Sugar helps plants to tell the time

The cacti in this sundial all have their own internal circadian clocks.

Clocks within a clock. The cacti in this sundial all have their own internal circadian clocks. Image by the author.

Plants can harness light energy to produce their own sugars from carbon dioxide and water in a process known as photosynthesis. Much of the sugar produced during the day is stored as starch to be used as an energy source overnight when photosynthesis is not possible. To enable the plant to maximise photosynthesis during the day and regulate the use of its starch energy stores at night, plants need to be able to “tell the time”. Plants have an internal “circadian” clock, which maintains 24-hour rhythms that modulate many plant processes, including photosynthesis.

At the core of the plant circadian clock are three proteins called CCA1, LHY and TOC1. These proteins (known as transcription factors) can regulate the production of other proteins. In the morning, the CCA1 and LHY proteins repress the production of TOC1 protein (see figure below). Over the course of the day, the levels of CCA1 and LHY proteins decline as their production is repressed by other transcription factors, including PRR7. With fewer CCA1 and LHY proteins to prevent its production, TOC1 levels rise at dusk and this further represses CCA1/LHY production. During the night TOC1 production falls and CCA1/LHY levels start to rise again.

circadian clock

At the core of the circadian clock is a loop of three proteins ( CCA1, LHY and TOC1) that regulate each others production. Image by the author.

The CCA1/LHY and TOC1 cycle (plus their regulators) creates fairly robust 24-hour rhythms of protein production that can regulate other processes. However, circadian clocks are only useful if they stay in sync with the environment. The 24-hour cycles of environmental day-night (a result of the Earth’s rotation on its axis) and circadian clocks are not exactly the same, so they would tend to drift apart over time. To keep them in sync with the external environment, circadian clocks are modulated by external factors like light and temperature.

Circadian regulation of photosynthesis and starch formation/breakdown leads to the amount of sugar in the plant rising and falling in 24 hour cycles, with sugar content peaking 4-8 hours after dawn. In a paper published last year in Nature, the authors found that sugar can modulate the circadian clock. The researchers lowered the sugar content of the plants by inhibiting photosynthesis (by growing the plants in the absence of carbon dioxide, or with a chemical inhibitor). In these conditions, the period of the circadian clock was lengthened by 2.5-3 hours, production of PRR7 protein was increased and CCA1 production decreased. A normal circadian clock period was restored when the plants were treated with sucrose (a sugar).

The effects of sucrose treatment on the circadian clock vary depending on time the day. When a dose of sucrose was given to plants growing in continuous low light at the time dawn would have been, it shifted the clock forward by 2 hours. However, when a dose of sucrose was given 12 hours later at the equivalent of dusk the clock was shifted back by 2 hours. The PRR7 protein was found to be required for this effect because sugars did not alter the length of the circadian period in plants that were lacking a functional PRR7 protein. Sucrose treatment at any time in the cycle led to the down-regulation of PRR7, but this was most pronounced in the morning. So at dawn, increases in plant sugar content can modulate the circadian clock, via the PRR7 protein, to move it into the day-phase but if sugar levels are still high later in the day, the onset of the night-phase can be delayed.

Model of sugar modulation of the circadian clock. Image by the author

Model of sugar modulation of the circadian clock. Image by the author

Although PRR7 production is decreased by sugars, it is increased by light. The opposite regulation of PRR7 by light and sugars might seem contradictory, but the authors propose a model for how it could work. Following the light-activation of PRR7 at dawn, which decreases CCA1/LHY production, the increase of sugars due to photosynthesis decreases PRR7 levels, leading to the increased production of CCA1/LHY proteins. Later in the day as sugar levels decline, production of CCA1/LHY are decreased by PRR7 regulation, leading to the increase in TOC1 at dusk.

References:

1) Haydon et. al (2013) Photosynthetic entrainment of the Arabidopsis circadian clock. Nature. Free access via PubMed Central.

Posted in Cell Signalling, Plants | 2 Comments

Farewell lab bench, hello writing desk

IMG_3675Friday was my last day at the John Innes Centre. Although I’m happy to be moving on to new things, I’m going to miss the place and the wonderful people I’ve been working with over the last 5 years. In the past few weeks I have been preparing for my departure by wrapping up my experimental work, clearing out my lab bench, fridge and freezer drawers, and putting together a research paper to send to a journal.

Overall, I’m pretty pleased by what I’ve achieved research-wise over the last few years. I’ve tried lots of things, some of them worked, and I’ve got some interesting data that I’m still really excited about. I will definitely be keeping an eye on the field in future to see what happens next!

