For plants, sunlight can be a double-edged sword. They need it to drive photosynthesis, the process that allows them to store solar energy as sugar molecules, but too much sun can dehydrate and damage their leaves. A primary strategy that plants use to protect themselves from this kind of photodamage is to dissipate the extra light as heat. However, there has been much debate over the past several decades over how plants actually achieve this.
Introverts take heart: When cells, like some people, get too squished, they can go into defense mode, even shutting down photosynthesis.
In a study recently published, a team at CU Boulder took advantage of a new microscopic technique to follow the lives of individual bacteria as they grew and divided in complex colonies.
The researchers discovered something unexpected in the process: Whenever these single-celled organisms, a type of cyanobacteria or blue-green algae, got too smushed, they began to switch off the machinery that was essential for them to turn sunlight into sugar.
“If a cell is between a rock and a hard place, internally everything says, ‘Yes, I have nutrients. I want to grow,’” said Cameron, also of the Renewable and Sustainable Energy Institute (RASEI) at CU Boulder. “But there also has to be a feedback that says, ‘I need to turn off photosynthesis so I don’t expand and rupture.’”
The findings, which appear in Nature microbiology, provide a new window on this process that sustains most life on Earth—and, in particular, how organisms regulate photosynthesis when the going gets tough. Cameron hopes that his team’s results will also help scientists to develop custom-made microbes that could one day turn light into electricity or even construct living buildings.
As Cameron explains, it all began almost by accident.
He launched this research several years ago with a deceptively simple goal in mind: He and his colleagues wanted to track the behavior of individual cells within a bacterial colony throughout their entire lifecycle.
That was a difficult feat—for simple-looking organisms, cyanobacteria can form pretty complex structures.
“The cells on the outside of a colony are exposed to a lot of light, while those on the inside have low light exposure,” Cameron said. “Over time as they grow and accumulate more cells, they shade themselves.”
To be able to see every single cell in a growing colony, Cameron developed a method of culturing cyanobacteria so that they spread out like flat pancakes. When he started peering at these two-dimensional growths under a microscope, he noticed something odd: The more the colonies grew, and the more the bacteria inside began to squeeze together, the more they began to glow, or “fluoresce,” under a certain type of light.
The microbes, he explained, were emanating heat out to their environment, almost like a person perspiring on a packed city bus.
“I dropped everything and spent the next four years figuring out what was happening,” Cameron said.
The key, he and his colleagues discovered, was that the cells in a colony weren’t all glowing the same. The cyanobacteria on the inside of a colony, for example, fluoresced a lot more than those on the fringes. They also grew a lot slower, dividing in two at about half the rate as their exterior cousins.
Put differently, when cells get squished, they shine.
“When the cells become confined, and they can’t expand, they become highly fluorescent,” Cameron said.
So why were those interior cells perspiring so much?
To answer that question, you have to get to know the phycobilisome. This teeny, protein-based structure is the antenna of the cell. Many of these phycobilisomes sit inside cyanobacteria where they collect sunlight, then transport it to the reaction sites where that energy can be converted into glucose, or sugar.
Or that’s what usually happens. Cameron and his colleagues found that when their cyanobacteria became too confined, they started to shed their phycobilisomes.
“If the cyanobacteria got more light than they could use for photosynthesis, these antennae would literally pop off,” Cameron said.
The microbes couldn’t use all that energy, so to keep from getting glutted, they turned off photosynthesis.
The group’s results, he said, show just how dynamic single-celled organisms can be: They have a lot of tools for staying healthy in a dynamic social environment.
Understanding that social environment could also one day help scientists put photosynthesis to work—tapping sunlight to design more sustainable buildings or make other biology-based tools.
“We might be able to develop small-scale machines that are using light to perform computations or work,” Cameron said.
It’s a whole new way to think about catching some rays.
Other coauthors on the new study include CU Boulder researchers Kristin Moore, a former graduate student in RASEI; Sabina Altus, a graduate student in the Department of Applied Mathematics; Jian Wei Tay, an image analysis specialist in RASEI; Janet Meehl, a professional research assistant in RASEI; Evan Johnson, lab manager; and David Bortz, an associate professor of applied mathematics.
Read the paper: Nature microbiology
Article source: University of Colorado at Boulder
Author: Daniel Strain
Image credit: Kristin A. Moore and Jeffrey C. Cameron
Photosynthesis, the process by which some organisms convert sunlight into chemical energy, is well known. But, it is a complex phenomenon, which involves a myriad of proteins. Now, scientists have uncover the location and functions of a new type of chlorophyll molecule for the first time.
A new study has identified a family of genes in cyanobacteria that help control carbon dioxide fixation. The discovery furthers our basic knowledge of photosynthesis. It also opens new doors to design systems for sustainable biotech production.
