Tracing the roots of an ancient friendship

 

Figure 1

An AM fungus (yellow) contacts the surface of a plant root. The nuclei of the plant cells are visible as blue spots. Image adapted from ref 3. Credit: Andrea Genre and Mara Novero (CC BY 3.0).

Plants need nutrients to be able to grow. Unfortunately, many of these nutrients can be scarce in the soil and therefore hard to get hold of. To get around this problem, most plants are able to form friendly relationships – known as symbioses – with soil microbes that can provide them with certain nutrients in exchange for sugars.

Today, around 80% of land plants form symbioses with a group of fungi known as arbuscular mycorrhizal (AM) fungi (1). Fossil evidence suggests that this symbiosis first emerged around 450 million years ago. This is around the same time that plants first started to colonise land. The transition from water to the dry and harsh environments on land would have presented many challenges to the early land plants, for example, how to avoid losing too much water. Another challenge would have been how to access essential nutrients that their ancestor (a type of green algae) would have gained directly from the water.

The liverworts, hornworts and mosses are thought to be the earliest groups of land plants (2). Since the AM symbiosis is widespread in these groups, it has been suggested that this symbiosis is one of the innovations that helped these primitive plants to survive on land.

Previous studies have identified many plant genes that are needed for AM symbiosis in legumes and other land plants. These genes can be split into two main groups: some are in a signalling pathway needed for the plant and fungus to communicate with each other, and others are activated later to allow the fungus to infect into the roots of the plant. Recently, Pierre-Marc Delaux and colleagues used a technique called phylogenetics to analyse genetic material from many different algae, liverworts, hornworts and mosses with the aim of finding out when the AM symbiosis genes first appeared (2).

Delaux et al. show that these plant genes emerged in stages, starting from before earliest plants colonised land. The signalling pathway genes appeared first, and are present in the algae that are thought to be the closest relatives of land plants, the Charophytes (2). On the other hand, the infection genes appear to be missing from the algae, but are present in the liverworts, hornworts and mosses.

These findings suggest that the algal ancestors of land plants were pre-adapted to interact with fungi. Currently, there is no evidence to suggest that the Charophytes are able to form AM symbioses themselves. Therefore, it is possible the signalling pathway evolved to allow algae to interact with other microbes and was later altered to allow the early land plants to interact with AM fungi.

Reference:

  1. Parniske, M. (2008). Arbuscular mycorrhiza: the mother of plant root endosymbioses. Nat Rev Microbiol, 6, 763-75.
(Good review of AM symbiosis, but unfortunately this article is hidden behind a paywall…)
  2. Delaux P, Radhakrishnan GV, Jayaraman D, Cheema J, Malbreil M, Volkening JD, Sekimoto H, Nishiyama T, Melkonian M, Pokorny L, Rothfels CJ, Sederoff HW, Stevenson DW, Surek B, Zhang Y, Sussman MR, Dunand C, Morris RJ, Roux C, Wong GK-S, Oldroyd GED, Ané JM. 2015. Algal ancestor of land plants was preadapted for symbiosis. Proceedings of the National Academy of Sciences of the United States of America. 2015, DOI: 10.1073/pnas.1515426112, PMID: 26438870
  3. Corradi N, Bonfante P. 2012. The Arbuscular Mycorrhizal Symbiosis: Origin and Evolution of a Beneficial Plant Infection. PLoS Pathog 8(4): e1002600. doi:10.1371/journal.ppat.1002600

An unsustainable trade

Guest post by Isabella Whitworth (@Orchella49).

DSCF5010 I Whitworth

Roccella gracilis on wool yarn that has been dyed with orchil made from Lasallia pustulata. Image credit: Isabella Whitworth .

Lichens are complex plant-like organisms made up of a fungus and an alga or cyanobacterium that live together in a mutually beneficial relationship (symbiosis). They are often found attached to rocks or trees and species can vary hugely in appearance, from flat, crusty forms to leaf-like growths. Certain species have been used as dyestuffs for millennia, although not all lichens produce dye.

My research into dye lichens was triggered by a chance mention of ‘an archive in the attic’ by local friends. Their forebears were dye manufacturers in nineteenth century Leeds in the UK and the company archive had been passed down three generations. The company’s initial fortunes came from the successful processing of orchil, a dye made from lichens. Continue reading

Yucca aloifolia, the plant that’s found a reverse gear

Guest post by Alun Salt

If you’re a plant, then having a pollinator you can rely on seems like a good idea. Yucca plants have a partnership with Yucca moths. Yucca plants have flowers that are specialised for Yucca moths. The payback is that Yucca moths can’t find food so easily anywhere else, because they’re now adapted to access Yucca flowers. A Yucca plant can provide food, knowing its pollen won’t be wasted by a pollinator that visits other species of plant. It sounds like a good deal, but it’s also a dead end.

