If you specialize in soil-plant interactions or agronomy, you most likely know about these opportunistic, avirulent fungi that enhance plant health. Like mycorrhizae, Trichoderma sp. do more than just providing their host with nutrients. They offer numerous fitness enhancing services that must not be overlooked. In today’s post I will briefly describe the ecological services provided by Trichoderma sp. and how we are using this group of fungi to enhance our own fitness.
Trichoderma is a genus of asexually reproducing fungi that inhabit soils from tropical and temperate biomes all around the world. For quite some time, this globally distributed genus has been known to strengthen a plants disease resistance. With its massive distribution, it has evolved to compete with numerous soil organisms using a diverse array of mechanisms. When Trichoderma sp. make their way into a plant’s roots, they defend against fungal parasites trying to steal a free meal using such mechanisms.
Rhizoctonia solani is a globally distributed pathogenic fungus that infects various plant species all over. Upon its invasion, R. solani causes several different diseases including collar rot, root rot, damping off, and wire stem. Luckily for the plant, when Trichoderma sp. are present, the plant has a better chance of withstanding this parasite. First, Trichoderma sp. detect compounds in the soil that have a fungal origin. They follow the increasing concentrations, growing towards the plant pathogen. Even before Trichoderma sp. make direct contact with R. solani it releases powerful enzymes known as exochitinase that initiate fungal degradation. When it makes physical contact, Trichoderma sp. latch on and secrete even more fungitoxic cell-wall-degrading enzymes, eventually killing R. solani and the threats it brings.
Underneath the forest floor, Trichoderma sp. not only out compete for space against other root opportunists, but actively reduce possible root pathogens through means of chemical exudates. These belowground interactions are difficult to elucidate, since the invasive act of studying these interactions disturbs the natural conditions at play but even still, there is a wealth of knowledge pertaining to these root-soil relationships. The ecological services of Trichoderma sp. are not limited to these belowground environments, as they extend their fitness enhancing properties to the aboveground habitat as well.
Foliar pathogens are as much a threat to plants as root pathogens. When asexual spores from Trichoderma sp. called conidia land on the leaves of plants, they ultimately reduce the probability of foliar pathogen invasion. The spores germinate, and hyphae extend over the leaf’s surface. When foliar parasites disperse to leaves colonized by Trichoderma sp. they meet the same fate as the root pathogens beneath the forest floor.
These fungal mutualists have direct effects on the plants they coexist with, but it doesn’t end here. Trichoderma sp. also have indirect effects on their host plants. Recent studies have described a novel mechanism of pathogen resistance between the two partners. When Trichoderma sp. have invaded the root system of a compatible plant, the plant actually produces more of its own defensive compounds. For example, cucumber plants with these fungal mutualists produced increased levels of the defensive compound phenylalanine ammonia lyase. Now, the plant could simply be responding to the invading symbiote as a possible threat, or there could be more complex signaling taking place. Regardless, the plant becomes more protected against potential threats.
In addition to enhancing disease resistance in plants both directly and indirectly, Trichoderma sp. have actually been shown to enhance plant growth. This seems counterintuitive, as initiating the production of plant compounds implies that the plant is allocating resources to the synthesis of these different chemicals instead of growing its own tissues. But this is just another one of those weird re-occurring interactions we see again and again within the rhizosphere. The T-22 strain of Trichoderma has been shown to significantly enhance the growth of maize roots, and other economically valuable crops. With these more robust roots, the plants interact with the soil more, as its roots engage with more mycorrhizal fungi, which further extends the services those organisms provide.
Not only do plants receive more nutrients through their enhanced mycorrhizal interactions, but Trichoderma sp. themselves actually absorb available soil nutrients. They have also been shown to enzymatically solubilize soil resources like rock phosphate, iron, copper, magnesium and zinc, thereby making them available to the plant.
Mycorrhizae receive much praise for their ability to scavenge and mine for soil nutrients, increase soil-water relations, and enhance forest connectivity. Though, the services Trichoderma sp. provide must not be overlooked. Trichoderma sp. carry out numerous roles, enhancing plant fitness by increasing their disease resistance both directly and indirectly, increasing plant growth, expanding the interactions between plant roots and mycorrhizae, and collecting and sending scant soil nutrients to their host plants themselves. These remarkable fungi have positive interactions with plant tissues beneath the forest floor, as well their above ground leaf tissues. The field of agronomy has known about these services Trichoderma sp. provide and have been inoculating crops with the beneficial fungi for decades. Using these naturally occurring, opportunistic fungal symbiotes, we have reduced the amount of pesticides and fertilizers we pump into our crop lands. All in all, by utilizing Trichoderma sp. our farming mechanisms have become more sustainable, and have yielded more food-two key goals we must continue to achieve during this Anthropogenic time period.