Another fantastic year of discovery is over – read on for our 2016 plant science top picks!
The year began with the publication of the fascinating eelgrass (Zostera marina) genome by an international team of researchers. This marine monocot descended from land-dwelling ancestors, but went through a dramatic adaptation to life in the ocean, in what the lead author Professor Jeanine Olsen described as, “arguably the most extreme adaptation a terrestrial… species can undergo”.
One of the most interesting revelations was that eelgrass cannot make stomatal pores because it has completely lost the genes responsible for regulating their development. It also ditched genes involved in perceiving UV light, which does not penetrate well through its deep water habitat.
Read the paper in Nature: The genome of the seagrass Zostera marina reveals angiosperm adaptation to the sea.
BLOG: You can find out more about the secrets of seagrass in our blog post.
Plants are known to form new organs throughout their lifecycle, but it was not previously clear how they organized their cell development to form the right shapes. In February, researchers in Germany used an exciting new type of high-resolution fluorescence microscope to observe every individual cell in a developing lateral root, following the complex arrangement of their cell division over time.
Using this new four-dimensional cell lineage map of lateral root development in combination with computer modelling, the team revealed that, while the contribution of each cell is not pre-determined, the cells self-organize to regulate the overall development of the root in a predictable manner.
Watch the mesmerizing cell division in lateral root development in the video below, which accompanied the paper:
Read the paper in Current Biology: Rules and self-organizing properties of post-embryonic plant organ cell division patterns.
In March, a Spanish team of researchers revealed how the anti-wilting molecular machinery involved in preserving cell turgor assembles in response to drought. They found that a family of small proteins, the CARs, act in clusters to guide proteins to the cell membrane, in what author Dr. Pedro Luis Rodriguez described as “a kind of landing strip, acting as molecular antennas that call out to other proteins as and when necessary to orchestrate the required cellular response”.
Read the paper in PNAS: Calcium-dependent oligomerization of CAR proteins at cell membrane modulates ABA signaling.
In April, we received an amazing insight into the ‘decision-making ability’ of plants when a Swiss team discovered that plants can punish mutualist fungi that try to cheat them. In a clever experiment, the researchers provided a plant with two mutualistic partners; a ‘generous’ fungus that provides the plant with a lot of phosphates in return for carbohydrates, and a ‘meaner’ fungus that attempts to reduce the amount of phosphate it ‘pays’. They revealed that the plants can starve the meaner fungus, providing fewer carbohydrates until it pays its phosphate bill.
Author Professor Andres Wiemsken explains: “The plant exploits the competitive situation of the two fungi in a targeted manner, triggering what is essentially a market-based process determined by cost and performance”.
Read the paper in Ecology Letters: Options of partners improve carbon for phosphorus trade in the arbuscular mycorrhizal mutualism.
The transition of ancient plants from water onto land was one of the most important events in our planet’s evolution, but required a massive change in plant biology. Suddenly plants risked drying out, so had to develop new ways to survive drought.
In May, an international team discovered a key gene in moss (Physcomitrella patens) that allows it to tolerate dehydration. This gene, ANR, was an ancient adaptation of an algal gene that allowed the early plants to respond to the drought-signaling hormone ABA. Its evolution is still a mystery, though, as author Dr. Sean Stevenson explains: “What’s interesting is that aquatic algae can’t respond to ABA: the next challenge is to discover how this hormone signaling process arose.”
Read the paper in The Plant Cell: Genetic analysis of Physcomitrella patens identifies ABSCISIC ACID NON-RESPONSIVE, a regulator of ABA responses unique to basal land plants and required for desiccation tolerance.
Sometimes revisiting old ideas can pay off, as a US team revealed in June. In 1930, Ernst Münch hypothesized that transport through the phloem sieve tubes in the plant vascular tissue is driven by pressure gradients, but no-one really knew how this would account for the massive pressure required to move nutrients through something as large as a tree.
Professor Michael Knoblauch and colleagues spent decades devising new methods to investigate pressures and flow within phloem without disrupting the system. He eventually developed a suite of techniques, including a picogauge with the help of his son, Jan, to measure tiny pressure differences in the plants. They found that plants can alter the shape of their phloem vessels to change the pressure within them, allowing them to transport sugars over varying distances, which provided strong support for Münch flow.
Read the paper in eLife: Testing the Münch hypothesis of long distance phloem transport in plants.
In July, an international and highly multidisciplinary team published the genome of 6,000-year-old barley grains excavated from a cave in Israel, the oldest plant genome reconstructed to date. The grains were visually and genetically very similar to modern barley, showing that this crop was domesticated very early on in our agricultural history. With more analysis ongoing, author Dr. Verena Schünemann predicts that “DNA-analysis of archaeological remains of prehistoric plants will provide us with novel insights into the origin, domestication and spread of crop plants”.
