Importing Legumes to Australia

How nitrogen fixation improves soil...
15 August 2013

Interview with 

John Howieson, Murdoch University


Soybean root nodules, containing billions of Rhizobium bacteria


A major priority for western Australia is sustainable food production. Faced with some of the worst soils of the world and the changing climate, scientists here are investigating how to improve soil fertility by growing legumes that put nitrogen into the ground. Most native legumes here are laced with toxins, so animals including sheep can't eat them. But it's not just a simple case of bringing in dessert-adapted species from other countries...

John - I'm Professor John Howieson. I'm at the Centre for Rhizobium Studies at Murdoch University. The end game of our research is sustainable food production. We focus on food production from legumes which are a very special group of plants that take nitrogen out of the air and with the help of bacteria in the roots called rhizobium, break the closely-bonded atoms of nitrogen gas and turn them into protein. And so, to deliver sustainable agricultural production in infertile soils, the overwhelmingly best way is through legume cultivation, along with the right rhizobia, the bacteria in the roots, and that's what we do inside this glass. See if you'd like to come and have a look, Chris... 

Chris - Let's do it.

John - We're exposed here very heavily to a changing climate. One of the most important impacts is the rain is falling in different times of the year and in different amounts at those times, and it's actually affecting our legume growth. One of the things we're doing here is trying to select new legumes that are already adapted to those changing environmental circumstances. We've collected them from environments around the world that we think we are transitioning towards, Chris. We're collecting the rhizobia that goes with them and screening both of those components for adaptation to the western Australian environment. In this glasshouse, we do experiments where we match particular strains of rhizobium and give it to the plant which is the legume.

Chris - So, you can't just have any old rhizobial bacteria with any old plant then.

John - No. It's a very sophisticated genetic relationship between legume and rhizobium, so you have to get those two sets of genes perfectly together to fix this nitrogen into proteins in the roots.

Chris - So, if I take a plant from one country and move it to Australia, because the bacteria that it needs in the soil to grow in that environment are not in Australia, it's not going to grow. Is that what you're saying?

John - Well, that's exactly right, Chris. Although Australia has a high number of legumes, their rhizobia are completely incompatible with most of the food legumes that are found elsewhere in the world. So, when we bring legumes into Australia and try and domesticate them or try and grow them, we have to bring the rhizobia in as well. And then we're faced with the really big challenge of matching that rhizobia to the soil that the whole farming system is going to be put into.

Chris - Because the bacteria are fussy. If you put them in the soil, they don't necessarily like it.

John - Absolutely. most of the soils in this state are the worst soils in the world in terms of being infertile and able to hold water, and able to hold nutrients. It's a very difficult background to introduce new bacteria into. Most of the legumes that have been developed around the world for agriculture have been developed in more fertile soils. So, they struggle when they get to western Australia and so do their rhizobium.

Chris - So, are you then trying to find a combination of bugs that will go well with the plants and will go well with the soil? Is that the challenge?

John - That's exactly the challenge because when you start with the bugs, not all of them have the genetic machinery to fix nitrogen perfectly. So, we have to first screen the bugs for those that are capable of high levels of nitrogen fixation. Once we've got a cohort of those bugs, we then take them to the field and over a period of several years, we look at how well they can persist and colonise our soils, year-in, year-out.

Chris - So, talk us through these experiments because you have a whole raft of plants growing in front of us, some of which look healthier than others.

John - This is actually an experiment, trying to match a strain of rhizobium that will fix nitrogen optimally with these bean plants. We have a pot of very infertile soil we have added all the nutrients that we need for the legume growth except nitrogen, and we've added different strains of the rhizobia to select the ones that are going to fix nitrogen best. We move over here to the right, many of these pots, whole beans that are rather short. They're yellow. Their leaves are dropping off. They're going brown. That's a symptom of a poor rhizobium in symbiosis with this particular bean. On your left here Chris, you can see some really dark green beans. They're healthy. There's not a spot on them. They've got a very good strain of rhizobium. Let me show you what these look like - pull one of these plants out of the pot. You can see these big round blobs on the roots, about the size of a thumbnail. That's a nodule. Now, I'm going to scrape that nodule open with my fingernail, and you'll see inside that nodule, a beautiful red piece of plant tissue that actually looks like a nice steak. It's that red.

