News

For hundreds of millions of years, plants and fungi have formed symbiotic relationships to trade crucial nutrients, such as phosphate and fatty acids. This relationship is extremely important to the growth and survival of both organisms, and solving the mystery of how they transfer molecules to each other could eventually help reduce the use of fertilizer in agriculture.

Now, researchers at the Boyce Thompson Institute (BTI) have uncovered structural networks of tubules at the plant–fungal interface that could shed light on the mechanisms of this natural partnership. Details of the study were published in Nature Plants.

An estimated 80% of vascular plant families form symbioses with a type of soil fungi called arbuscular mycorrhizal fungi. The fungi penetrate the outermost cells of a plant’s roots and grow intricate branch-like structures called arbuscules. Each host plant cell then grows a membrane that envelops an arbuscule, and nutrient exchange takes place within the space between the plant membrane and the fungal cell wall.

In an attempt to understand the fundamental interaction between plants and the fungi, BTI faculty member Maria Harrison and postdoctoral scientist Sergey Ivanov joined forces with R. Howard Berg, director of the Donald Danforth Center’s Integrated Microscopy Facility. They used advanced electron microscopy techniques to image arbuscules present in the roots of the legume Medicago truncatula colonized by the fungus Rhizophagus irregularis, and were surprised by the results.

“Our understanding of the sub-cellular structural basis of the interaction relies on studies performed 30-40 years ago. Many of them indicated that the material around the fungus but inside the plant membrane would be an amorphous matrix of carbohydrate material,” said Harrison, the paper’s corresponding author. Instead, the researchers found a network of round, tubular and dumbbell-shaped structures constructed of lipid membranes, nearly all of which appeared to connect back to the plant cell’s membrane.

The researchers were further surprised to find another network of membrane tubules in the space between the fungal cell membrane and the fungal cell wall. “It was totally unexpected to see such an extensive proliferation of fungal membrane, particularly knowing that the fungus is starving for lipids,” explains Ivanov, the paper’s lead author.

Harrison and Ivanov speculate that the networks are related to the transfer of lipids.

“Somehow lipids are released from the plant cell and fed to the fungus, and we wondered how they move through what we thought was an aqueous matrix between the plant cell membrane and the fungal cell wall,” says Harrison. “But maybe this space isn’t so aqueous after all, and perhaps this membrane-rich environment facilitates the movement of lipids between the organisms.”

Given the plant and fungal membrane networks’ close physical proximity to each other, the fungal network could be involved in lipid absorption in order to optimize the process. The researchers suspect the network is not involved with transferring phosphate to the plant because the membrane networks are more abundant near the larger branches of the arbuscule, whereas phosphate uptake likely occurs near the smaller branches.

Harrison believes that newer technologies are to thank for finding these networks of tubules. “High-pressure freezing fixation of samples gives better membrane preservation than older techniques. I think that is the reason that these extensive membranes were not seen before,” she says. “Plus, 3D electron tomography is very powerful and let us visualize the networks, which didn’t look connected on 2D images.”

Ivanov, worked closely with Berg, who has expertise with cryofixation and electron microscopy, and Jotham Austin II at the University of Chicago, who is an expert in tomography.

Another research group led by Uta Paszkowski of the University of Cambridge in the UK performed similar imaging studies in rice colonized with R. irregularis, and found similar structures. Those results were published in the same issue of Nature Plants.

Read the paper: Nature Plants

Article source:Boyce Thompson Institute

Image credit: Jotham Austin II and R. Howard Berg

The findings suggest wild grasses are naturally genetically modifying themselves to gain a competitive advantage.

Understanding how this is happening may also help scientists reduce the risk of genes escaping from GM crops and creating so called “super-weeds” – which can happen when genes from GM crops transfer into local wild plants, making them herbicide resistant.

Since Darwin, much of the theory of evolution has been based on common descent, where natural selection acts on the genes passed from parent to offspring. However, researchers from the Department of Animal and Plant Sciences at the University of Sheffield have found that grasses are breaking these rules. Lateral gene transfer allows organisms to bypass evolution and skip to the front of the queue by using genes that they acquire from distantly related species.

“Grasses are simply stealing genes and taking an evolutionary shortcut,” said Dr Luke Dunning.

“They are acting as a sponge, absorbing useful genetic information from their neighbours to out compete their relatives and survive in hostile habitats without putting in the millions of years it usually takes to evolve these adaptations.”

Scientists looked at grasses – some of the most economically and ecologically important plants on Earth including many of the most cultivated crops worldwide such as: wheat, maize, rice, barley, sorghum and sugar cane.

The paper, published in the journal Proceedings of the National Academy of Sciences, explains how scientists sequenced and assembled the genome of the grass Alloteropsis semialata.

Studying the genome of the grass Alloteropsis semialata – which is found across Africa, Asia and Australia – researchers were able to compare it with approximately 150 other grasses including rice, maize, millets, barley and bamboo. They identified genes in Alloteropsis semialata that were laterally acquired by comparing the similarity of the DNA sequences that make up the genes.