Friday was also my last day of being a “Research Scientist”. In September I start a new job with the life and biomedical sciences journal eLife as an Assistant Features Editor. I will be editing and writing magazine-style content including “eLife digests”, which are summaries of the journal’s research articles aimed at non-scientists. I’m really excited about the job as I will be working in science communication both within the scientific community and also to wider audiences.

I want to discuss my reasons for leaving science research because, unfortunately, some people consider it to be a negative thing, like an admission of failure or a waste of all that training. I am NOT leaving research because I dislike labwork, or even because I don’t feel that I’m good enough to stay. I’ve loved the research work I’ve been doing over the last few years, but I’ve also really enjoyed all the science communication activities I’ve been involved in. I’ve decided to leave research because in the long-term, I think I will find a career in science communication more fun and rewarding. Far from being a waste, the understanding of science I’ve gained during my research training will help me communicate the workings and findings of science to wider audiences.

What does my new job mean for this blog? Don’t worry, it will be business as usual here as I intend to continue posting regular articles about plants and microbes. Given that I will be more exposed to a wider variety of life science areas, it is possible that I may stray to other topics from time-to-time to write about other interesting things I come across. However, I will retain a focus on plants and microbes because I still love plants, and I intend to continue learning about them in my spare time. Also, I think plants and non-medical microbes (those that don’t cause animal diseases) tend to receive less attention online than they perhaps deserve. If this blog contributes even a little to raising the profile of some of these organisms then I’ll be very pleased.

 

Posted in Life, Science Communication, the universe and everything else | 9 Comments

Sunflowers are turning heads

Sunflowers in Fargo, North Dakota, USA. Image released into the public domain by the United States Department of Agriculture.

Sunflowers in Fargo, North Dakota, USA. Image released into the public domain by the United States Department of Agriculture.

It is August and the sun is shining here in Norwich*, so what better plant to be the Organism of the Month than the sunflower?

Sunflowers are the subjects of some of Vincent Van Gogh’s most famous paintings, but they are cultivated for more than just their beauty. Sunflower oil, made from compressing sunflower seeds, is commonly used in cooking and to make biodiesel. It is the 4th most highly consumed oil in the World (behind palm, soybean and oil seed rape) (1). The seeds are also edible and are promoted as good sources of some vitamins, minerals and cholesterol-lowering compounds.

Sunflowers come from from the Americas and have been cultivated there for thousands of years (2). Native American tribes found a variety of uses for sunflowers. The seeds were ground up to make flour and the fleshy flower head used as a vegetable in cooking. Red, blue, purple and black pigments were extracted from the seeds to make dyes. The fibrous leaves and stems were used for the weaving of fabric and baskets. Sunflowers even had a role in worship of the Aztec’s Sun God. Although sunflowers first arrived in Spain in the early 16th century, it was Russia who first started producing sunflower oil commercially in the late 18th century (2). Russia remained the World’s largest producer of sunflower oil until the end of the 20th century, when it was overtaken by Argentina.

Image by Tim Bartel  via Wikimedia Commons. Licensed under CC BY-SA 2.0.

Image by Tim Bartel via Wikimedia Commons. Licensed under CC BY-SA 2.0.

Sunflowers (latin name Helianthus annuus) belong to the Daisy (Compositae/Asteraceae) family  of plants. The members of this family have what are known as compound flowers, where the flower heads are actually made up of many individual tiny flowers. The centre of a sunflower flower head is full of black florets that produce the sunflower seeds. Around the outside of the flower-head is a set of ray florets, which each have a single large yellow petal. The ray florets are infertile and their main function is to attract insect pollinators.

If you observe sunflowers over the course of the day, you will notice that the flower heads move to track the sun in the sky overhead. In the morning they face East for sunrise and then slowly change position so that by sunset they face West. Overnight, the flower heads reposition to face East again. How this process, known as heliotropism, works in sunflowers is a bit of a mystery. Since the flower heads stop tracking the sun when they mature, it has been suggested that the turning of the flower head might be due to localized growth of cells on one side of the plant stem near the flower head.

How is heliotropism in sunflowers regulated? It is assumed that light-sensing plays a role, but it isn’t the only factor because flower heads still track East to West on a cloudy day when the sun isn’t visible. Also, if you rotate sunflowers by 180 degrees overnight so that the flower heads face West at dawn instead of East, the flower heads continue to move the way they would have done in the old position. It takes a few days for them to change their movement to track the sun again. This indicates that the day-night cycle of flower head movement in sunflowers is mostly regulated by the plants own internal (circadian) clock, with light-sensing making adjustments when needed.

