“EXCITING BIOLOGY” REWRITES TEXT BOOKS IN UNCOVERING PLANTS’ HIGH-FAT DIET FOR FUNGAL BENEFACTORS

One of biology’s most charismatic relationships, credited with helping plants to colonise land more than 400 million years ago, has yielded a fundamental survival secret with implications for agriculture and biotechnology.

Plant scientists have discovered that a particular form of fungi, which invades plant roots and then helps the colonised plants to absorb nutrients from soil, receive life-sustaining carbon from their symbiotic hosts in the form of long-chain fatty acids, a building block for essential lipids.

Previously, scientists had thought that the fungi received carbon from their hosts only in the form of carbohydrates, which the fungi used to make their own fatty acids and then the more complex lipids necessary for survival.

It’s now clear that the main source of carbon from the host plants are fatty acids, and that these fatty acids are necessary before the fungi can create the more complex lipids that are needed for storing energy, for signalling and for cellular membranes.

The latest work, published online today [7pm BST, Thursday, 8 June 2017] by Science, comes from a joint team of scientists from Rothamsted Research and the John Innes Centre. This field of science is very competitive currently, with research groups in China, Germany and the US all chasing similar breakthroughs.

“This is exciting biology. We’ve discovered that the fungus is effectively re-programming the plant to pump out lipids,” says Peter Eastmond, senior biochemist in the Department of Plant Sciences at Rothamsted and co-leader of the research team.

“There are important implications for sustainable agriculture, particularly in nutrient-poor soils where you need to make the most of resources,” says Eastmond. “And also for biotechnology, in creating green pathways to produce lipids in plants, for biofuels and for precursor chemicals for industrial applications, as an alternative to fossil fuels.”

The symbiotic relationship at the heart of this research, and one of the most widespread associations in nature, is that between the great majority of plants, at least 80% of them, and arbuscular mycorrhizal fungi, which create special feeding structures within the plants’ roots called arbuscules. (TOP: Microscopy image (x200) shows fungal arbuscules (green blocks, about 60 micrometers long) and hyphae (green tubes) containing lipid drops (red spots) inside the plant root (background); ABOVE: Legumes, such as this crop of field (broad) beans, are among the many plants capable of developing symbiotic associations with arbuscular mycorrhizal fungi in the soil.)

The fungi, which develop hyphae to increase the roots’ surface area, can provide the plant with up to 80% of its nutrients from soil while the plants can yield up to 30% of the carbon they derive through photosynthesis to the fungi.

Rothamsted began to investigate the relationship’s metabolism in 2011 after ground-breaking genetic studies at the JIC had been the first to identify and isolate two genes essential for sustaining the symbiosis, RAM (Required for Arbuscular Mycorrhization)1 and RAM2.

“We grappled with understanding why these genes were so important until we came up with the hypothesis that the symbiosis created a lipid factory in the plant that fed the fungus,” recalls Eastmond. “This went against what the literature said...we were proposing to overturn what’s in the text books.”

But this is what they did. The team came up with a trio of robust and ingenious experiments that got around the inseparable union to distinguish whether one or both of the symbiotic partners were producing lipids. Each experiment independently endorsed the hypothesis. 

“There’s a lot more work to do in following up this discovery,” says Eastmond. “It will have a lasting impact on the understanding of the metabolism of this symbiotic relationship and lead researchers in many new directions.”

Giles Oldroyd, project leader in cell and developmental biology at JIC, and co-leader of the research team comments: “It has long been thought that the plant delivered sugars to the symbiotic fungi. Our research demonstrates that in addition the plant delivers lipids to the fungus...We hope that through a better understanding of these plant/fungal symbioses we may be able to improve their use in agriculture and thus increase the sustainability of agricultural systems.”