“We also collected samples of Alloteropsis semialata from tropical and subtropical places in Asia, Africa and Australia so that we could track down when and where the transfers happened,” said Dr Dunning.

“Counterfeiting genes is giving the grasses huge advantages and helping them to adapt to their surrounding environment and survive – and this research also shows that it is not just restricted to Alloteropsis semialata as we detected it in a wide range of other grass species.”

“This research may make us as a society reconsider how we view GM technology as grasses have naturally exploited a similar process.

“Eventually, this research may also help us to understand how genes can escape from GM crops to wild species or other non-GM crops, and provide solutions to reduce the likelihood of this happening.

“The next step is to understand the biological mechanism behind this phenomenon and we will carry out further studies to answer this.”

Read the paper: PNAS

Article source: University of Sheffield

Image credit: University of Sheffield

Plant researchers at Martin Luther University Halle-Wittenberg (MLU)_ are providing new insights into basic cell division in plants. The scientists have succeeded in understanding how processes are coordinated that are pivotal in properly separating daughter cells during cell division. In the renowned scientific publication “The EMBO Journal “, they describe the tasks of certain membrane building blocks and how plants are impacted when these building blocks are disrupted.

For their study, the plant researchers examined the roots of the thale cress plant, Arabidopsis thaliana. They cultivated normal plants and plants in which they artificially switched off certain enzymes that affect the composition of the membranes. “We wanted to find out which membrane building blocks are important for cell division and why,” explains Professor Ingo Heilmann from MLU.

For plants to develop, their cells have to divide. First, the genetic material located in the cell nucleus divides. Two whole new cell nuclei are formed from the duplicated genetic material. The other components of the cell, for example the chloroplasts and mitochondria, are distributed between the two future daughter cells. All this takes place in the parent cell.

Only then the daughter cells will be separated by a new cell wall. The whole process can be compared to a construction site. First, a temporary scaffold made of protein fibres, the so-called phragmoplast, forms in the middle of the cell. Like railway tracks, these fibres guide the building materials needed for the cell wall. Small bubbles gradually transport new cell wall material along the rails. This is assembled together by a complex fusion machinery to form a larger structure: the cell plate. The cell plate continues to grow at its edges from the centre of the cell outwards until a cell wall disc completely separates the daughter cells from one another. “The fusion machinery has to correctly coordinate the protein fibres for everything to function properly, otherwise the freight cars will transport the cell wall material to the wrong spot or at the wrong time and cell plate formation will cease,” explains Heilmann.

Using biochemical and cell biology experiments, his research group was able to show that PI4P, a membrane building block, plays two roles during cell division: PI4P not only controls the activity of the fusion machinery, it also ensures the new material is transported in the right direction. For the first time, the researchers were able to show that PI4P helps to ensure that the protein scaffold of the phragmoplast is assembled and disassembled in the right places. In normal plants, this results in regular cells that fit together perfectly and give the plant its needed stability.

In mutated plants, however, the scientists observed severe defects in cell division: They found enlarged cells containing several cell nuclei as a result of the unsuccessful separation of the daughter cells. Some cells were unable to divide completely, the cell tissue was chaotic, and there were enormous differences in the sizes of the individual cells. “This is not happy tissue. It makes the entire plant more unstable, reduces its size and impacts how it adapts to environmental stimuli,” explains Heilmann.

The results of the research group from Halle help to better understand the dynamics of the plant’s cytoskeleton of microtubules. The cytoskeleton not only determines the direction of cellular transport processes during cell division, but also directs general plant growth. Therefore, the new findings could have far-reaching consequences, for example on the deposition of cellulose in plant cell walls and thus on biomass and cellulose production. However, it must first be determined whether the findings can also be applied to other plants and how the activity of the enzymes investigated here can be specifically regulated.

Read the paper: The EMBO Journal

Article source:Martin-Luther-Universität Halle-Wittenberg

Image credit: Ingo Heilmann (The EMBO Journal)

A relatively unknown fungus, accidentally found growing on an Acacia tree in the Northern Cape, has emerged as a voracious wood-munching organism with enormous potential in industries based on renewable resources.

The first time someone took note of Coniochaeta pulveracea was more than two hundred years ago, when the South African-born mycologist Dr Christiaan Hendrik Persoon mentioned it in his 1797 book on the classification of fungi.

Now C. pulveracea has had its whole genome sequenced by microbiologists at Stellenbosch University (SU) in South Africa, and henceforth made its debut in cyberspace with a few tweets and a hashtag. All because this relatively unknown fungus has an extraordinary ability to degrade wood – hence the descriptor “pulveracea”, meaning powdery.

In the age of biotechnology, biofuels and the usage of renewable raw materials, this is an important fungus to take note of, says Prof Alf Botha, a microbiologist in the Department of Microbiology at SU.

Over the past 25 years, there has been a number of reports on the ability of species in the Coniochaeta genus to rapidly degrade lignocellulose into fermentable simple sugars. But thus far Prof Botha‘s lab is the only one to be working on C. pulveracea.

