How a virus gives back to its host

 

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Image by F_A (CC BY 2.0)

A study by UK scientists has shown that tomato plants infected with a virus are more attractive to bumblebees than healthy plants. Why would a plant virus want to change the behaviour of bumblebees?

The virus in question – cucumber mosaic virus (CMV) – can infect many different species of plant including tomatoes and a model plant called Arabidopsis thaliana. In tomatoes it causes many symptoms including yellowing, mottling, leaf distortion and can reduce the yield of seeds. As a result there is pressure for populations of plants to evolve better defences against the virus. Since CMV can only multiply within plant cells you might expect that, over time, CMV might become less common, but this doesn’t appear to be the case. One way the virus might be able to combat this problem is to compensate for the decrease in seed production in infected plants by encouraging pollinators, such as bumblebees, to visit the flowers.

Bumblebees fertilise tomato flowers by a process called buzz pollination, in which sounds produced by the bees shake the flowers to release pollen. Although tomato flowers can fertilise themselves without help from the bumblebees, buzz pollination makes the process more efficient and also leads to the transfer of pollen between flowers. Volatile compounds (molecules that easily become gases) released from the plants may help to guide the bees to the flowers. CMV infection can change the mix of volatile compounds that plants produce, but it was not clear whether this changes the behaviour of the bees.

Simon Groen, Sanjie Jiang, Alex Murphy, Nik Cunniffe et al. found that the bees are more attracted to the volatiles produced by CMV-infected tomato plants than those produced by healthy, uninfected plants. In the absence of buzz pollination, CMV-infected plants produce fewer seeds than healthy plants. However, mathematical modeling indicates that, in the “wild”, the bee’s preference for virus-infected flowers may help to compensate for this so that CMV-infected plants may produce more seeds than uninfected plants. Further experiments in A. thaliana suggest that molecules of micro ribonucleic acid (or miRNA for short) produced by the plants might regulate the mix of volatiles that plants produce.

These findings suggest that in some environments it may be in a virus’ interest to help its host plant by making the plant more attractive to bumblebees or other pollinators. Bumblebees are important pollinators for many crop plants so these findings may help us to develop new ways to increase crop yields in the future.

Reference: Groen SC, Jiang S, M, Murphy AM, Cunniffe N, Westwood JH, Davey MP, Bruce TJA,  Caulfield JA, Furzer O, Reed A, Robinson SI, Miller E, Davis CN, Pickett JA, Whitney HM,  Glover BJ, Carr JP. 2016. Virus Infection of Plants Alters Pollinator Preference: A Payback for Susceptible Hosts? PLOS Pathogens http://dx.doi.org/10.1371/journal.ppat.1005790

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Bramble: friend or foe?

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Blackberries change colour from red to black as they ripen. Image by Thomas’ pics (CC BY 2.0 via Flickr)

In England at this time of year, the hedgerows along country lanes are full of delicious fruits called blackberries. Just last week I spent an enjoyable afternoon with friends gorging on blackberries along the route of an old railway line in Norwich (now a footpath and cycleway). The berries are a good source of vitamin C and antioxidants, and are commonly used in desserts and preserves. Although I love collecting and eating blackberries, I have a bit of a love-hate relationship with the plant that produces them, the bramble (Rubus fruticosus agg.).

Rubus fruticosus agg. isn’t a single species, but instead is a group (or aggregate; agg) of around 200-300 very similar species of shrub in the rose family that are very hard to tell apart (1). Like roses, brambles are covered in sharp thorns that help to protect the plant from herbivores (and humans). The thorns also help to make brambles a safe haven for many small birds and other wildlife.

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Brambles are pollinated by insects. Image by Roger Bunting (CC BY-NC-ND 2.0 via Flickr)

Brambles grow wild across most of Europe and in the UK they can thrive in most environments (1). The white or pinkish flowers are self-fertile and can still produce seeds even in the absence of fertilization (a process called apomixis) to produce an army of clone plants (2). Furthermore, brambles can produce suckers – new shoots from buds in the roots – which helps them rapidly cover an area of ground. As a result, brambles are often among the first plants to colonise abandoned plots of land. This is great for wildlife and the casual blackberry picker, but it’s not so helpful if you are trying to work on said piece of abandoned land…

When some friends and I took on an allotment this year, our plot had been neglected for a while and contained quite a lot of brambles. We removed a lot of the plants but have left some to be our own personal blackberry patch. Removing brambles is not a fun business as the thorns can cut through clothes (and gardening gloves). For several weeks in the spring my arms and legs were covered in scratches and I often found bramble thorns impaled in my fingers. If you don’t manage to completely remove the whole root, the bramble is quite capable of growing a fresh shoot so we’ve had a few cheeky brambles reappearing in the vegetable beds.

