La Trobe University researchers have discovered a genetic regulator that shuts down the process in which legume plants convert nitrogen from the atmosphere into nutrients. Farms & Farm Machinery speaks to the research leader to find out how this discovery could increase crop growth and yield, while reducing reliance on nitrogen fertilisers
Legume plants like beans, peas and lentils are unique compared to other crops, due to their special ability to interact with soil bacteria – known as rhizobia – and convert or ‘fix’ nitrogen into a usable form of nutrients.
Farmers can benefit from this process, as legumes fix nitrogen from the atmosphere and leave some behind in the soil for the next crop a farmer plans to grow – reducing the need for synthetic fertilisers.
However, when there is already “enough” nitrogen in the soil through natural processes or synthetic fertilisers, legumes will decrease their rate of nitrogen fixation.
This is about to change though, as a recent discovery by a team of international scientists, led by La Trobe University researchers, has identified that a genetic regulator inside legumes can be removed to enable the plants to continue with their special ability – regardless of soil nitrogen levels.
Identified for the first time, a genetic ‘off switch’ reduces fixation when nitrogen rates are already high in soil.
Research leader and La Trobe University lecturer in plant science Dugald Reid says that when the gene was removed, legumes fixed significantly more nitrogen than the unmodified plants – creating new opportunities for farmers.
“The higher the amount of nitrogen in the soil, the lower the amount of fixation that occurs in unmodified legumes,” he says.
“There is significantly more fixation when we remove the genetic regulator.”
Discovery
The team discovered the regulator – dubbed Fixation Under Nitrate (FUN) – after screening 150,000 individual legume plants in which the gene had been removed to identify how plants control the switch from nitrogen fixation to soil nitrogen uptake.
Initial research was done with a model legume species known as Lotus japonicus, and by increasing the nitrogen levels available to it, the team was able to identify the impaired nitrogen fixing regulation – uncovering the FUN regulator.
The gene was previously unknown, with Reid saying other research teams had tried to modify the legume’s nitrogen fixation by increasing the number of nodules on a plant – where the fixation occurs.
“In those cases, the modifications had a detrimental effect on the plant’s growth,” he says.
“The plants weren’t growing as well because they were spending too much energy on the process, but when you remove the FUN gene, the plant’s growth isn’t effected.”
Reid says the next step in the project is to try and remove the genetic regulator in other legume crops such as faba bean, soybean and cowpea.
“If we see benefits of removing the genetic regulator in one legume crop, then it’s quite likely that it’s going to translate into other legumes as well,” he says.
The team has since started to generate cowpea mutants and currently has plants growing in the lab where mutations can be detected.
“We should have a cowpea mutant in the next six months or so and then it’s about getting enough seed to take it into the field and trial it,” Reid says.
In terms of faba bean, Reid says the team has three seeds that were retrieved from a mutant population, and they are currently being tested to see if they perform the same way as the model species.
A mutation of soybean could also be on the horizon, but Reid says it might still take a couple of years to develop.
Growth and yield
With legumes fixing more nitrogen, Reid says the removal of the genetic ‘off switch’ could result in an increase in crop growth and yield.
“Once we test these other crops, we’ll assess the yield in different conditions in the field,” he says.
These tests will look at conditions where there are high levels of nitrogen in the soil as well as low levels of soil nitrate.
The team will then consider the difference in yield as well as in protein content, which Reid says will measure how much nitrogen in the plants comes from fixation, as opposed to nitrogen from the soil.
“If we can increase the nitrogen that the legumes are fixing by 10 per cent, then that’s 10 per cent more nitrogen that could get left behind, or it’s 10 per cent more nitrogen that can get allocated to yield,” he says.
“Both of these scenarios provide potential benefits for farmers, as they’ll either get an increased yield, which has direct economic benefits on the legume crop, or increased nitrogen, which means they can reduce the amount of nitrogen they need on their wheat or their canola the following year.”
Reducing fertiliser use
Since legumes can leave nitrogen behind in the soil once it has fixed it, farmers can reduce their reliance on nitrogen fertilisers, which can be detrimental to the environment.
By removing the genetic regulator and allowing legumes to fix more nitrogen, the use of fertilisers could be even further reduced.
There are several different costs associated with fertilisers, Reid says, including economic costs, environmental factors in production and the environmental factors when applying the fertiliser.
“Urea is an expensive fertiliser and the production of it involves natural gases that produce a lot of emissions – contributing to a high carbon footprint,” Reid says.
“Once you put it in the paddock, it ends up getting converted into nitrous oxides, which is a very potent greenhouse gas and makes up a large portion of Australia’s emissions.”
Reid hopes that his research will lead to less reliance on these nitrogen fertilisers – mitigating the costs associated with them.
“This helps lay the foundations for future research that provides new ways for us to manage our farming systems to reduce nitrogen fertiliser use, increase farm incomes and reduce the impact of nitrogen fertiliser use on the environment,” he says.