The work started in 2011, when he quite randomly snapped a brittle twig, covered in lichen, from a decaying Acacia tree. At the time, he was holidaying with family on a farm in the Northern Cape. “At the time we were looking for fungi and yeasts that can break down wood, so I knew this was something special when I decided to keep the twig,” he explains. But to date, despite numerous attempts, they have not been able to find it again.

However, back in the lab there was great excitement when they observed that this species in the Coniochaeta genus was literally munching its way through birchwood toothpicks. Even more astounding was its ability to change form between a filamentous fungus and a yeast, depending on the environment.

“This is highly unusual for a fungus. We’d typically expect this kind of behavior from some fungal pathogens,” explains Botha.

Over the past decade Botha and his postgraduate students focused on unraveling this yeast-like fungus’ behavior. In 2011 Dr Andrea van Heerden found that it produced enzymes that degraded the complex structures of wood into simple sugars, feeding a community of surrounding fungi that do not have the ability to degrade wood. In 2016, she published the results of her investigation into its ability to switch to a yeast-like growth. Understanding this process would be important to the potential use of this fungi in industrial processes.

In the latest study, MSc student CJ Borstlap worked with Dr Heinrich Volschenk, an expert molecular biologist, and Dr Riaan de Witt from the Centre for Bioinformatics and Computational Biology at SU, to produce the first draft genome sequence of C. pulveracea. With a genome size of 30 million nucleotides and over 10 000 genes, this was no easy task. In the process he picked up the necessary coding skills to identify and name all 10 053 genes, and to identify those responsible for the wood-degrading character of the fungus.

Dr Volschenk says the next step is to understand the fungus’ mechanism of breaking down wood and producing sugars on a molecular level: “With the genetic blueprint now available, we can study the network of genes and proteins the fungus employs to convert wood and other similar renewable resources into more valuable products,” he explains.

The sequence data for C. pulveracea have been deposited at the DNA Data Bank of Japan (DDBJ), the European Nucleotide Archive (ENA) at Cambridge, and GenBank in the United States of America, under the accession number QVQW00000000 and is freely available to all researchers in this field.

Read the paper: Microbiology Resource Announcements

Article source: Stellenbosh University

Image credit: Heinrich Volschenk

Scientists at the Max Planck Institute for Plant Breeding Research in Cologne have revealed that direct physical associations between plant immune proteins and fungal molecules are widespread during attempted infection. The authors’ findings run counter to current thinking and may have important implications for engineering disease resistance in crop species.

Fungal diseases collectively termed powdery mildew afflict a broad range of plant species, including agriculturally relevant cereals such as barley, and result in significant reductions in crop yield. Fungi that cause powdery mildew deliver so-called effector molecules inside plant cells where they manipulate the host’s physiology and immune system. In response, some plants have developed Resistance (R) genes, usually intracellular immune receptors, which recognize the infection by detecting the fungus’ effectors, often leading to plant cell death at the site of attempted infection to limit the spread of the fungus. In the prevailing view, direct recognition of effectors by immune receptors is rather a rare event in plant-pathogen interactions, however, and it has been thought instead that in most cases recognition proceeds via other host proteins that are modified by the pathogen.

In barley populations, one of the powdery mildew receptors, designated mildew locus a (Mla), has undergone pronounced diversification, resulting in large numbers of different MLA protein variants with highly similar (>90%) DNA sequences. This suggests co-evolution with powdery mildew effectors, but the nature of the evolutionary relationship and interactions between immune receptor and effectors remained unclear.

To address these questions, Isabel Saur, Saskia Bauer, and colleagues from the department of Paul Schulze-Lefert first isolated several effectors from powdery mildew fungi collected in the field. Except for two, these proteins were all highly divergent from one another. When the authors expressed the effectors together with matching MLA receptors this led to cell death not only in barley but also in distantly related tobacco cells, already suggesting that no other barley proteins are required for recognition. Using bioluminescence as a marker for direct protein-protein interactions, the scientists indeed found specific associations of effector-MLA pairs in extracts of tobacco leaves. Similarly, a protein-protein interaction assay in yeast also revealed interactions only of matching effector-MLA pairs. These results suggest that highly sequence-related MLA receptor variants directly detect unrelated fungal effectors.

Plant disease is a major cause of loss of yield in crops. Transfer of plant R genes between species is a potentially powerful approach to generate disease-resistant crops. The authors’ discovery that multiple variants of the same resistance gene are able to bind dissimilar pathogen proteins, also in distantly related plant species, underlines the potential of this approach and chimes in with the earlier finding that wheat versions of Mla, Sr33 and Sr50, provide disease resistance to the stem rust isolate Ug99, a major threat to global wheat production.

Paul Schulze-Lefert sees further exciting applications on the horizon: “Our discovery that direct receptor-effector interactions are widespread for MLA receptors suggests that it may be feasible to rationally design synthetic receptors to detect pathogen effectors that escape surveillance by the plant immune system.”

Read the paper: eLife

Article source:Max Planck Institute for Plant Breeding Research

Image credit: Takaki Maekawa