James White from Rutgers University on Plant Biology and Soil Health

GN: Hello, everyone. It’s Steve Melito from Go Natural Education here. Today,  we’re talking to Professor James White of the Department of Plant Biology at Rutgers University. He is the author of more than 180 articles, the author and editor of numerous reference works, and an expert in endophytes: microbes that inhabit the internal tissues of plants without causing disease. The conversation that we’ll be having is about plants but it’s also about soil, the subject of a recent series of blogs on our website at  gonaturaleducation.info and we invite you to check them out.

James White welcome to the podcast.

JW: Thank you, Steve. I’m happy to be here.

GN: James, some of our folks who are listening are students and some of these students are evaluating different careers or are just interested generally in the natural environment. So, they’re wondering – as am I – what does a plant biologist do? Would you please explain?

JW: Yes. Plant biologists – also called plant scientists, by the way – study all aspects of plants. They’ll study the physiology of plants and how plants work internally. Plant biologists will also study things like photosynthesis or, in my case, we study endophytes. These are microbes that go into plants. So, really anything that has to do with the biology of a plant is what plant biologists study.  

GN: Excellent. At the top of the podcast, the definition that I gave of endophytes was very basic. Would you please explain what they are and how they relate not just to plants but to the soil?

JW: Yes. All plants have endophytes. The term “endo” means internal and “phyte” means plant. These are basically microbes that plants take into their tissues, and many of these endophytes start out as soil microbes. They are absorbed into the plant through the roots typically. The plant uses endophytes to get nutrients and to become more stress-tolerant and hearty. Because they are soil microbes, endophytes will often carry nutrients from the soil into the plant. Then the plant absorbs these microbes from the soil.

GN: Got it. Many folks who’ve spent some time in the garden are familiar with the back of a fertilizer package and nitrogen and potassium, but plants need a lot of different nutrients. What are some of the things that they need?  

JW: They need macronutrients, which are the nutrients in the N-P-K that you see on your fertilizer package, and calcium, and some other very important nutrients that plants need in large quantities. There are also micronutrients, and these are things like molybdenum and iron, manganese, and zinc – the metals. Plants need to have the macronutrients to build much of what they’re made of, but these micronutrients are needed in smaller quantities. Plants need them to run their enzymes because the enzymes have little metallic components called cofactors. So, plants need to get those metals and those macronutrients to make their enzymes work.

GN: Do the macronutrients and micronutrients refer to the size and, if so, how small are these particles?

JW: It’s not the size that the macro and the micro refers to. Rather, it’s the amount. Macronutrients are needed in higher quantities than micronutrients, which are needed in just tiny quantities. Still, just because it’s a tiny quantity doesn’t mean it’s not important. In fact, you can’t do photosynthesis if you don’t have magnesium and iron, and some other micronutrients. Oftentimes, these micronutrients are hard to get. So, plants will use endophytes to get these nutrients. They also use them to get macronutrients, but micronutrients are especially important for endophytes.

GN: Plants seem to have a symbiotic relationship with endophytes. Do they give endophytes anything back? Is there an exchange of any type?

JW: They do, actually. Plants cultivate the endophytes and give them some carbon from photosynthesis. Then they’ll frequently take the nutrients from the endophytes in smaller quantities. So, it’s an exchange. What do the microbes get? Well, the microbes get replicated. They get a home. They deliver nutrients and, in some cases, they’re degraded by the plant.

I haven’t yet mentioned this and, Steve, I don’t know if you were planning to get to it, but there’s another process that involves endophytes called rhizophagy: the rhizophagy cycle. The term “rhizo” means root and “phagy” refers to eating. Basically, it’s the root’s eating cycle. What this involves is where roots that go into the soil will secrete what we call exudates. They’re sugars and organic acids, and there may also be some amino acids and some other nutrients that plants secrete at their root tips into the soil. These are attractants for microbes, and the microbes can detect the nutrients that the roots are secreting.

Then the microbes swim to the root tip or otherwise get to the root tip because that’s a source of food. So, they get to that root tip and when they’re there, the plant will absorb the microbes into the root cells at the tips. They’ll take them into their cells and will secrete or expose the microbes to superoxide. This is a highly potent form of reactive oxygen. Superoxide is an especially powerful oxidant and it will oxidize the cell walls off of the bacteria they get absorbed into the root cell, take their cell walls off, and make them into protoplasts. The reactive oxygen effectively extracts nutrients from these microbes and then the microbes are replicated in the root cells and especially in the root hairs.