Along with not understanding HOW heliotropism in sunflowers works, we don’t understand WHY it is beneficial. One possibility is that the increase in light falling on the flower head leads to increased photosynthesis (plant food production using light energy) in the flower head. Another possibility is that by tracking the sun, the flower heads heat up more over the course of the day, which could help them attract insect pollinators, and/or increase the speed of seed formation.

* Or at least it was when the author wrote this. Sunshine is not guaranteed, especially if you live in the UK!

References:

Posted in Organism of the Month, Plants | Tagged , , | 10 Comments

My PhD in pictures

IMG_3736

Image by the author’s mother

Last Friday I donned a strange-looking outfit to graduate with my PhD. I had a lovely day with my family, and many of my friends who came back to Norwich to graduate. Despite having a certificate (and photographic evidence), it is still quite hard to believe that my PhD is now well and truly finished.

The last 5 years have been a mix of fun, exciting, busy, challenging and frustrating times. Overall, the good times more than outweigh the bad and I have learnt loads about science and myself along the way, so the experience was definitely worth the hard work.  To mark the occasion, I have put together some photos that represent my PhD journey. Continue reading

Posted in Life, the universe and everything else | 4 Comments

Leishmania parasites: neglected tropical killers

Later this year I will be visiting The Gambia in West Africa to work on a GirlGuiding community project. Before I go I will need to have some vaccinations to protect me from several of the diseases found there. Unsurprisingly, this has rekindled my interest in tropical diseases. This week, I have strayed from my usual subjects to write about the leishmaniases, a group of tropical diseases found across Africa, Asia and the Americas. Included in the World Health Organisation’s (WHO) list of Neglected Tropical Diseases, the leishmaniases are a big burden on public health services in many countries, and were responsible for an estimated 50,000 deaths worldwide in 2010 alone.

Leishmaniases are caused by parasitic single-celled organisms of the genus Leishmania. The parasites are carried between mammal hosts by several species of blood-sucking sandflies. The parasites live within the mouthparts of the sandflies and can be transmitted to mammals when the sandflies bite. The parasites exist in different forms in sandflies and mammalian hosts. In the sandfly, Leishmania cellsbecome promastigotes and move around the insect gut using string-like structures called flagella. Within a mammal host, the parasites “hide” within host immune cells (called macrophages) and lose their flagella to become amastigotes.

Leishmania parasite lifecycle requires sandfly and mammalian hosts. This image is a work of the Centers for Disease Control and Prevention (US Federal Government). The image is in the public domain (via Wikipedia).

The Leishmania parasite lifecycle requires both sandfly and mammalian hosts. This image is a work of the Centers for Disease Control and Prevention (US Federal Government). The image is in the public domain (via Wikimedia Commons).

Continue reading

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Sabotage of plant cell communication by invading bacteria

Leaf speck on a tomato leaf caused by Pseudomonas syringae infection. Image by Alan Collmer via Wikimedia Commons (CC0).

Leaf speck on a tomato leaf caused by Pseudomonas syringae infection. Image by Alan Collmer via Wikimedia Commons (CC0).

To protect themselves from infection by disease-causing microbes, plants have systems that detect potentially harmful microbes and activate defence responses. Disease-causing microbes can overcome these defences by producing proteins called effectors that can enter host plant cells and disrupt them. Understanding what these effectors do in host plants could be useful for the development of more disease-resistant crop plants. Unfortunately, the roles of many effector proteins are not yet understood.

One of the ways effector proteins can interfere with plant defence responses is to prevent the relay of danger messages from the site of microbe detection at the plasma membrane to other locations in the cell. For the signal relays to function, the various protein components need to be located in the right places in the cell (plasma membrane, cytoplasm, nucleus, vacuole etc.). The cytoskeleton, consisting of filaments of the protein actin, is required for this organisation and moves proteins contained within (or on) small membrane-bound structures called vesicles. Continue reading

Posted in Bacteria, Cell Signalling, Plants | 3 Comments

Making waves at the Plant Calcium Signalling meeting

Last week I went to the Plant Calcium Signalling Meeting in Münster, Germany. I really enjoyed the meeting and it was a great opportunity to get an update on the most recent research in the area.

I have a guest blog on Annals of Botany blog about my personal highlights of the conference click the link to read it.

If you haven’t seen it already, read the article I posted earlier this week about a new drought-tolerant barley variety that has been developed by some of my colleagues at the John Innes Centre in collaboration with researchers at the University of Jordan.

And don’t forget that the Organism of the Month here at Plant Scientist is the poppy. There are still loads in flower in the UK at the moment to take a look if you can. If you want to know more read Kirsty Jackson’s article.

 

Posted in Cell Signalling, Plants, Science Communication | Leave a comment