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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Poison in the garden

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The gates of Alnwick Poison Garden, north-east England. Image by Jacqui (CC BY-NC-ND 2.0) via Flickr

While giving my undergraduate class a tour of a botanic garden, a university professor said that “we should only eat the parts of a plant that the plant wants us to eat”. He was referring to the fruit, which many plants encourage animals to eat in order to spread their seeds in the environment (though not all fruits are edible). I don’t think he meant us to take his advice literally, but it is sensible to eat plants with caution. Alongside famous poisons including belladonna and hemlock, plants produce a variety of other molecules that aim to deter animals from eating them. Some of these molecules – such as ricin, which is produced by the castor oil plant – are so poisonous that tiny quantities can kill you. Others, like caffeine or the anti-malaria drug quinine, have less dramatic effects on the human body that we may find desirable or useful.

I recently visited The Alnwick Garden in north-east England, which has a special garden dedicated to educating visitors about the potential dangers of plants. In fact, some of the plants on display in the Poison Garden are so dangerous that visitors can only enter as part of a guided tour. I really enjoyed the tour and if you are ever in the area I recommend you pay the garden a visit.

The tour included some well-known poisonous plants, but the main message I took home from the tour was that many common garden plants are also potentially dangerous if they touch your skin or you accidently eat them. Below are a few examples of common plants that aren’t as benign as they might first seem:

Rhubarb (Rheum x hybridium)

While the pink fleshy stalks of the rhubarb plant are safe to eat and are commonly used in desserts, the leaves are highly toxic (1). This is thought to be due to the presence of high levels of oxalic acid, which can interfere with chemical reactions in the body by combining with calcium and other metals.

Common ivy (Hedera helix)

This rapidly growing vine is a haven for wildlife and attracts at least 70 species of nectar-feeding insects in its native range of Europe and Western Asia (2). Contact with ivy can cause an allergic skin reaction in some people, due to a natural pesticide in the leaves called falcarinol (3). Regardless of whether you are allergic to ivy or not, you should avoid eating this plant because its leaves contain saponins, which can cause vomiting, convulsions and even death.

Common nettle (Urtica dioica)

Children quickly learn that contact with common nettles results in a painful stinging sensation and skin inflammation. This is due to a cocktail of molecules including histamine, serotonin and oxalic acid, which is released from hairs on the surface of the leaves. For more information check out this cool infographic by Compound Interest.

Common laburnum (Laburnum anagyroides)

All parts of this small tree are poisonous, due to the presence of a molecule called cytisine, which has a similar structure to nicotine and has similar effects on the body. Laburnam is a member of the pea family and cases of laburnam poisoning are often caused by individuals mistaking laburnum seeds for peas and eating them (4). Mild cases may cause nausea and vomiting, but laburnum poisoning can also lead to insomnia, convulsions and coma.

These are just a few examples of common garden plants that can be harmful to humans and other animals. Fortunately, you can protect yourself against these and other poisonous plants by taking simple precautions, such as wearing gloves while gardening and carefully identifying edible plants when foraging.

Author’s note: Sorry for the long silence on this blog. My life has been quite chaotic in the last few months due to several events (expected/not expected, good/bad) and so the blog has had to take a back seat. Things are calming down a bit now so I’m hoping to get back into posting regularly, probably about twice a month. As ever, I’m always keen to receive guest posts so if you are interested in writing for Plant Scientist, please do get in touch.

References:

  1. The Poison Garden blog: Rheum x hybridium http://www.thepoisongarden.co.uk/atoz/rheum_x_hybridum.htm
  2. Wikipedia: Hedora helix https://en.wikipedia.org/wiki/Hedera_helix
  3. Compound interest advent calendar http://www.compoundchem.com/2014advent2/
  4. The Poison Garden blog: Laburnam anagyroides http://www.thepoisongarden.co.uk/atoz/laburnum_anagyroides.htm

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

Sudden oak death – a disease as ominous as its name

Guest post by Monica Lewandowski (@MMLewandowski52)

Sudden oak death is a disease that has killed millions of oaks (Quercus spp.) and tanoaks (Notholithocarpus densiflorus) in the western United States. First detected in California in the mid 1990s, it continues to steadily spread through northern California and Oregon forests, with the potential to wreak more havoc in forests and landscapes across the world.

The underlying cause of sudden oak death is a fungal-like organism, Phytophthora ramorum. The spores of P. ramorum are spread by wind, rain and human movement of infected plants. And more bad news – P. ramorum can infect much more than oaks. A strain of P. ramorum that infects larch trees is making headlines in the United Kingdom, where it’s better known as larch tree disease. Several species of trees and shrubs, herbaceous plants and even maidenhair fern are on P. ramorums “host” list (view regulated plant list in the United States). This is a cause for concern as losing one or more key plant species in a forest can lead to dramatic changes for both the flora and fauna of an ecosystem. Continue reading

Where’s the plant science in beef?

Guest post by Erin Sparks (@ErinSparksPhD)

Four years ago I became a first generation beef farmer. I had just started a postdoc studying the development of plant roots when my husband told me that his parents intended to give us beef cows as a wedding present. Whoa. Wait. What?!?!?! First of all, we live in a very small apartment – where are we going to put cows? Second of all, we know nothing about farming. Fear not fair reader, the good news is that my in-laws keep the cows for us and they are “many”-generation beef farmers so they know what they’re doing. Through their tutelage, I’m slowly becoming a beef farmer. I’ve learned about herd management, breeding, economics and more. Although all aspects of farming fascinate me, I wanted to tell you specifically about how plant science contributes to our farm.

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One of Erin’s cows and her new twins. Much like humans, twins are a rarity for bovine. Image credit: E. Sparks

We run a cow-calf operation, which means that we keep a herd of cows (100+ in total) and three bulls on the farm. These animals are bred and their calves are then sold to market. What do these animals eat? Feeding cattle is a basic cost-benefit analysis. If you pay more to feed your animals than the profit you gain, you can’t make a living. Although it is not as simple as that, because beef prices are constantly fluctuating, so you also have to consider market projections. On our farm, we strive to be self-sufficient for feeding our animals. This means we grow over 200 acres of hay that is rolled and stored. In the summer, the animals are grazing in the fields, but come winter, when the fields freeze over, the animals get fed these hay bales. Alternatively, you can raise animals on grain feed, but this is exceedingly more expensive. We save grain feed for the calves after weaning, and to increase growth before selling. Continue reading