The universe of fungi that inhabit plants
STRI/DICYT When he first arrived at the research station on Barro Colorado Island (BCI) as a postdoctoral student, Camilo Zalamea never imagined that he would work in Panama for eight years. The Colombian botanist loves to observe things calmly and, as plants do not move around, they allow him to do exactly that. Now, in addition, he has fallen in love with microorganisms. Without them, he confesses, we cannot fully understand how forests work.
For one of his latest projects as a post-doctoral fellow, before starting his next stage as an assistant professor at the University of South Florida, Camilo integrates his two passions. In mid-2019, he can be found walking the island’s trails in search of seed bags that he buried a year ago. In each bag he placed 35 seeds of one of the four species he is studying: three of the genus Cecropia and one of the genus Jacaranda. Half are buried just below their parental tree. The others, 30 meters away.
Camilo chose these four species because they are ‘pioneers’: they need light to germinate and they grow very fast. In addition, their seeds can spend years in the ground without dying, waiting for just the right conditions to germinate.
“That’s a long time for these tiny seeds, in a place where it rains so much, where there are so many pathogens and so many potential sources of mortality,” Camilo explains.
Pioneer species are also ideal for restoring deforested areas, because in a short time they develop and generate the necessary shade for other species to establish and grow.
Through previous studies on BCI, Camilo and his collaborators found that the seeds of pioneer species use different methods to survive. Some invest their energy in physical barriers, while others produce compounds that are unpleasant for their predators. Finally, there are those that do not seem to defend themselves in any way, such as the ones he is digging up after twelve months.
“Our hypothesis is that they have positive associations with fungi, which somehow help them survive,” he says. “We know that the fungi that colonize these seeds vary according to the seed species and, with this project, I want to better understand those interactions between the seeds and their fungi.”
But not only is there variation between the fungal species that colonize the different seeds, but the effect that a single fungus has on one seed species may be different for another species. A fungus that kills one seed may protect another, or simply have no effect.
After retrieving the bags from under the parental tree, Camilo looks for bags buried further away. Trees and seeds share pathogens, which are usually around the adult tree. The farther the seeds are dispersed from the parental, the more likely they are to survive and grow.
Back in the laboratory, he cuts each seed in half, checks to see if it is alive or dead and looks at the fungus species associated with it. He mainly uses two different methods for this: traditional culture and high-end sequencing. By DNA sequencing he can see the wide diversity of fungi inhabiting a single seed. Whereas only some of them grow when cultured on a petri plate.
“The good thing about having cultures is that we can later use them to do experiments,” Camilo adds. “For example, if we find that a fungus species is usually associated with dead seeds, we can begin to infer that it is pathogenic for the species from which it is being isolated. Then, we can put that fungus on other seed species to see its effect.”
Although it may seem very specific, Camilo’s research at BCI has broad and important implications. In agriculture, it could serve as a guide to prevent pathogenic infections in the seeds of food crops or help to preserve seeds that are stored for long periods before sowing.
On the other hand, understanding the basic biology of pioneer species and their pathogens may help to invent more effective ways to restore tropical forests in areas where they have been destroyed.