The Good Endophyte

John Kempf and Dr. James White discuss how plants receive much of their nutrient requirement via endophytes — from microbes inside the plants

John Kempf: James, you’ve been a pioneer in the development of the rhizophagy cycle — the concept of plants feeding and getting nutrition from microbes. This is one of the fundamental pieces that we need to understand; we need to develop a completely different paradigm for how plants get nutrients. 

The contemporary model has been based on the idea that plants get their nutrition primarily from soluble ions in the soil solution. Yet we know that model has all kinds of problems because it’s obviously not how wild plants and undomesticated ecosystems get nutrition, because soluble nutrients don’t exist in large concentrations in unmanaged soils. 

I’ve been really interested in understanding this phenomena and in seeing how we can develop agronomic management systems that can give us exceptional levels of plant health and productivity without degrading our soil ecosystems with salt-based fertilizers. You’ve described how organisms such as Fusarium, which we consider to be pathogenic, can actually develop symbiotic or collaborative relationships with plants that are not pathogenic, dependent on the soil environment. A lot of people are starting to pay attention, and I’m really excited about that.

Dr. James White. Thank you, John. Just to add to what you said about Fusarium and how microbes impact the behavior of pathogenic fungi, all fungi take in endophytic bacteria. If you look inside their hyphae, you’ll see bacteria in there. They utilize those endophytes for nutrients — nitrogen mostly — that’s what we’re finding.

Of course, the big picture is that if you consider a pathogen, the behavior of that pathogen changes when it has bacteria inside it. We think that’s because of the need for nutrients — for nitrogen in particular — and that if a pathogen can get its nutrients from endophytic bacteria, it doesn’t have to attack plants to get those nutrients. It’s the principle that you can use microbes to protect plants and to build soils that are healthy; microbes don’t cause disease in those kinds of soils. 

Kempf. I’ve thought of disease-suppressive soils as soils that contain suppressive bacteria — suppressive microbial populations that suppress pathogens from expressing themselves. But what you’re describing is actually that these are soils that have such abundant bacterial populations that they can supply the nutrients that the fungus requires, and that alters the nature of the relationship. So, in fact, a disease-suppressive soil is not so much about specific types of bacteria or specific strains as it is the ability to provide bacterial nitrogen to the fungus. 

White. Yes, that’s what we’re seeing. There’s not any kind of silver bullet — that you could take one microbe and give it to a plant, and that’s going to be everything the plant needs. Same with fungi. There’s no silver bullet. A suppressive soil is a soil that’s rich in microbes that work with fungi and plants.

Kempf. I learned recently that fungal mycelia is a single cell, and I’ve been wondering about this. I’ve marveled at the speed at which bacterial endophytes can move through fungal hyphae, and so it makes sense that it’s not cell-to-cell transport. And I’ve also learned that the DNA is identical in a single mycorrhizal fungi that is in some cases miles across. How does that translate to fungi being a single-celled organism or not?

White. Well, you have to think of the fungal structure. The main fungus that I’m going to talk about now are the ascomycetes. It’s a group of fungi that have septa or cross walls in the hyphae — the chain — the tubular growth of the fungus. It has cross walls, but the cross walls have holes in the middle. They have a pore in the middle. It’s just a wall that goes down and then goes back, and then there’s a hole right in the middle. 

There’s constant movement of nuclei through that hole. And even bacteria go through that hole, and they can move between the hyphae. It’s going to slow them down a little bit because of the septa that are there, but they still have free movement. There isn’t that hindrance of movement until you look at the mushrooms — the basidiomycetes — because they have plugs in their septa, and it’s more difficult to move bacteria between the cells, but they still do it, because those bacteria are important.

Kempf. So there are multiple cells, but the cell membranes in some of these groups are very open and facilitate free movement back and forth.

White. There are multiple cells, but there are holes in the walls between the cells. In a sense it’s a single cell, but that can be a little bit confusing because there are multiple genetically different nuclei in many of those hyphae. They have this process where they just move DNA from one hypha to the next, and so many times you’ll get a hyphal network that has multiple nuclei in it — multiple sets of DNA.