Journal of Theoretical Biology 240 (2006) 1–8 Biomechanical model for appressorial design in Magnaporthe grisea Anthony Tongen 1 , Alain Goriely, Michael Tabor à Program in Applied Mathematics and Department of Mathematics, University of Arizona, Building #89, Tucson, AZ 85721, USA Received 16 December 2004; received in revised form 5 July 2005; accepted 18 August 2005 Available online 3 October 2005 Abstract The fungus Magnaporthe grisea, commonly referred to as the rice blast fungus, is responsible for destroying from 10% to 30% of the world’s rice crop each year. The fungus attaches to the rice leaf and forms a dome-shaped structure, the appressorium, in which enormous pressures are generated that are used to blast a penetration peg through the rice cell walls and infect the plant. We develop a model of the appressorial design in terms of a bioelastic shell that can explain the shape of the appressorium, and its ability to maintain that shape under the enormous increases in turgor pressure that can occur during the penetration phase. r 2005 Elsevier Ltd. All rights reserved. Keywords: Rice blast fungus; Bioelastic shell; Nonlinear elasticity 1. Introduction Rice is one of the most important commodities world- wide for both food and barter, and the fact that each year the fungus Magnaporthe grisea causes losses of between 10% and 30% of the approximately 520 million metric tons of rice harvested per year (Talbot, 2003), makes this lethal microorganism an important topic of study. The com- monly used term rice blast fungus is highly appropriate given that it deploys enormous turgor pressures to operate a violent ‘‘breaking and entering’’ mechanism to infect its host. There have been many studies of the fungus in the mycological literature that have raised interesting questions concerning the biomechanical aspects of its operation (Bastmeyer et al., 2002), and it is some of these questions that we attempt to address here. A detailed description of all the biological processes involved is, of course, beyond the scope of this paper, but it is appropriate to give a summary of some the key steps relevant to our study. A simple cartoon of the overall process is given in Fig. 1. 1. A conidium lands on the rice leaf surface and attaches to it using material known as spore tip mucilage. Once attached, it is very hard to dislodge the conidium. 2. A germ tube grows out of the conidium, hooks into the surface of the rice leaf, and begins to form an appres- sorium. Experimental evidence (Money and Howard, 1996) indicates that turgor pressures of the order 3–5 atmospheres develop in the appressorium. The mechan- isms by which the appressorium is initially built and the interplay with the initial turgor pressure are not well understood. 3. The germ tube is eventually shut off from the appressor- ium leaving the latter as a separate, independent, unit. 4. The appressorium adheres to the plant surface by means of a highly potent adhesive in a ring around its base. The inner region of contact between the appressorium and leaf surface, termed the appressorial pore, appears to lack any obvious appressorial cell wall structure. 5. A melanin layer develops within the appressorium (but not over the appressorial pore). The ‘‘melanization’’ process is signaled by a darkening of the appressorial wall. The melanin layer allows only water molecules to permeate the wall and, on interaction with an internal ARTICLE IN PRESS www.elsevier.com/locate/yjtbi 0022-5193/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jtbi.2005.08.014 à Corresponding author. E-mail addresses: tongen@math.jmu.edu (A. Tongen), goriely@math.arizona.edu (A. Goriely), tabor@math.arizona.edu (M. Tabor). URL: http://math.arizona.edu/goriely. 1 Current address: Department of Mathematics and Statistics, James Madison University.