Logging Switchgrass onto the Wood Wide Web
A group of scientists have allowed switchgrass, an important plant for biofuel production, to tap into the nutrients and signals transmitted by underground fungal networks (sometimes known as the wood wide web). This research was led by Zhenzhen Qiao at the Oak Ridge National Laboratory in Oklahoma.
Ectomycorrhizal fungi are underground helpers
The term wood wide web refers to the network of fungal filaments, called hyphae, that are found in the soils of forests and fields. Some of these fungi can grow into and around plant roots, forming a close, mutually beneficial symbiotic relationship. The fungi act as a sort of extension to the plants’ root systems, helping them acquire water and nutrients such as nitrogen and phosphorus from the soil. In exchange, the plants can provide the fungi with carbon-based compounds from their photosynthetic processes, like sugars and fats. Many microscopic organisms contribute to this wood wide web, but in temperate climates like Europe and North America, the dominant organisms in the forest web are ectomycorrhizal fungi (EMF, for short)(1).
The wood wide web has ectomycorrhizal fungi (shown in blue, not to scale) connecting root systems of different trees. The inset image (adapted from Molina et al. 1993) shows a cross-section of a tree root where EMF have grown around and into the root, allowing nutrient exchange.
EMF have important benefits to plants as they help not only with nutrient acquisition but also with resource transfer and signalling between plants. Mycorrhizal fungi (from the Greek mykes, fungus, and rhiza, root) can supply their host plant with up to 20% of its nitrogen in grasslands, and up to 80% in forests. Furthermore, the fungal partners can provide approximately 75% of the host’s phosphorus(2). The fungi also help the plant cope with various environmental stressors, including drought and pests(3). Scientists have known since the 1980s that the fungi don’t just help individual plants; the fungal strands can connect with the roots of several different trees, forming networks that allow the plants to exchange nutrients and water amongst themselves(4). In a more well-known example, birch trees growing in full sun were able to transfer carbon-based nutrients through hyphae to fir trees growing in shade which were not able to photosynthesize as well(5). Evidence also shows that plants under attack by herbivores can use mycorrhizal networks to pass signals to other plants allowing them to prepare a defence(6).
Not an open-access network
As important and beneficial as EMF are, the number of species with which they form a successful symbiotic relationship is actually quite limited - only about 2% of plants can form these associations, primarily woody trees(7). In plants that are hosts, EMF grow around and into the tips of the roots, forming a web that surrounds the root cells, known as the Hartig net. This allows for a large contact surface between the two organisms and facilitates exchange of water and nutrients. Generally, most fungi trying to enter plants are harmful pathogens, and plants have defence mechanisms to prevent them from entering their tissues. In the case of beneficial fungi, these secrete specific molecules that the plant can sense and “know” not to activate its defences, so that the symbiosis can be established(8). Not all plants are capable of sensing the helpful fungal signals, and therefore not all plants can host EMF, though they may be able to form symbioses with other types of fungi that contribute to the wood wide web.
Why switchgrass?
Switchgrass
growing in the field. Image credit: OMAFRA
One plant that is not a host is switchgrass. This tall grassy plant was primarily used as forage for grazing livestock, but its potential for bioenergy production has been realised more recently. For efficiency and economic purposes, switchgrass needs to be able to grow in marginal environments, to leave areas with rich, fertile soil for food crop production(9). Droughts, which will become increasingly more common with climate change, also reduce the yield of switchgrass(10). Because of this, it would be valuable if switchgrass were able to obtain the benefits of supplementary water and nutrients from an ectomycorrhizal network.
Introducing a poplar gene into switchgrass
Scientists at the Oak Ridge National Laboratory have taken a gene from trees, which are able to form EMF symbiosis, and introduced it into switchgrass. This short sequence of DNA, named PtLecRLK1, is copied from poplar trees (Populus trichocarpa, Pt) and encodes a lectin (Lec) receptor-like kinase (RLK). The gene allows the plant to produce a protein that sits in the cell membrane and is involved in receiving signals related to plant immunity and defence. In general, LecRLKs sense sugars, and are known to be involved in plant symbioses with bacteria and fungi, though the exact mechanisms aren’t yet known(11). In some cases, a LecRLK can bind to sugars in beneficial bacteria’s cell walls to facilitate symbiosis(12). For poplar, PtLecRLK1 is important for allowing EMF to enter the plant and form symbiosis with it(13).
In wild-type switchgrass (the kind found in nature) EMF may loosely surround the roots but can’t enter between the cells. However, the lab-grown switchgrass transformed to have PtLecRLK1 allowed the EMF to form a thick layer around the roots and enter several cell layers deep into them to form a distinct Hartig net. This shows that the gene is responsible for allowing the previously incompatible symbiosis to occur. Furthermore, transformed plants that associated with EMF had more nitrogen-containing metabolites, but less fatty acids, suggesting the successful exchange of carbon for nitrogen between the plant and the fungus.
Cross-section of switchgrass roots under a microscope. The switchgrass cells are coloured pink while the EMF are green. a) shows wild-type switchgrass with little entry of EMF while b) shows transformed PtLecRLK1 switchgrass strongly associating with EMF. c) and d) are close ups of a) and b), respectively. Image credit: Qiao et al., 2021.
After confirming that their transformed plants were able to form EMF symbiosis, the group ran some experiments to see whether it benefitted the plants. They grew wild-type and PtLecRLK1 switchgrass in environments with limited or normal amounts of phosphorus. The wild-type plants were about 30 cm shorter when they did not receive enough phosphorus, but the PtLecRLK1 plant heights were not affected. Furthermore, the transformed plants produced on average twice as many tillers (side stems) in low-phosphorus conditions compared to the wild-type. These are both characteristics that indicate that when switchgrass can form connections with EMF, it can grow better in low-nutrient conditions, which would make it more profitable for bioenergy production.
The researchers also looked at gene usage between the wild-type and transformed plants. Gene usage gives clues as to which genes the plant needs under certain circumstances. When in the presence of EMF, the transformed PtLecRLK1 plants activated more genes related to nutrient uptake and plant-microbe recognition, which suggests that the presence of PtLecRLK1 turns on other genes that help make the switchgrass more susceptible to forming a symbiotic relationship with the EMF. As well, the transformed plants had less proteins needed to make jasmonic acid, a plant hormone important in defence against fungal pathogen entry. This decrease of defence is something that is also seen when poplars establish symbiosis with EMF.
Finally, the group investigated why switchgrass is not usually a host, and how PtLecRLK1 helps it become one. First, they compared the DNA sequence of the PtLecRLK1 gene to genes naturally present in switchgrass. Though there were similar receptors present, the parts that would recognise molecules secreted by EMF differed. By looking at which genes are expressed when, they deduced that when PtLecRLK1 is in switchgrass, it senses a signal from the EMF and sets off a cascade of further signals throughout the plant cells that eventually lead to a compatible symbiotic interaction.
Looking to the future
"We’ve engineered switchgrass to grow where it would typically struggle, that is, marginal land that is unsuitable for food crops. The fungus allows the switchgrass to absorb minerals from the soil.” – Jay Chen, scientist at the Oak Ridge National Laboratory
The effects of EMF association here were measured under very controlled circumstances; it will take larger-scale experiments in more realistic field conditions to confirm that the PtLecRLK1 gene and EMF association allow better growth and resilience in switchgrass. However, the results from these experiments suggest that inserting this gene allows switchgrass to form a symbiosis with EMF that was previously impossible, and leads to increased yields. This opens avenues of investigation towards transforming other crop plants to allow them to tap into the wood wide web as well.
For more information, see the original article here:
Reference list
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