Yes we CAM!
By Tegan Armarego-Marriott
You can’t kill a cactus
We all know that certain house plants tend to shrivel up and die the second you turn your back (I’m looking at you, ferns), while others can thrive despite weeks or even months without watering. Many of these water-wise survivor species, including cacti, aloe vera, agaves, many orchids and even pineapple, are known as CAM plants. CAM, which stands for ‘Crassulacean acid metabolism’, is the photosynthesis-related secret to these plants’ water-wise ways. New research by Susanna Boxall and colleagues at the University of Liverpool (Boxall et al. 2017) delves into the inner workings of the succulent CAM plant Kalanchoë fedtschenkoi (shown above).
CO2 and H2O (in the presence of light) makes O2 and sugar
Photosynthesis involves taking carbon dioxide from the atmosphere and fixing it into sugars. Because this fixation requires both CO2 and sunlight, a lot of plants spend the hottest part of their day with their pores, or stomata, wide open, which lets in the air, but simultaneously lets out a lot of water. CAM plants have found a way to cheat the system: they first take up CO2 in the dark of night and convert it into a carbon-containing organic acid, namely malate, that is stored inside the cell. Then, during the day, they close their stomata, convert the malate back into CO2, and our good-old classic photosynthesis equation can proceed in the sunlight!
It might sound simple, but such simplicity requires extreme planning and precise timing. At night, the plants rapidly convert CO2 into malate using the enzyme phosphoenolypyruvate carboxylase (PPC). In the day, however, the re-released CO2 must be used for photosynthesis, and not immediately turned back into malate in a futile cycle. This means that the hungry PPC enzyme must be efficiently turned off during the day and back on again at night.
The on-off switch
One way to turn an enzyme on or off might be to make it only when needed and destroy it when it isn’t (i.e. the protein is synthesized during the day and degraded at night). But it turns out that PPC is present day and night, at pretty much the same level, and with pretty much the same desire to work (specific activity). Another way could be to have a third party that controls enzyme activity, a method that can be particularly effective when this third party is the product of the enzymatic reaction. This method, feedback control, is used in the case of PPC, which is inhibited by the accumulation of its downstream product, malate.
Interestingly, how well malate inhibits PPC is in turn controlled by yet another player: PPC kinase, or PPCK. PPCK makes a change to PPC (phosphorylation), which alters the physical properties of the enzyme, allowing PPC to cope with a greater amount of malate before it’s inhibited. It’s basically the plant-enzyme equivalent of that little zip on your suitcase that extends the suitcase size. Except that while unzipping allows you to pack in a couple of extra socks, phosphorylation of PPC by PPCK makes it ten times less sensitive to malate.
How do we know PPCK is important?
Surprise-surprise! PPCK appears at night (facilitating maximal malate production), and disappears again during the day, suggesting that PPCK might play a major role in controlling the timed carbon metabolism of CAM plants. In order to test this theory, Boxall and her co-workers recently knocked down the levels of PPCK in the succulent plant Kalanchoë fedtschenkoi (pictured above). In their plants, lack of accumulation of PPCK at night corresponded to PPC being stuck in the dephosphorylated (zipped) position. Because of this, PPC was more rapidly inhibited by malate, meaning the plants could fix less CO2 and store less malate during the night. This made it clear that PPCK is necessary to optimize night-time fixation of CO2 in CAM plants.
The ingenuity of this system is that one PPCK enzyme can modify a whole lot of PPC. In turning over only a small amount of the regulatory PPCK, instead of a much larger amount of PPC, the plant saves time and energy, making control by PPCK an extremely energy-efficient and flexible solution.
Controlling the controller
If PPCK is controlling the activity of PPC, the obvious question to ask is — what controls PPCK? It’s long been known that plants, like most organisms, have an internal timekeeper. This circadian clock tracks the day and activates or deactivates certain processes depending on their need. This includes things such as breaking down storage compounds, like starch, in order to fuel growth at night. Or for example defining when enzymes like PPCK accumulate.
