Why do plants wiggle? New study provides answers

In a new study, physicists from the United States and Israel may have gotten to the bottom of a quirky behavior of growing plants -- and a mystery that intrigued Charles Darwin himself during the later decades of his life.

For many humans, plants might seem stationary and even a little dull. But green things actually move a lot. If you watch a timelapse video of a sunflower seedling poking up from the soil, for example, it doesn't just shoot straight up. Instead, as the sunflower grows, its crown spins in circles, twists into corkscrews and, in general, wiggles around -- albeit very slowly.

Now, researchers co-led by Orit Peleg at CU Boulder and Yasmine Meroz at Tel Aviv University have discovered one role for these chaotic movements, also known as "circumnutations." In greenhouse experiments and computer simulations, the group showed that sunflowers take advantage of circumnutations to search the environment around them for patches of sunlight.

"A lot of people don't really consider the motion of plants because, as humans, we're usually looking at plants at the wrong frame rate," said Peleg, a co-author of the study and an associate professor in the BioFrontiers Institute and Department of Computer Science.

The team published its findings Aug. 15 in the journal Physical Review X.

The findings could one day help farmers to come up with new strategies for growing an array of crops in more efficient arrangements.

"Our team does a lot of work on social interactions in insect swarms and other groups of animals," said Chantal Nguyen, lead author and a postdoctoral researcher at BioFrontiers.

"But this research is particularly exciting because we're seeing similar dynamics in plants. They're rooted to the ground."

Darwin's cucumbers

Nguyen added that plants don't usually shift around like animals but, instead, move by growing in different directions over time. This phenomenon enchanted Darwin long after he returned from his voyage on the HMS Beagle, according to historical accounts.

In the 1860s, Darwin, who was then suffering from a range of ailments that limited his own mobility, spent days observing plants at his home. He planted seeds from cucumbers and other species, then traced how their crowns moved around from day to day -- the resulting maps look wild and haphazard.

"I am getting very much amused by my tendrils -- it is just the sort of niggling work which suits me," he wrote a friend in 1863.

Amused or not, Darwin couldn't explain why some of his tendrils twisted.

It's a mystery that has also perplexed Meroz, a physicist by training. One 2017 study pointed her in the right direction. In it, scientists led by the University of Buenos Aires grew lines of sunflowers under cramped conditions. They discovered that the plants naturally and consistently arranged themselves into a zig-zag pattern, almost like the teeth of a zipper. The arrangement likely helps the plants maximize their access to sunlight as a group.

Meroz wondered if plant wiggles could be the engine that drives such patterns in plant growth.

"For climbing plants, it's obvious that it's about searching for supports to twine on," said Meroz, a professor of plant sciences and food security. "But for other plants, it's not clear why it's worth it."

Here comes the sun

To find out, she and her colleagues grew five, one-week-old sunflowers in rows. Then, like Darwin before them, they mapped out how the plants moved over the course of a week.

Next, Nguyen and Peleg developed a computer program to analyze the patterns behind the sunflower growth. The researchers could also use their computer simulations to see what would happen if the sunflowers moved more or less -- in other words, if they wiggled haphazardly or in a slow and steady pattern.

If the digital plants didn't wiggle at all, the group discovered, they would all wind up all leaning away from each other in a straight line. If they wiggled too much, in contrast, they would grow in a random pattern. If they moved with just the right amount of randomness, however, the sunflowers formed that tell-tale zig-zag, which, in real life plants, provides a lot of access to sunlight. Nguyen explained that plants seem to circumnutate to find where the best light is coming from, then grow in that direction.

"When you add a little bit of noise into the system, it allows the plant to explore its surroundings and settle into those configurations that allow each plant to find maximum light exposure," she said. "That happens to lead to this nice zig-zag pattern that we see."

In future experiments, the researchers will test out how sunflowers grow in more complicated arrangements. Meroz, for her part, is glad to see plants get some credit for the movers and shakers they really are.