Read the paper in Nature Genetics: Genomic analysis of 6,000-year-old cultivated grain illuminates the domestication history of barley.
Another exciting cereal paper was published in August, when an Australian team revealed that C4 photosynthesis occurs in wheat seeds. Like many important crops, wheat leaves perform C3 photosynthesis, which is a less efficient process, so many researchers are attempting to engineer the complex C4 photosynthesis pathway into C3 crops.
This discovery was completely unexpected, as throughout its evolution wheat has been a C3 plant. Author Professor Robert Henry suggested: “One theory is that as [atmospheric] carbon dioxide began to decline, [wheat’s] seeds evolved a C4 pathway to capture more sunlight to convert to energy.”
Read the paper in Scientific Reports: New evidence for grain specific C4 photosynthesis in wheat.
September marked an historic event. Professor Stefan Jansson cooked up the world’s first CRISPR meal, tagliatelle with CRISPRy fried vegetables (genome-edited cabbage). Jansson has paved the way for CRISPR in Europe; while the EU is yet to make a decision about how CRISPR-edited plants will be regulated, Jansson successfully convinced the Swedish Board of Agriculture to rule that plants edited in a manner that could have been achieved by traditional breeding (i.e. the deletion or minor mutation of a gene, but not the insertion of a gene from another species) cannot be treated as a GMO.
Read more in the Umeå University press release: Umeå researcher served a world first (?) CRISPR meal.
*You may also be interested in the upcoming meeting, ‘New Breeding Technologies in the Plant Sciences’, which will be held at the University of Gothenburg, Sweden, on 7-8 July 2017. The workshop has been organized by Professor Jansson, along with the GPC’s Executive Director Ruth Bastow and Professor Barry Pogson (Australian National University/GPC Chair). For more info, click here.*
Phytochromes help plants detect day length by sensing differences in red and far-red light, but a UK-Germany research collaboration revealed that these receptors switch roles at night to become thermometers, helping plants to respond to seasonal changes in temperature.
Dr Philip Wigge explains: “Just as mercury rises in a thermometer, the rate at which phytochromes revert to their inactive state during the night is a direct measure of temperature. The lower the temperature, the slower phytochromes revert to inactivity, so the molecules spend more time in their active, growth-suppressing state. This is why plants are slower to grow in winter”.
Read the paper in Science: Phytochromes function as thermosensors in Arabidopsis.
In November, a Chinese team published the genome of Ginkgo biloba¸ the oldest extant tree species. Its large (10.6 Gb) genome has previously impeded our understanding of this living fossil, but researchers will now be able to investigate its ~42,000 genes to understand its interesting characteristics, such as resistance to stress and dioecious reproduction, and how it remained almost unchanged in the 270 million years it has existed.
Author Professor Yunpeng Zhao said, “Such a genome fills a major phylogenetic gap of land plants, and provides key genetic resources to address evolutionary questions [such as the] phylogenetic relationships of gymnosperm lineages, [and the] evolution of genome and genes in land plants”.
Read the paper in GigaScience: Draft genome of the living fossil Ginkgo biloba.
The year ended with another fascinating discovery from a Danish team, who used fluorescent tags and microscopy to confirm the existence of metabolons, clusters of metabolic enzymes that have never been detected in cells before. These metabolons can assemble rapidly in response to a stimulus, working as a metabolic production line to efficiently produce the required compounds. Scientists have been looking for metabolons for 40 years, and this discovery could be crucial for improving our ability to harness the production power of plants.
Read the paper in Science: Characterization of a dynamic metabolon producing the defense compound dhurrin in sorghum.
Another amazing year of science! We’re looking forward to seeing what 2017 will bring!
P.S. Check out 2015 Plant Science Round Up to see last year’s top picks!
Four eighth grade students from The Nueva School in Hillsborough, California are releasing “They Grow”, a scienceified version of the popular Drake song “Headlines” (warning: Headlines contains explicit language!). In the video the students rap about photosynthesis, starting with the basics and moving on to the intricate processes of the light reaction and the Calvin cycle.
With help from their science teacher, Tom McFadden, these four students wrote and performed their lyrics, then planned, shot and edited their music video. Tom’s “Science Rap Academy” class meets twice a week for an hour each Tuesday and Thursday. The students have been working on this project since January and are very excited to finally release it to the world. They hope that their video will help students better understand the complex process known as photosynthesis.
Watch the video:
More from the students:
“I think that this song and video that we have created will help provide students learning about photosynthesis with a fun, engaging and relatable way to learn how plants grow,” – Alex, coproducer and rapper.