When plants and pollinators become dependent, a threat to one can wipe out the other. For example, if a new blight hit Yucca plants, the Yucca moth would find its food source had disappeared, and it would starve to extinction itself. Once a species has specialised it’s very hard to dismantle adaptations and build new ones. Continue reading

How to live with a legume

Peas, beans and other members of the legume family of plants can form friendly relationships (symbioses) with nitrogen fixing bacteria from the soil. Guided by the plant, the bacteria infect into the root and colonise plant organs called nodules, which supply sugars to the bacteria. Nodules also provide conditions that enable the bacteria to efficiently convert nitrogen gas (N2) into a form of nitrogen that plants can use to grow.

Endophytic bacteria (red) in surrounded by nitrogen-fixing bacteria in a nodule from the legume Lotus japonicas. Scale bar = 50 um. Image from Figure 1 by Zgadzaj et al (2015) (CC BY 4.0)

Endophytic bacteria (red) in surrounded by nitrogen-fixing bacteria (green) in a nodule from the legume Lotus japonicas. Scale bar = 50 um. Image from Figure 1 by Zgadzaj et al (2015) (Licenced under CC BY 4.0)

To set up a symbiosis, the plant and bacteria exchange signals to enable them to identify each other. The plants release molecules called flavonoids into the soil and, in return, the nitrogen-fixing bacteria produce molecules called Nod factors. These Nod factors activate signalling pathways that trigger many responses in the plant and allow the bacteria to enter. The bacteria need to produce the correct Nod factors to gain admittance and so most legumes are only able to team up with a few species of bacteria. Other bacterial molecules such as exopolysaccharides also play important roles in establishing the symbioses. Continue reading

Studying plant genes with a paintbrush and a baking tin

Image by J. Murray. Used with permission.

Image by J. Murray. Used with permission.

A paintbrush and a baking tin might seem unlikely equipment for scientists to use to study plant genes but both feature in a study recently published in the journal Plant Cell. Intrigued? Let me explain…

Imagine for a moment that you are a research scientist studying how legume plants (e.g. peas, beans) set up friendly relationships with soil bacteria called rhizobia. The rhizobia provide the plants with much needed nitrogen. In return, the plant provides the rhizobia with carbohydrates and a home within the plant roots in special organs called nodules. Continue reading

Soybean: producing protein on a massive scale

US Department of Agriculture. Released into the public domain.

US Department of Agriculture. Released into the public domain.

This month’s organism is the soybean (Glyine max), a globally important crop plant that originates from Asia. The seeds (called soybeans) are rich in protein (40% of dry weight) and contain a good mix of essential amino acids needed by humans (1). Not surprisingly, this makes soybeans and their products popular with vegetarians and vegans as a source of non-animal protein. However, soybean-protein is also widely used as the main protein source for intensive farming of animals including chickens, cows and pigs.

Growing soybean a very efficient way to produce protein in terms of land-use. Soybeans produce twice as much protein per area of land than other vegetables or grain, and around 15 times more than land set aside for meat production (2).  The beans can be eaten whole after cooking (as in the Japanese dish edamame), but the majority of soybeans are processed to make a variety of soy-based food products, for example soya milk or tofu. Soy products are also added to many processed foods. Along with being rich in protein, soybeans are also rich in oil (20% of dry weight). The oil is extracted and used mainly for cooking with the remaining protein-rich pulp used as animal feed. Continue reading

Guest post. Arbuscular mycorrhizal fungi are old friends of plants

By Kirsty Jackson (@kjjscience)

Plants, unlike animals, are sessile. This means that they are unable to move from where they grow. If you are a plant that has germinated into a nice nutrient rich soil with plenty of food and water this is not such a bad thing.  However, if you find yourself in a spot which has a low nutrient content or is too dry, then you might find the inability to move yourself to a new patch a bit of a problem. One way to get yourself out of a sticky situation is to call on a friend who might be able to lend a hand.  Plants call on their friends the arbuscular mycorrhizal fungi (AMF).

AMF are a group of fungi which form a close relationship with plants for mutual benefit.  They are able to get nutrients such as phosphorus and nitrogen out of the soil and exchange them for sugars that the plant makes by photosynthesis.  AMF get their name from the specialised structures they form within plant roots called arbuscules.  Arbuscle translates from Ancient Greek as “little tree” which is exactly what these structures look like. They are highly branched fungal hyphae. The branches increase the surface area between the fungus and the plant that in turn increases the amount of nutrient-sugar exchange that can occur. The second half of their name, mycorrhiza, translates as “mushroom roots”. The fungal hyphae that grow out into the soil act like an extension of the plant roots. Continue reading