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CO2 worsens wildfires by helping plants grow

By fueling the growth of plants that become kindling, carbon dioxide is driving an increase in the severity and frequency of wildfires, according to a UC Riverside study.

The worldwide surge in wildfires over the past decade is often attributed to the hotter, drier conditions of climate change. However, the study found that the effect of increasing levels of carbon dioxide (CO2) on plants may be a bigger factor.

"It's not because it's hotter that things are burning, it's because there's more fuel, in the form of plants," said UCR doctoral student in Earth and planetary sciences and study author James Gomez.

This conclusion, and a description of the eight model experiments that produced it, have been published in Communications Earth & Environment.

To convert light into food in a process called photosynthesis, plants require CO2. Burning fossil fuels for heat, electricity, and transportation is adding increasing levels of CO2 into the atmosphere. Plants use the extra CO2 to make carbohydrates that help them grow, leading to an increase in biomass that burns.

Certainly, heat waves and drought occur more frequently in today's climate than they did 50 years ago. These are conditions that cause plants to wither and die. As they dry out and die, they burn more easily. The models accounted for these effects on plants, as well as for different types of plants, and for the increase in atmospheric CO2.

"Warming and drying are still important fire factors. These are the conditions that make the extra plant mass more flammable," said UCR professor of Earth sciences Robert Allen.

The models analyzed by the research team all assumed an idealized 1% per year increase in atmospheric CO2 concentrations since 1850. The idealized increase is meant to isolate the effects of the greenhouse gas on wildfire activity.

"These experiments are mainly looking at the contribution of CO2 to changes in wildfire activity," Gomez said. "That's the only thing that's changing in these models. Other drivers of climate change and wildfire activity do not change through time," Gomez said. "This includes, for example, changes in other greenhouse gases like methane, as well as changes in land use."

Seasons are still important factors in promoting wildfires, and fires still occur more often during "fire seasons." Dry, windy conditions help spread the flames faster, increasing the size of the burned area. "However, our study shows the increase in fires during hotter seasons is driven by fuel load rather than an increase in the number of what some consider 'fire weather' days," Gomez said.

This means megafires can often happen outside of what is considered fire season. As an example, the biggest wildfire on record in Texas, with more than a million acres burned, occurred this past February.

The researchers hope that their results inspire others to conduct additional studies of the factors driving the increase in wildfires. In addition, they hope that policymakers recognize the urgent need to decrease the amount of CO2 that people release into the atmosphere.

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Two of the Milky Way's earliest building blocks identified

Astronomers have identified what could be two of the Milky Way's earliest building blocks: Named "Shakti" and "Shiva," these appear to be the remnants of two galaxies that merged between 12 and 13 billion years ago with an early version of the Milky Way, contributing to our home galaxy's initial growth. The new find is the astronomical equivalent of archeologists identifying traces of an initial settlement that grew into a large present-day city. It required combining data for nearly 6 million stars from ESA's Gaia mission with measurements from the SDSS survey. The results have been published in the Astrophysical Journal.

The early history of our home galaxy, the Milky Way, is one of joining smaller galaxies, which makes for fairly large building blocks. Now, Khyati Malhan and Hans-Walter Rix of the Max Planck Institute for Astronomy have succeeded in identifying what could be two of the earliest building blocks that can still be recognized as such today: proto-galactic fragments that merged with an early version of our Milky Way between 12 and 13 billion years ago, at the very beginning of the era of galaxy formation in the Universe. The components, which the astronomers have named Shakti and Shiva, were identified by combining data from ESA's astrometry satellite Gaia with data from the SDSS survey. For astronomers, the result is the equivalent of finding traces of an initial settlement that grew into a large present-day city.

Tracing the origins of stars that came from other galaxies

When galaxies collide and merge, several processes happen in parallel. Each galaxy carries along its own reservoir of hydrogen gas. Upon collision, those hydrogen gas clouds are destabilized, and numerous new stars are formed inside. Of course, the incoming galaxies also already have their own stars, and in a merger, stars from the galaxies will mingle. In the long run, such "accreted stars" will also account for some of the stellar population of the newly-formed combined galaxy. Once the merger is completed, it might seem hopeless to identify which stars came from which predecessor galaxy. But in fact, at least some ways of tracing back stellar ancestry exist.

