Penetration of hard substrates by a fungus employing enormous turgor pressures.

Abstract

Many fungal pathogens penetrate plant leaves from a specialized cell called an appressorium. The rice blast pathogen Magnaporthe gnsea can also penetrate synthetic surfaces such as poly (vinyl chloride). Previous experiments have suggested that penetration requires an elevated appressorial turgor pressure. In the present report we have used nonbiodegradable Mylar membranes, exhibiting a range of surface hardness, to test the proposition that penetration is driven by turgor. Reducing appressorial turgor by osmotic stress inhibited penetration of these membranes. The size of the turgor deficit required to inhibit penetration was a function of the surface hardness. Penetration of the hardest membranes was inhibited by small decreases in appressorial turgor, while penetration of the softer membranes was sensitive only to large decreases in turgor. Similarly, penetration of the host surface was inhibited in a manner comparable to penetration of the hardest Mylar membranes. Indirect measurements of turgor, obtained through osmotically induced collapse of appressoria, indicated that the infection apparatus can generate turgor pressures in excess of 8.0 MPa (80 bars). We conclude that penetration of synthetic membranes, and host epidermal cells, is accomplished by application of the physical force derived from appressorial turgor.The mechanism of host surface penetration by plant pathogenic fungi has been debated for nearly a century (1-6). The potential role of extracellular enzymes, to facilitate perforation of the host cuticle or cell wall during fungal invasion, is poorly understood (with one exception) due to the complex and ill-defined chemical nature of plant surfaces (7). On the other hand, an essential role for mechanical force during host surface penetration has been proposed for the rice blast fungus Magnaporthe grisea (Hebert) Barr (8). This pathogen produces unicellular infection structures, called appressoria, which adhere tightly to the host surface and produce slender infection pegs that pierce the underlying cell wall. The cell walls of appressoria contain a dense layer of pentaketidederived melanin whose presence is correlated with a build-up of appressorial turgor pressure (8) and is essential for penetration (8,9). In this study, we have inhibited penetration by exposing appressoria to solutions of high osmotic pressure. This approach was used to reduce the hydrostatic pressure (or turgor) within the infection apparatus and to estimate the magnitude of the turgor involved in penetration. Our results offer unequivocal evidence for an extraordinary mechanical component of the mechanism by which appressoria penetrate hard surfaces, but do not exclude a role in host penetration for some other factor such as extracellular enzymes. MATERIALS AND METHODSOrganism and Growth Conditions. These studies were conducted with strain 042 (see ref. 8) of M. grisea (Hebert) Barr, telomorph of Pyricularia grisea Sacc. (10). The time course of infection-structure development in vitro has been well documented ...

The cAMP-dependent protein kinase catalytic subunit is required for appressorium formation and pathogenesis by the rice blast pathogen Magnaporthe grisea.

“…into the underlying tissues (Howard et al, 1991). Artificial surfaces with properties similar to the rice leaf surface are also conducive to appressorium formation (Lee and Dean, 1994).…”

Regulatory network of genes associated with stimuli sensing, signal transduction and physiological transformation of appressorium inMagnaporthe oryzae

“…Surface hydrophobicity and nutrient starvation are prerequisites for inducing the autophagic machinery in the conidium through which stored nutrients like glycogen and lipids are metabolised to empower the morphogenetic transformation during the appressorium development (Kershaw and Talbot 2009; Liu et al 2016). The matured appressorium accumulates highly concentrated glycerol of ≈3.2 M, resulting in the generation of 8 MPa turgor pressure that creates physical force needed to breach the plant cuticle (Howard et al 1991; De Jong et al 1997). After gaining entry into host cells, M. oryzae invades in a hemibiotrophic lifestyle.…”

A nuclear contortionist: the mitotic migration ofMagnaporthe oryzaenuclei during plant infection

“…It remains unknown if extracellular cues trigger mitosis during appressorium development. Accumulation of turgor pressure in the appressorium in coordination with septin-dependent cytoskeletal rearrangements at the appressorial pore leads to formation of the penetration peg (Howard et al 1991; Dagdas et al 2012). The penetration peg is a specialised hypha that physically breaches the leaf cuticle, allowing the fungus to enter rice cells approximately 24 h post-inoculation (Kankanala et al 2007).…”