther to reach phloem for long-distance communication. (c) Environmental signals (e.g. day length or light intensity) or challenges (e.g. biotic stresses caused by microbial pathogen infection) perceived in the leaves are processed in the receptive cells (dark-green or yellow patch, respectively) and transported through plasmodesmata for local communication within a tissue [6]. These signals can then enter phloem (broken red arrows) for inter-organ signaling and are transported to distantly located target cells and tissues, such as the shoot or root tips, to bring about appropriate biochemical, physiological, and/or developmental changes. Broken blue arrows indicate xylem transport. Trends in Plant Science 2011 16, 201-210DOI: (10.1016/j.tplants.2011.01.004) Copyright © 2011 Elsevier Ltd Terms and Conditions

Figure 2 Role of plasmodesmata in defense against microbial pathogens. Hypothetical models illustrating the potential role of plasmodesmata (PD) in interactions with: (a) viral, (b) fungal, and (c) bacterial pathogens. (a) Non-tubule-forming viruses encode MPs, which modulate the plasmodesmal SEL to allow transport of infectious materials, whereas tubule-forming viruses encode MPs that remodel the plasmodesma by forming self-assembled tubules, through which virions pass. Cells respond to viral infection by inducing callose deposition at the plasmodesma, which can deter viral spread. Some of the callose deposition could lead to plasmodesmal occlusion, which might help controlled cell death to occur by symplastic isolation of infected cells during the HR. (b) Hemibiotrophic fungal IH and effector molecules might need to move through plasmodesmata during cell-to-cell infection. One of the possible mechanisms underlying defense against fungal pathogens might also involve the regulation of plasmodesmal permeability by callose deposition. (c) Bacterial pathogens do not directly encounter plasmodesmata because their habitat is mostly limited to intercellular spaces within plants. However, to counteract plant defense responses, bacterial pathogens might use plasmodesmata to transport their specific effector molecules. To prevent this from happening, the plant might close the plasmodesma using callose or by other as yet unknown means. Trends in Plant Science 2011 16, 201-210DOI: (10.1016/j.tplants.2011.01.004) Copyright © 2011 Elsevier Ltd Terms and Conditions

Figure I Callose deposition at plasmodesmata. (a) A false-colored confocal image of aniline blue fluorescence shows plasmodesmata as punctate signals (arrowheads) at the epidermal cell junctions of Arabidopsis leaf tissue. Scale bar = 20μm. (b) Immunogold detection of callose (arrowheads) at a plasmodesma (PD). Scale bar = 200nm. Abbreviations: CW, cell wall; Cyt 1, cell 1 cytoplasm; Cyt 2, cell 2 cytoplasm; ML, middle lamella. Reproduced, with permission, from J-Y. Lee. Trends in Plant Science 2011 16, 201-210DOI: (10.1016/j.tplants.2011.01.004) Copyright © 2011 Elsevier Ltd Terms and Conditions

Figure II Transient callose accumulation during sieve plate pore formation. A simplified model depicting development of sieve plates [79]. (a) Plasmodesmata (PD) at the future pore sites. (b) A massive callose deposition occurs at the plasmodesmata, which is followed by (c) degeneration of appressed ER and widening of the plasmodesmal cytoplasmic space. Callose plugs are degraded, and open pores with residual callose are formed in the mature sieve plate (d). Trends in Plant Science 2011 16, 201-210DOI: (10.1016/j.tplants.2011.01.004) Copyright © 2011 Elsevier Ltd Terms and Conditions

Figure I A hypothetical model illustrating a potential role for plasmodesmata during defense against bacterial (e.g. Pseudomonas syringae) infection. Trends in Plant Science 2011 16, 201-210DOI: (10.1016/j.tplants.2011.01.004) Copyright © 2011 Elsevier Ltd Terms and Conditions