Help comes from basic physics. When galaxies collide and their stellar populations mingle, most of the stars retain very basic properties, which are directly linked to the speed and direction of the galaxy in which they originated. Stars from the same pre-merger galaxy share similar values for both their energy and what physicists call angular momentum -- the momentum associated with orbital motion or rotation. For stars moving in a galaxy's gravitational field, both energy and angular momentum are conserved: they remain the same over time. Look for large groups of stars with similar, unusual values for energy and angular momentum -- and chances are, you might find a merger remnant.

Additional pointers can assist identification. Stars that formed more recently contain more heavier elements, what astronomers call "metals," than stars that formed a long time ago. The lower the metal content ("metallicity"), the earlier the star presumably formed. When trying to identify stars that already existed 13 billion years ago, one should look for stars with very low metal content ("metal-poor").

Virtual excavations in a large data set

Identifying the stars that joined our Milky Way as parts of another galaxy has only become possible comparatively recently. It requires large, high-quality data sets, and the analysis involves sifting the data in clever ways so as to identify the searched-for class of objects. This kind of data set has only been available for a few years. The ESA astrometry satellite Gaia provides an ideal data set for this kind of big-data galactic archeology. Launched in 2013, it has produced an increasingly accurate data set over the past decade, which by now includes positions, changes in position and distances for almost 1.5 billion stars within our galaxy.

Gaia data revolutionized studies of the dynamics of stars in our home galaxy, and has already led to the discovery of previously unknown substructures. This includes the so-called Gaia Enceladus/Sausage stream, a remnant of the most recent larger merger our home galaxy has undergone, between 8 and 11 billion years ago. It also includes two structures identified in 2022: the Pontus stream identified by Malhan and colleagues and the "poor old heart" of the Milky Way identified by Rix and colleagues. The latter is a population of stars that newly formed during the initial mergers that created the proto-Milky Way, and continue to reside in our galaxy's central region.

Traces of Shakti and Shiva

For their present search, Malhan and Rix used Gaia data combined with detailed stellar spectra from the Sloan Digital Sky Survey (DR17). The latter provide detailed information about the stars' chemical composition. Malhan says: "We observed that, for a certain range of metal-poor stars, stars were crowded around two specific combinations of energy and angular momentum."

In contrast with the "poor old heart," which was also visible in those plots, the two groups of like-minded stars had comparatively large angular momentum, consistent with groups of stars that had been part of separate galaxies which had merged with the Milky Way. Malhan has named these two structures Shakti and Shiva, the latter one of the principal deities of Hinduism and the former a female cosmic force often portrayed as Shiva's consort.

Their energy and angular momentum values, plus their overall low metallicity on par with that of the "poor old heart," makes Shakti and Shiva good candidates for some of the earliest ancestors of our Milky Way. Rix says: "Shakti and Shiva might be the first two additions to the 'poor old heart' of our Milky Way, initiating its growth towards a large galaxy."

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Stellar winds regulate growth of galaxies

Galactic winds enable the exchange of matter between galaxies and their surroundings. In this way, they limit the growth of galaxies, that is, their star formation rate. Although this had already been observed in the local universe, an international research team led by a CNRS scientist has just revealed -- using MUSE, an instrument integrated into the European Southern Observatory's (ESO) Very Large Telescope -- the existence of the phenomenon in galaxies which are more than 7 billion years old and actively forming stars, the category to which most galaxies belong.

The team's findings, to be published in Nature on 6 December 2023, thus show this is a universal process.

Galactic winds are created by the explosion of massive stars.

As they are diffuse and of low density, they are usually hard to spot.

To see them, the scientists combined images of more than a hundred galaxies obtained through very long exposure times.