“This song brings energy into the classroom while effectively communicating the perplexing process of photosynthesis. I think that this song will be engaging and entertaining, and was a blast to film,” – Stanley, coproducer and rapper.
“Our number one goal with this song was to make learning about science, specifically photosynthesis, fun to learn. I think that we were able to achieve that by scienceifing a popular song that many kids know, so they can really connect to it. We also made the video and song easy to follow and understand so people of all ages can learn from it,” – Jason, coproducer and filmmaker.
“This entire process was very fun, writing and singing our lyrics and filming the video. I’m really thrilled to share this final product that we have been working hard on to the world and I hope that people enjoy it,” – Quincy, coproducer and rapper.
It’s the ancient story of plant evolution: photosynthetic algae moved to damp places on land, eventually evolving more complex architecture, and spreading across almost all terrestrial habitats. To cope with the drier conditions, plants developed roots to absorb water, and vascular tissue to transport it; a waxy cuticle coating their surfaces to prevent evaporation; and microscopic pores called stomata that open to allow carbon dioxide to diffuse in for photosynthesis but close to prevent excessive water loss.
How, then, does eelgrass (Zostera marina) fit in to this tale? It’s a monocot descended from the flowering plants, but it has turned its back on dry land and returned to the sea; a rare feat that only appears to have happened on three occasions. The recent sequencing of the eelgrass genome has revealed several interesting insights into the dramatic genetic changes that have allowed it to adapt to what lead author Professor Jeanine Olsen described as, “arguably the most extreme adaptation a terrestrial (and even a freshwater) species can undergo.”
Sayonara to stomata
If you live in the sea, conserving water isn’t your main concern. Eelgrass was known to lack stomata, but genetic comparisons to other species, including its freshwater relative Spirodela polyrhiza, revealed the first surprise of the study: eelgrass has lost not only its stomata but also the genes involved in their development and patterning. “The genes have just gone, so there’s no way back to land for seagrass,” said Olsen.
A difference in defense
When angiosperms are attacked by herbivores or pathogens, their defense response typically involves the release of volatile secondary metabolites through their stomata. How can eelgrass release these compounds without stomata? The answer is: it doesn’t. The genome study found that eelgrass is missing crucial genes involved in making ethylene (an important hormone release in times of stress), as well as those responsible for producing non-metabolic terpenoids, which act to repel pests.
Selective pressures of the marine environment differ greatly from those of terrestrial habitats, so different pathways may be involved. Second, eelgrass has a wide repertoire of pathogen resistance genes, which suggests that it is exposed to a very different set of pathogens that may not respond to typical immune responses. Third, volatile secondary metabolites are often involved in attracting pollinators; this is not believed to be necessary in eelgrass, where submarine pollination occurs using the water itself.
Changing the cell wall
Eelgrass is subject to extremely salty conditions, and it’s had to adapt to osmotic stress. Unlike typical plant cell walls, eelgrass has engineered its cell wall matrix to retain water in the cell wall, even during low tide. This involves depositing sulfated polysaccharides and low methylated pectins in the cell wall matrix, but until its genome was sequenced no-one knew exactly how. It turns out that eelgrass has rearranged its metabolic pathways: “They have re-engineered themselves,” Olsen explains.
Living with a lack of light
Some species of Zostera can grow in water 50m deep, where light levels are reduced and shifted into a narrow wavelength range; ultraviolet (UV), red and far-red light have particularly low penetration after the first 1–2m of seawater. In a classic eelgrass ‘use it or lose it’ response, it has lost the UVR8 gene, which is responsible for sensing and responding to UV damage, as well as the phytochromes associated with red and far-red receptors. It does, however, retain the photosynthetic machinery, including photosystems I and II.
Unravelling angiosperm evolution
The recent eelgrass publication has revealed how this plant has either lost or adapted typical angiosperm traits to suit its needs, by ditching its stomata, volatile secondary metabolites and certain light sensing genes, or by altering the structure and function of the cell wall. It also developed adaptations that enable gas exchange, help pollen stick to submerged stigmas, and promote nutrient uptake.
Could these adaptations be useful in crop breeding? While a lack of defense compounds would probably be a step backwards, it would be extremely useful to understand how eelgrass copes with biotic stresses without them. Removing light receptors would also be problematic, but could eelgrass help us to develop crops that can grow in shaded conditions, perhaps in intercropping systems? What can we learn from eelgrass’ nutrient uptake and salt-tolerant adaptations?
Now that we have seen some of the secrets of eelgrass, how can we best make use of them?
Read the paper: The genome of the seagrass Zostera marina reveals angiosperm adaptation to the sea (Open Access)
Read the editorial: Genomics: From sea to sea (paywall)
Read the press release: Genome of the flowering plant that returned to the sea