The interactions between protons and calcium ions in this process were previously largely unknown. An article in the journal Science by a team led by biophysicist Professor Rainer Hedrich from Julius-Maximilians-Universität (JMU) Würzburg in Bavaria, Germany, has now shed new light on this subject.

Using a sophisticated optogenetic approach, the researchers have discovered a previously unknown endogenous acid sensor in plant cells. And they have discovered in the guard cells of leaves that there is a calcium store that plays an important role in processing proton signals in cellular responses.

Why Such Simple Elements as Protons and Calcium Ions Act as Signals

In the course of evolution, cells have designed their metabolism to utilise energy-rich phosphates. This results in a problem: at the predominantly neutral cellular pH value, the valuable phosphates can be bound by calcium ions (Ca²⁺) and converted into an insoluble and therefore no longer usable form (calcium dihydrogen phosphate).

To avoid this, cells keep their internal calcium level very low; in their environment, however, it is 10,000 times higher. Outside the cells, the concentration of protons (H⁺) and therefore the acidity is also much higher. Due to this concentration gradient, both types of ions have a strong urge to flow into the cells — making them ideal for use as messenger substances.

“The stimulus-dependent opening of calcium and proton channels in the cell membrane results in a temporary intracellular increase in both messenger ions,” explains Rainer Hedrich. “The cells understand this as a signal, which they translate into a biological reaction using calcium- and proton-binding enzymes.”

Light Switch Controls the Flow of Protons Into the Cell

How do plant cells react to the influx of protons and the associated acidification of their cell plasma? Until now, this could only be investigated with great experimental effort and even then only indirectly.

This is now much easier thanks to an appropriately equipped thale cress (Arabidopsis thaliana), which Hedrich’s team has developed using optogenetic methods: A light-sensitive proton channel from a fungus, the channelrhodopsin KCR2, was optimised for use in plant cells. This means that protons can now be specifically sent into the cells in response to a light pulse.

Furthermore, they expressed KCR2 together with the genetically encoded pH reporter pHuji. This makes it very easy to measure the current pH value in the cell upon KCR2 activation.

Shouguang Huang, the first author of the Science publication, next scrutinised the guard cells of the new Arabidopsis mutant. “When I stimulated them with blue light for a second, they depolarised, just as I had expected from a light-activated proton channel,” says the researcher. During the subsequent experiments, the Würzburg ion channel specialists made a far-reaching discovery.

KCR2 Activation Acidifies the Cell and Causes Calcium to Rise

Their electrophysiological studies on guard cells showed that when the light stimulation began, the membrane potential immediately depolarised and the pH reporter pHuji signalled an acidification of the cell interior.

“However, we were astonished when the depolarisation and acidification continued for a good minute after the end of the light pulse,” says Hedrich. “This could only mean that the light activation of KCR2 and the acidification had activated the sphincter cell’s own ion channels.” These are the long-known guard cell anion channels SLAC1 and SLAH3, whose activation, however, also requires the presence of calcium.

Endoplasmic Reticulum as a Calcium Store

“Taking all the facts together, it could be assumed that the proton currents carried by KCR2 and the associated acidification of the cell interior must also have generated a calcium signal,” summarises the JMU professor.

His team was able to prove that the rapid acidification of the guard cells is followed by a calcium signal that lasts for 150 to 200 seconds. And they discovered that this calcium does not come from outside the cell, but is released from an endogenous store, the endoplasmic reticulum. This is a network of membrane tubes and cisterns that run through the cytoplasm.

Future studies will now focus on analysing the molecular nature of the H+-sensitive calcium channel of the endoplasmic reticulum and investigating its proton-activated on/off switch. Overall, these studies are important in order to better understand how plant cells react to external stimuli such as infections or drought.

It was with this in mind that the European Space Agency (ESA) launched the Gaia mission ten years ago. Data collected by the Gaia satellite are opening up a window into the near Universe, providing astronomic measurements — such as position, distance from the Earth and movement — on nearly two billion stars.

At EPFL, the Standard Candles and Distances research group headed by Prof. Richard Anderson is aiming to measure the current expansion of the Universe and sees Gaia as a valuable tool. “Gaia increased by a factor of 10,000 the number of stars whose parallaxes are measured thanks to a massive gain in accuracy over its predecessor, the ESA Hipparcos mission,” he says. Today, scientists use parallaxes to calculate the distance to stars. This method involves measuring parallax angles, with the help of the satellite, through a form of triangulation between Gaia’s location in space, the Sun and the star in question. The farther away a star, the more difficult the measurement because parallax gets smaller the larger the distance.

Despite the resounding success of Gaia, the measurement of parallax is complex, and there remain small systematic effects that must be checked and corrected in order for Gaia parallaxes to reach their full potential. This is what scientists from EPFL and the University of Bologna, in Italy, have been working on, through calculations performed on over 12,000 oscillating red giant stars* — the biggest sample size and most accurate measurements to date.

“We measured the Gaia biases by comparing the parallaxes reported by the satellite with parallaxes of the same stars that we determined using asteroseismology,” says Saniya Khan, a scientist in Anderson’s research group and the lead author of a study published today in Astronomy & Astrophysics.

Stellar earthquakes

In the same way that geologists study the Earth’s structure using earthquakes, astronomers use asteroseismology, and specifically stars’ vibrations and oscillations, to glean information about their physical properties. Stellar oscillations are measured as tiny variations in light intensity and translated into sound waves, giving rise to a frequency spectrum of these oscillations.

“The frequency spectrum lets us determine how far away a star is, enabling us to obtain asteroseismic parallaxes,” says Khan. “In our study, we listened to the ‘music’ of a vast number of stars — some of them 15,000 light-years away!”

