SCIENCE TO THE GROWER: Sync and Swim Pythium and Phytophthora don’t mind asking for directions

by Richard Evans

It’s summer! To celebrate, my wife and I took our grandkids to the public swimming pool, but the joy was quickly dampened (so to speak) by voices that started at one corner of the pool and traveled toward us in a wave. When the wave reached us, we learned that someone had discovered a turd floating in the water. We hopped out of the pool to escape the spreading cloud of microorganisms, and I took advantage of the down-time to ponder the aquatic pathogens that infest irrigation water and substrates in greenhouses and nurseries.

Most of the root diseases of plants growing in hydroponic systems or soilless substrates have been attributed to the genera Pythium and Phytophthora, which are fungus-like microorganisms that taxonomists have reclassified as oomycetes. In fact, they are more akin to aquatic organisms like brown algae than to fungi. Both Pythium and Phytophthora love aquatic environments, and they produce asexual single-celled zoospores that are outfitted with two appendages, called flagella, which they use to swim. One flagellum works like an oar to propel the zoospore through water. The other flagellum serves as a rudder for steering. These zoospores serve as the primary dispersal and infection agents, and they can swim nearly 3 feet per hour. They may not be as fast as Mark Spitz, but they can beat anything Mark spits.

Researchers have known for many years that Pythium and Phytophthora have a “homing response” — they can locate targets for infection and swim toward them. Early evidence for this was the discovery that they follow chemical trails, a process called chemotaxis. Some of the attractive chemicals are nonspecific ones, like amino acids and ethanol that may be released by injured roots. Others are compounds unique to a particular kind of plant, and their presence, even at exceedingly low concentrations, enables the pathogen to identify and infect a host species. Some chemicals may even warn the pathogens of trouble ahead, causing them to turn and swim away, just as you might slip out the back door when you detect your mother-in-law’s perfume in the foyer of your house.

Researchers have reported that plant infection by Pythium and Phytophthora usually doesn’t occur unless a threshold density — on the order of millions of zoospores per ounce of water — is present. It appears they need to summon the troops in order to invade a plant, but it turns out that isn’t necessarily true. Indeed, just like those humans who communicated the presence of something unpleasant in the pool, Pythium and Phytophthora can communicate with each other! Researchers at Virginia Tech (Kong and Hong 2010) found that Phytophthora zoospores (and perhaps other forms of Phytophthora) can communicate through a process called quorum sensing. The pathogen produces, and releases into the water, a chemical that can be detected by its neighbors, enabling them to count their numbers. Kong and Hong filtered a solution containing Phytophthora to isolate the zoospores from the chemicals they were releasing. They called this a zoospore-free fluid. Then they exposed plant tissue to a single zoospore. Infection rarely occurred in the absence of the zoospore-free fluid, but a single zoospore provided with the zoospore-free fluid almost always infected the plant. 

Even more remarkable is that different species of Phytophthora and Pythium are able to communicate with each other. Hong’s group at Virginia Tech found that zoospore-free fluids collected from several species of Phytophthora and one species of Pythium could stimulate plant infection by another Phytophthora species (Kongand others 2010). The researchers suggested that cooperative behavior among oomycetes gives them a competitive advantage over other groups of pathogens, especially when food is in short supply. This signaling among oomycetes also may explain why individual species of Phytophthora or Pythium sometimes cannot be detected before serious disease outbreaks occur.

Communication among oomycetes may also help to account for a strange phenomenon called pattern swimming. Despite their microscopic size, Phytophthora and Pythium zoospores can swim in synchrony, forming patterns that are visible to the naked eye. Speaking of things visible to the naked eye, I wonder if it’s safe for me to hop back into the pool.

Richard Evans is UC Cooperative Extension Environmental Horticulturist, Department of Plant Sciences, UC Davis.

References

Kong P, Hong C. 2010. Zoospore density-dependent behaviors of Phytophthora nicotianae are autoregulated by extracellular products. Phytopathology 100:632–637.

Kong P, Tyler BM, Richardson PA, Lee BW, Zhou ZS, Hong C. 2010. Zoospore interspecific signaling promotes plant infection by Phytophthora. BMC Microbiology 10:313