The connectivity between plants and their environment is vast and it is those relationships that are essential for the proper growth and development of many leafy organisms. This is particularly so for large forests of trees that have special interactions with the soil microbiome in order to grow their roots and take up the copious amounts of nutrients they require to sustain their tall physiques. Both bacteria and fungi can play a mutualistic role, but it is fungi that we will be looking at today and how genetic engineering can allow us to co-opt this relationship into the unassuming switchgrass.
Sitting In A Tree, F-U-N-G-I
We should first discuss and simplify the complex relationship that these ectomycorrhizal (ECM) soil fungi have with their tree partners. The fungi themselves will grow into the tree root cells and form a net of fungal structures that go throughout the root cells and also form a sheathe around the exterior of the root tip itself. This then allows the fungi to assist in the collection and transport of water into the roots while being supplied with the sugars the tree produces through photosynthesis. Understanding how these interactions form on the biochemical and genetic level is key to both determining how they benefit plant growth beyond nutrient collection and how we might extend this relationship to other plants that aren’t normally included.
A member of the ECM that has received extended focus has been Laccaria bicolor due to its more blatant impacts on the growth and development of its mutualistic trees and due to having a fully sequenced genome for analysis. It is known that L. bicolor is able to help with directional root growth and the uptake of specific molecular needs, such as phosphorus. This research was primarily done in its connection to the poplar tree. Which has led to a deeper investigation of how these signaling methods between the fungi and the poplar root cells are conducted and what genetic regulatory pathways are used to facilitate the interaction.
When comparing genetic maps of different poplar trees and mutations in their genome, it was discovered that a particular gene called PtLecRLK1 would see a reduction in root colonization by the fungi if the gene was knocked out. This gene codes for a type of plant protein known for being involved in the complicated immune response system of plants. The homologous version of the gene in the model organism plant Arabidopsis was shown to be activated in a variety of conditions, such as when the plant was injured or under different kinds of stresses. Which brings us to the research at hand.
Carbon Capture and Biofuels
Scientists at the Oak Ridge National Laboratory have been looking into how to improve the usage of bioenergy crops, plants grown to increase carbon intake from the air and deposition of the carbon into the soil alongside the usage of the plants themselves as a biofuel source to reduce reliance on fossil fuels. In order to not take up room on rich soiled lands used for agriculture, bioenergy crops are usually planted in more marginal soils with less nutrients and a poor consistency. But because of this, the amount of water and nitrate fertilizer required for them to grow well is increased. Poplar as a large woody plant is included in this category of crops, along with switchgrass.
The desire is to be able to develop bioenergy crops that are able to sustain themselves on fewer inputs from farmers, while still providing the benefits of carbon capture and biofuel production. Traits for water retention and greater utilization of the soil microbiome are key, which is why the poplar and L. bicolor relationship is so prized. Switchgrass itself has its own connected fungi, referred to as arbuscular mycorrhizal fungi (AMF) and these also assist with nitrogen uptake and the ability to grow in heavy metal contaminated soils, which allows switchgrass to also be used to clean up such soils over time through phytoremediation (bioremediation) processes. So, as can be seen, improving the growth and development of switchgrass further would be highly beneficial.
Prior studies published by the researchers that has helped to identify the PtLecRLK1 gene as being responsible for the fungal mutualism also showed that they could induce the connection in Arabidopsis through transgenic gene insertion. They wished to do the same experiment with switchgrass and used Agrobacterium and a gene cassette as the transfer method and they observed that expression of the gene also resulted in activating regulatory systems that increased metabolite production and interaction genes related to fungal colonization, while downregulating genes connected to the immune system response including jasmonic acid and ethylene.
To ensure no negative effects, they tested and confirmed that this period of immune system downregulation to allow the L. bicolor to colonize the switchgrass roots did not have any impact on the plant’s ability to defend itself against other pathogens. They also wanted to make sure that this dual fungal relation with AMF and ECM fungi would not compromise either, but determined that future experiments would have to look into those effects in the long term.
Overall, they were able to successfully create a new symbiotic relationship in switchgrass with the fungal colonies of L. bicolor that are normally found to interact with woody trees like poplar. The full benefits this may have on the growth of switchgrass are unknown, but are already being tested. We can certainly hope that this will result in a new form of switchgrass that is even more suitable for use in marginal soils to improve CO2 capture from the air and produce larger amounts of biofuels for many uses.
Photo CCs: Panicum virgatum Shenandoah 6zz from Wikimedia Commons