Chris - It does.

John - Well, that red is leg haemoglobin. It's the same sort of protein that we have in our blood called haemoglobin and it carries oxygen. The enzyme that the bacteria uses to break that tightly-bonded nitrogen molecule is sensitive to oxygen.

Chris - So, the plant soaks up all the available oxygen using that leg haemoglobin.

John - Yes. This bright red leg haemoglobin soaks up the oxygen out of the air and trickles it back into the nodule where it's used by the rhizobium for respiration, but it's in not in great enough quantity to denature the nitrogenised enzyme.

Chris - And if I were to zoom in on this nodule with a microscope, where would the bacteria be?

John - Okay, so the bacteria are located in special pockets called symbiosomes that the nodule cell forms specifically for the rhizobium. So, once you have this genetic conversation between rhizobium and plant working well, the symbiosome will develop and form a beautiful little house for the rhizobium that then becomes a nitrogen production factory.

Ron - Just mind yourself on this fence. It's to keep the bandicoots out and the rabbits out of the plot area. My name is Ron Yates and I'm from the Department of Agriculture and Food in Western Australia. We're at the back of Murdoch University. This is our little trial plot and this is where we test some of our legumes that we brought from overseas.

Chris - What are we looking at?

Ron - What we're looking here is Lebeckia Ambigua. It's a perennial legume from the Western Cape of South Africa. It's very interesting to us because it comes from a dry, arid environment and we think it has potential to grow in Western Australia.  

Chris - Very thin spindly stems, very low leaf area, looks like a thing I would describe as a succulent with nice yellow flowers on the top in long thin spears.

Ron - Yes. Just the habit of the plant, it looks very crowd-adapted, thin leaves, so less transpiration and less water loss from the plant. Most of the growing points under the ground which makes it very adaptable to grazing pressure - mainly sheep in Western Australia.

Chris - Were I to put a fork into the ground and I'm not going to do that, don't worry. And dig this up.  What would I see?

Ron - What you'd see is this big tap root, at least 3 meters.

Chris - Three meters! It's not a very big plant.

Ron - No, but most of its energy in the first 2 years goes into its root system.

Chris - This is I suppose a metre across. How much nitrogen can that fix? 

Ron - One ton of effectively nodulated legume is equivalent to 65 kilograms of urea or in another way is, 20 units of nitrogen.

Chris - To put that another way, to make fertilisers artificially, it has a very high carbon cost. We have to use fossils fuels, don't we, so there must be a carbon-saving of doing this.

Ron - Yes, the artificial way of making nitrogen, 1 ton of fossil fuel makes 1 ton of urea. That's least a lot of emissions of particular CO2.

Chris - You're doing field trials with this now. When are we going to see this going out there into the wheat belt and enriching land for farmers so they're not spending they're not spending their hard-earned cash on boosting the carbon dioxide level of the atmosphere.

Ron - It all depends. We'd probably have to get a company onboard to produce the seed and then there'd be another company that produces the inoculant, and then the next thing is cost, and the farmers would have to say that it is cost-effective to grow this plant.

Chris - What about the environmental impact because no country knows the cost of bringing in things from overseas better than Australia? This is a sort of double whammy because it's a non-native species and you're also rearing bacteria which are not going to be natural bacteria that would naturally be in the soil. They're bacteria that are right for this plant, aren't they?

Ron - Yes. There's lots of hurdles in getting plants and their bacteria into Australia. Luckily, we've gone through these hurdles for this plant. The big problem that we do worry about is it, coming a pest or a weed. This plant has ticked the box that we believe and they believe that it's not going to become a problem.

Chris - Ron Yates and before him, Murdoch University's John Howieson.


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