To turn sounds into distance measurements, the research team started with a simple fact. The speed with which sound waves propagate across space depends on the temperature and density of the star’s interior. “By analyzing the frequency spectrum of stellar oscillations, we can estimate the size of a star, much like you can identify the size of a musical instrument by the kind of sound it makes — think of the difference in pitch between a violon and a cello,” says Andrea Miglio, a full professor at the University of Bologna’s Department of Physics and Astronomy and the study’s third author.

Sophisticated analyses

Having thus calculated a star’s size, the astronomers then determined its luminosity and compared this figure to the luminosity perceived here on Earth. They coupled this information with temperature and chemical-composition readings obtained from spectroscopy and ran these data through sophisticated analyses to calculate the distance to the star. Finally, the astronomers compared the parallaxes obtained in this process with those reported by Gaia in order to check the accuracy of the satellite’s measurements.

“Asteroseismology is the only way we can check Gaia’s parallax accuracy across the full sky — that is, for both low- and high-intensity stars,” says Anderson. And the future of this field is bright, as Khan outlines:

“Upcoming space missions like TESS and PLATO intended to detect and survey exoplanets will employ asteroseismology and deliver the required datasets across increasingly large regions of the sky. Methods similar to ours will therefore play a crucial role in improving Gaia’s parallax measurements, which will help us pinpoint our place in the Universe and benefit a plethora of subfields of astronomy and astrophysics.”

In the sun-soaked islands in the Caribbean, however, drought conditions can occur much more rapidly, with warning signs appearing too late for mediation strategies to limit agriculture losses or prevent stresses on infrastructure systems that provide clean water to communities.

Such occurrences — known as flash droughts — are the focus of a new paper authored by Assistant Professor Craig Ramseyer of the College of Natural Resources and Environment and published in the Journal of Hydrometeorology. The paper’s finding is that Caribbean Islands are uniquely susceptible to sudden droughts, and Ramseyer advocates for alternative methodologies to more accurately measure dry conditions in the region.

“The tropics have extremely intense solar radiation, so atmospheric processes tend to be expedited,” said Ramseyer, who teaches in the Department of Geography. “Despite often receiving daily rainfall, island ecosystems are particularly vulnerable to drought conditions.”

Ramseyer, whose research focuses on tropical rainfall and severe weather impacts in the Caribbean, utilized a new drought index that considers the atmospheric demand for moisture to identify drought risk conditions instead of more traditional soil moisture measurements.

“This new drought index is really developed to try to identify the first trigger of drought by focusing on evaporative demand,” said Ramseyer, who collaborated on the paper with Paul Miller ’12, M.S. ’14, an assistant professor at Louisiana State University. “Evaporative demand is a measure of how thirsty the atmosphere is and how much moisture it can collect from soil or plant matter.”

Ramseyer, who received funding for this research through a grant from the National Oceanic and Atmospheric Administration’s Climate Program Office, stressed that identifying drying conditions earlier is a key step to limiting the impacts of droughts.

“A lot of drought observation is based on soil moisture, but in tropical environments, a decline in soil moisture is a response to other things that have already happened so you’re further down in the chain of events,” he said. “We can mitigate a lot of losses in, say, agriculture, by being able to forecast sudden, anomalous increases in evaporative demand.”

The impacts of drought conditions extend beyond agriculture: Tropical ecosystems are also strongly impacted by dry atmospheric weather conditions, and access to fresh water is a necessity for both communities in the region and a tourism industry that is a central driver for economies in the Caribbean.

A new position for atmospheric research

To better understand how that interplay of meteorological patterns impacts drought conditions, Ramseyer utilized 40 years of data from a long-term ecological research project in the El Yunque National Forest. He found that flash droughts have routinely occurred in the Caribbean and that occurrences of drought are not limited to traditional dry seasons on the island.

“In terms of climate, Puerto Rico is situated at a crossroads, buffered on the west by the El Niño southern oscillation and by the cooler North Atlantic oscillation on the east,” said Ramseyer. “Because of that, Puerto Rico has a unique geography for researching atmospheric changes.”

The looming concerns over global warming have only accelerated the need for meteorologists to better understand drought occurrences in the Caribbean and enhance monitoring of moisture conditions in the region.

“A warming planet results in more moisture available in the atmosphere overall, which means that the kinds of short-term precipitation events common to the Caribbean will increase in intensity,” said Ramseyer. “Meanwhile, droughts are becoming higher in magnitude, so climate change is altering both extremes.”

Ramseyer, who helped secure Virginia Tech’s membership in the University Corporation for Atmospheric Research this year, said developing clearer criteria for flash drought conditions is an important first step toward addressing the infrastructure challenges that Caribbean communities are likely to face.

“The key current and future issue for the Caribbean is all about finding a way to capture rainfall successfully and draw it out slowly to mitigate evaporation losses,” said Ramseyer. “Puerto Rico and all of the Caribbean have water infrastructure challenges that must be addressed to accommodate these trends.”

Geography department chair Tom Crawford said Ramseyer’s paper reflects a utilization of big data in tackling climate and meteorological challenges.

“Dr. Ramseyer’s research applies advanced computing and geospatial science to make significant contributions to the problem of flash droughts and precipitation variability broadly,” said Crawford. “In addition to his research impact, his course on Climate Data Analysis and Programming is training the next generation of researchers on cutting edge computational techniques applied to the changing climate.”

Ramseyer advocates for additional research into understanding the relationship between flash drought events and economic losses and how future drought events can be better communicated to stakeholders and communities.