Plants need to keep their air passages clear and healthy, just as animals do. Plants “breathe” by taking in air through holes (called stomata) on the surface of their leaves. These pores are often surrounded by raised ridges. Stomata are necessary for a plant to survive, but they are also a point of entry for potential pathogens—microorganisms that can cause disease. Fungi like powdery mildew and rusts use these holes to enter and infect leaves.
After landing on a leaf, fungal spores germinate and put out thin thread-like tubes (called hyphae). These tubes explore the leaf’s surface looking for stomata. When the hyphae find a pore, they enter and fill it. From there they grow into the plant tissue, digesting and consuming it.
How do fungi find these pores? It turns out they respond to the texture of the leaf surface. Rusts recognize ridges on the plant surface and grow at right angles to them, increasing their chance of locating stomata, while powdery mildew recognizes the shape of special cells around the stomata and uses them as a cue.
Mulberry trees live in wet climates that can put plants at greater risk of infection by fungi. Some species of mulberry, however, have evolved unique texturing on their leaves. These surface patterns confuse fungal hyphae, masking the regular features that otherwise signal the presence of stomata.
Fungal hyphae cannot penetrate directly through the leaf surface that they land on and must enter through stomata. And hyphae can’t revert to spore form once they germinate. By masking their stomata, mulberry leaves cause fungal spores to exhaust their energy and die before they can find a way into the leaf for food.
The mulberry’s approach to protecting its stomata could inspire new ways of controlling fungal growth on other surfaces.Edit Summary
“Leaf surfaces of M. laevigata and M. serrata appeared very rough under SEM with prominent surface sculptures and abundant epidermal outgrowths (Table 1; Figures 2a,b). Stomata were mostly sunken or hidden amidst the cuticular elevations. Germination of conidia appeared normal and the germ tubes emerged towards the leaf surface and grew appressed to it. Penetration of hyphae occurred only rarely and the hyphae generally passed over stomata without entering them (Figure 2c). The surface undulations also appeared to mislead the hyphae which produced penetration hyphae over local depressions outside stomatal complexes. Occasionally several stomatopodia were found to have developed in quick succession indicating the complexity of the tropic signals from the highly undulated epidermal surface (Figures 2d, e and f). The hypha frequently passed over stomata without producing stomatopodia over them, but produced stomatopodia immediately after them over some grooves.” (Babu et al. 2003:209)
Tropic failure of Phyllactinia corylea contributes to the mildew resistance of mulberry genotypesMycopathologia, 156: 207–213October 1, 2003
“The germ tubes of rust fungi possess a highly sophisticated capability for sensing host topographical signals and this plays an important role in achieving successful infection of a host plant. During the pre-penetration phase on the leaf surface, rusts respond in specific ways to different topographical stimuli” (Read et al. 1997:163)
“In this study, we found that 91±99% of germ tubes of the cereal rusts P. graminis tritici (wheat stem rust) and P. hordei (barley brown rust) dierentiate appressoria upon encountering host stomata. Using artificial microfabricated substrata we have shown, for the first time, that a reproducibly high percentage (83±86%) of germ tubes of these rusts can be induced to dierentiate appressoria in response to topographical signals alone. Germ tubes of these species dierentiated over closely spaced, multiple ridges and grooves, but not to a significant extent over single ridges.” (Read et al. 1997:164)