Then they are ejected. They stimulate the growth of the root hairs and as those root hairs grow, they’re ejected from the tips of the root hairs. Every time there is a little growth spurt in the root hair, some bacteria are ejected out of it. So, you have a continual growth spurt as those root hairs elongate and with each growth spurt, you get more bacteria injected back into the soil.

From the tips of those root hairs, the bacteria will then reform their cell walls once they’re ejected out of the tip of the root hair. Then they go swimming back out into the soil to get more nutrients to be later on attracted back to the root tip. That’s the cycle part of the rhizophagy cycle. Plants just keep doing this: cycling these microbes go back to the soil and then attracting them back inside the plant as an endophyte. Nutrients are extracted and they are ejected back out of the root hairs. It’s a continual cycle, kind of like a little machine, farming these microbes in the soil to get nutrients from them. It’s a fantastic process.

GN: It is fantastic. Are there things that can interrupt this cycle in a bad way? For example, are there human activities that can interrupt this cycle and, if so, what are some of the consequences?

JW: If we, for example, sterilize soils or damage soils so that we don’t have good microbiology in the soils. If we’re doing deep turning of soils, if we’re using sterilization. Sometimes people sterilize soil to kill nematodes or some other kind of soil pathogens. Well, that has a non-target effect where it hurts all of the microbes. And then you’ve messed up your soil, your soil health, and the soil biology of your plants since they can’t get the microbes from the soil and they can’t use these microbes to get nutrients back. They’re just not there.  

Some of the microbes go in seeds.  So if you sterilize seeds, you’re killing the important microbes that the mother plant, the parental plant, actually puts on seeds. These are microbes that the seedling seeds need in order to grow. So, if you sterilize those seeds, and if you killed the microbes, the seedlings won’t grow well initially. They’ll also be more susceptible to disease and some other issues without their microbes. They’ll be less stress-hardy, and so forth.

If you remove all the microbes, what you do is addict plants to chemical fertilizer because, otherwise, you can’t grow them if you have no microbes there. They really need those microbes. Plants in nature don’t need our fertilizers. They are perfectly well on their own with their microbes.

GN: Wow. So what about manure? Because that’s something we have written about and covered at Go Natural Education quite a bit. Is it a good source of nutrients for plants, especially compared to the chemical options that are available?

JW: Manure really is a good source of nutrients. You do have to be a little bit careful in that if it’s green manure, it’s too microbial-rich. When you overwhelm a plant, that slows it down. That plant can take in so many microbes but when you have an overabundance of microbes from green manure, for example, it could be an issue. It’ll slow your plant. It doesn’t hurt your plant, but it slows down growth for a little while as the plant becomes adapted to it.

Under normal circumstances, if you treat that manure and age it a little bit before you put it on, then you’re perfectly fine and the plants love it. It has plenty of microbes for the plant. There’s no overwhelming and plants grow much better.

GN: Excellent. One last question for you about soil and endophytes. I read recently – although the research wasn’t recent – that endophytes could be used in bioremediation to remove contaminants from soil. Is that accurate? Is there anything new to report on that front?

JW: Yes, there is. The reason that endophytes can be used for bioremediation is because of the increased hardiness of plants. Under some circumstances, plants don’t have as many endophytes. Let’s say you’re typically using a cultivated plant where endophytes have been lost. The plants are not hardy. So, they won’t grow in certain contaminated soils – heavy lead, for example.  When an endophyte is present, plants become tolerant to stress. They become oxidatively stressed tolerant and that enables them to grow in those soils. Then, because they’re able to grow in those contaminated soils, they will absorb more of the contaminants into the tissues and then they can be removed by cutting the plant material and removing it to different site. Essentially, this cleans out the soil.

The other thing that’s happening is, at least with some contaminants, they will break down with reactive oxygen. Because the endophytes in plants and plant cells have an oxidative interaction, the plants are oxidizing the microbes then, and what happens is that reactive oxygen will also break down certain kinds of contaminants so they will reduce the content of the contaminants themselves.

So, there are two mechanisms really.  The main one of the increased hardiness of plants, which allows them to grow in the contaminated site and absorb those toxins.

GN: Fascinating stuff. James White, thank you so much for being with us on the podcast.