Fungi feel their way to feast.

Fungi feel their way to feast

From a fungus's point of view, thesurface of a bean leaf is a complex terrain of ridges and valleys. Yet the bean rust fungus seems to know exactly where it is going in its quest to invade the leaf's stomata, or pores. Cornell University researchers now have shown that, unlike many other organisms that rely heavily on chemical signals to survive, the bean rust fungus uses very specific, tactile clues to navigate and infest bean leaves. This finding may give agricultural researchers a new weapon for battling plant fungal diseases, which annually cause hundreds of millions of dollars worth of damage to U.S. crops.

In the March 27 SCIENCE, Harvey C.Hoch and his colleagues report that a plant's step-like ridges of a specific height induce the fungus to form the stomatal infection structures from which it launches its attack into the leaf. They found that the fungus's hyphae, or threadlike cells that explore the leaf's surface, are most likely to balloon into the infection structure when they encounter a 0.5-micron-high ridge.

The researchers tested the response ofthe fungus by letting it wander over plastic surfaces that were molded from silicon wafers etched with ridges, ranging in height from 0.03 to 5 microns. According to Hoch, a plant pathologist at Cornell's New York State Agricultural Experiment Station in Geneva, N.Y., infection structures formed at 0.5-micron-tall ridges more than 70 percent of the time; no structures grew over any ridges that were shorter than 0.1 micron or taller than 1 micron. The spacing between ridges is also important both for directing the hypha's path along the surface and in determining whether an infection structure will be constructed.

Hoch's co-workers at Cornell in Ithaca,N.Y., include plant researcher Richard C. Staples and Brian Whitehead, Jerry Comeau and Edward D. Wolf, who made the submicron terrains.

The fungus's preference for 0.5-micron-highridges on the silicon terrains makes sense because, as Hoch's group later discovered with a scanning electron microscope, the "guard cells' of bean leaft stomata have lips or ridges that measure about 0.49 microns high. In previous work with chemically inert plastic replicas of bean leaves, Willard Wynn, a plant pathologist at the University of Georgia in Athens, had suggested that the fungal infection structures are triggered solely by the topography of these stomatal cells. But, writes Hoch's group, "the nature, size and location of these topographical signals have until now only been surmised.'

Now that they have identified the topographicaltrigger, researchers can use this knowledge to foil the fungus by developing bean strains that have either no stomatal lips or lips that are taller or shorter than 0.5 microns, says Hoch. "This would make the plant completely immune [to bean rust fungus],' says Wynn. "It's not just a matter of being resistant. It would be completely disease free. It's really quite important.'

Today, bean plants are designed toresist rust fungus by walling it off or by releasing toxins once the fungus has invaded the plant through the stomata. The problem with this approach, notes Hoch, is that "the fungus can [quickly] change genetically to overcome the chemical resistance of the plant. . . . But we think a physical feature is something that it may not adapt to so fast'.

Apart from its importance to agriculture,Wynn says, Hoch's work is fascinating from a biological point of view. "I don't think that in animals or bacteria or higher plants, there is any other recognition system quite the same [as the rust fungus's tactile response],' he says, although two other classes of plant fungi do seem to take similar physical clues from their environment.

Hoch and his colleagues are now studyingthe detailed mechanism that enables the rust fungus to translate a physical stimulus into a message to build an infection structure. Hoch says they've identified a half dozen proteins and genes that seem to be involved in the construction of infection structures. They plan to investigate the function of these genes by putting them into other kinds of fungi and seeing whether these fungi grow structures when they encounter physical stimuli.

Photo: Bean rust hyphae balloon into infectionbulbs when they encounter 0.5-micron-high ridges of an artificial terrain.