Despite my moaning about brambles I must say that the blackberry crop from the allotment has been great. It is kind of ironic that our most successful crop this year is something we weren’t deliberately growing. All in all, if I had to summarize my relationship with the bramble at the moment, I would say: “it’s complicated”.

 

References:

1) Wikipedia: Blackberry https://en.wikipedia.org/wiki/Blackberry

2) Brambles (Rubus fruticosus) http://www.woodlands.co.uk/blog/flora-and-fauna/brambles-rubus-fructicosus

Image links:

Bramble by Thomas’ pics

Canal: Morse to town 7 June ’11 by Roger Bunting

Lab Girl by Hope Jahren

51miGrYIk-LIn Lab Girl, scientist Hope Jahren has cleverly weaves a memoir of her own life with passages about the lives of plants, her scientific passion. From her childhood in a small town in Minnesota to her current position as a Professor at the University of Hawai’i, she gives a candid account that includes some of the adventures, funny incidents, obstacles, and shifts in her scientific thinking that happened along the way. The book is a fascinating window into the life of a gifted, passionate, yet (reassuringly) human scientist. If you haven’t read it yet, then I highly recommend you get your hands on a copy.

If you aren’t convinced by my mini-review, then I suggest you check out this longer review from the NY times.

Tracing the roots of an ancient friendship

 

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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

The catch-22 of being a carnivorous plant

Guest post by Sonja Dunbar (@PlantSciSonja)

Plants, like any other organism, want to reproduce. The usual way that plants achieve this is known as sexual reproduction, where an egg cell and sperm from two different individuals fuse and then develop into a new plant. However, since plants are generally anchored to one spot, they can’t meet up to reproduce. Instead, they rely on a variety of more indirect methods to transport sperm to other plants. For example, many flowering plants (also known as angiosperms) recruit insect messengers to carry their sperm, safely packaged in pollen grains, from one plant to another. They use colourful, sometimes scented, flowers to attract potential pollinators and often reward them with a sugary drink, nectar, while coating them in the pollen the plant wants them to carry. But what if you are a plant that also eats insects?

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Some of the most well-known pollinators; bees and butterflies. Image credit: S. Dunbar

Carnivorous plants obtain nutrients from trapped insects to help them cope with a lack of important nutrients in their environment, such as nitrogen, that they need to grow (1). There are several different trap types, from snap traps, to flypaper traps and pitfall traps. The fact that carnivorous species are found in multiple different plant families suggests this strategy has arisen several times. Continue reading

On the origin of chloroplasts

Guest post by Joram Schimmeyer

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Chloroplasts in plant cells are easily identified under a microscope by their green colour. Image: J. Schimmeyer.

Of all the biological processes found on Earth, photosynthesis could be considered one of the most important. During photosynthesis, the energy from sunlight is used to build up sugars in the cells of plants, algae and some bacteria. These sugars can then be metabolised by the cells or other organisms that feed on them. Also, photosynthesis produces oxygen gas as a by-product, which is needed by most forms of life on earth. Without photosynthesis, life as we know it would not be possible.

In plants and algae, photosynthesis is carried out in tiny compartments inside cells called chloroplasts. This compartment contains a green pigment called chlorophyll, which is used to harvest light energy and is responsible for plants appearing green in colour. Chloroplasts vary greatly in shape and size, but they are all enclosed by two membranes and filled with even more membranes known as the thylakoid membrane system. The key players of photosynthesis are located within these thylakoid membranes; large groups of proteins use the light energy from chlorophyll to convert carbon dioxide form the atmosphere into sugars. The sugars can then be broken down to provide energy to drive growth and other cellular processes. Continue reading

An unsustainable trade

Guest post by Isabella Whitworth (@Orchella49).

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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