Surprisingly, Boxall and co-workers found that plants that didn’t accumulate PPCK at night also had disrupted accumulation of central parts of the clock itself. Which suggests that while the clock might control PPCK, PPCK, or more likely one of its metabolic consequences, might also be sending some sort of feedback to control the clock!
The future
Given the current context of climate change, and the proposed increases in global temperatures and droughts, interest in CAM plants and their water-saving method of carbon fixation has intensified. The work of Boxall and co-workers has underlined the importance of the enzyme PPC kinase and its temporal control, in optimising CAM carbon fixation.
While this work has moved us one step closer to understanding these organisms, it has also revealed an unexpected complexity, highlighting the need for continued research into CAM processes before we can fully comprehend and exploit their ways.
Interview with author Susanna Boxall
What’s the most amazing thing about CAM metabolism?
CAM is amazing because it is a special type of photosynthesis that allows plants such as succulents and cacti to thrive in deserts and regions prone to long term drought where there is very little water. CAM plants can be severely drought stressed and then bounce back when it rains.
What is a challenge and benefit of working with a species like K. fedtschenkoi?
The wonderful thing about working with species like Kalanchoë is that they have evolved such remarkable adaptations to their extreme environment, including CAM, that are not found in mainstream model species such as Arabidopsis, rice, wheat, and maize. A key challenge is that Kalanchoë are relatively large plants and they grow slowly compared to some other model plant species. This means our research takes a little longer to achieve exciting new outcomes when compared to research with species that achieve more rapid growth and swifter life cycles. One of the biggest challenges in the current academic and research climate is persuading grant funders and departmental heads that Kalanchoë research should not be judged on the same timescale as research from colleagues working with E. coli, yeast, fruit flies or even Arabidopsis. People just need to accept that some research takes longer to reach fruition, but that doesn’t make it any less valuable or interesting.
What do you find the most fascinating about the plant clock?
The complexity and intricacy of the way it controls metabolism in single cells, allowing reactions to occur at precise times and thus preventing futile cycling.
I’m also fascinated by the fact that metabolism affects the clock and the clock affects metabolism; I find it so elegant that everything is so beautifully interconnected!
Fun facts / further reading
Crassulacean metabolism is named after the plant it was first studied in: Crassula, a genus of succulent plants. The word comes from the latin crassus, meaning ‘thick’ after the leaves of the plant. Crassulacean is a bit of a mouthful, so feel free to remember the group as ‘Cacti, Aloe and More’. Or just go with CAM.
CAM metabolism is found in more than 35 plant families, including a wide range of flowering plants and even some ferns. It’s a type of convergent evolution, which means that this mechanism has arisen independently multiple times.
The Nobel Prize for Physiology or Medicine went to Jeffrey Hall, Michael Rosbash and Michael Young for their breakthroughs in understanding the inner workings of the circadian clock in fruit flies https://www.nobelprize.org/nobel_prizes/medicine/laureates/2017/press.html
To read more about the clock, and how it also controls things like flowering time, see Plants Get Jetlag Too: A Play on Flowering Time
Tegan Armarego-Marriott
Max Planck Institute of Molecular Plant Physiology
Potsdam, Germany
Armarego@mpimp-golm.mpg.de
ORCID: 0000–0002–8745–9468
Read the research paper on which this story is based:
Boxall, S.F., Dever, L.V., Knerova, J., Gould, P.D., and Hartwell, J. (2017). Phosphorylation of Phosphoenolpyruvate Carboxylase is Essential for Maximal and Sustained Dark CO2 Fixation and Core Circadian Clock Operation in the Obligate Crassulacean Acid Metabolism Species Kalanchoë fedtschenkoi. Plant Cell. Published on September 13, 2017. DOI: https://doi.org/10.1105/tpc.17.00301
Photo credits:
Image of Kalanchoë fedtschenkoi courtesy of James Hartwell.