By studying magnesium atom emission signals, the team was also able to map the morphology of these winds, which appear as cones of matter perpendicularly ejected from both sides of the galactic plane.

In the future, the researchers hope to measure how far these winds extend and the quantity of matter they transport.

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Heat tolerant coral may trade fast growth for resilience

Algae living within the soft tissue of coral supply much of the energy needed by their hosts, and some symbiotic algae help coral withstand warmer water better than others. In a recently published study led by the University of Hawai'i at Manoa, researchers found that there was a tradeoff for corals dominated by the thermally sensitive algae -- they have higher growth, but only in cooler water.

"As the ocean continues to warm, understanding how symbionts and environmental factors affect coral growth and health will help predict reef futures and inform conservation interventions where coral stocks are selected for specific traits or symbionts," said Shayle Matsuda, a doctoral student at the Hawai'i Institute of Marine Biology in the UH Mānoa School of Ocean and Earth Science and Technology at the time of the research.

The study was co-led by Matsuda, now a postdoctoral fellow at the Shedd Aquarium, and Mariah Opalek, who conducted the experiment for her undergraduate thesis at UH Mānoa. The research team investigated whether rice corals hosting symbiotic algae that can tolerate warmer water may grow more slowly, which could impact survivorship and competition for space on the reef, compared to coral hosting symbionts that are more susceptible to bleaching when ocean waters warm.

Over a two-month study period, the researchers measured the growth of rice corals dominated by heat tolerant or heat sensitive symbiotic algae. Additionally, they tested growth across decreasing light levels to see if the tradeoff between growth and tolerance to warm water would be affected by light, which is a major driver of the distribution of these symbionts in Kāneʻohe Bay, Hawai'i.

"This research shows us the complexity of coral growth on a reef," said Opalek, who is now a grant support assistant at Kaua'i Community. "A coral's competitive advantage could be lost in a matter of a few degrees depending on what type of symbiont they associate with."

During the first month, when water temperatures were warmer, the symbiont present did not affect growth. However, over the cooler second month, corals with heat sensitive algae grew up to 77% faster than corals dominated by heat tolerant algae, and this growth advantage increased in higher light treatments, which correlates to shallower depths on a reef.

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Study links 'stuck' stem cells to hair turning gray

Certain stem cells have a unique ability to move between growth compartments in hair follicles, but get stuck as people age and so lose their ability to mature and maintain hair color, a new study shows.

Led by researchers from NYU Grossman School of Medicine, the new work focused on cells in the skin of mice and also found in humans called melanocyte stem cells, or McSCs. Hair color is controlled by whether nonfunctional but continually multiplying pools of McSCs within hair follicles get the signal to become mature cells that make the protein pigments responsible for color.

Publishing in the journal Nature online April 19, the new study showed that McSCs are remarkably plastic. This means that during normal hair growth, such cells continually move back and forth on the maturity axis as they transit between compartments of the developing hair follicle. It is inside these compartments where McSCs are exposed to different levels of maturity-influencing protein signals.

Specifically, the research team found that McSCs transform between their most primitive stem cell state and the next stage of their maturation, the transit-amplifying state, and depending on their location.

The researchers found that as hair ages, sheds, and then repeatedly grows back, increasing numbers of McSCs get stuck in the stem cell compartment called the hair follicle bulge. There, they remain, do not mature into the transit-amplifying state, and do not travel back to their original location in the germ compartment, where WNT proteins would have prodded them to regenerate into pigment cells.

"Our study adds to our basic understanding of how melanocyte stem cells work to color hair," said study lead investigator Qi Sun, PhD, a postdoctoral fellow at NYU Langone Health. "The newfound mechanisms raise the possibility that the same fixed-positioning of melanocyte stem cells may exist in humans. If so, it presents a potential pathway for reversing or preventing the graying of human hair by helping jammed cells to move again between developing hair follicle compartments."

Researchers say McSC plasticity is not present in other self-regenerating stem cells, such as those making up the hair follicle itself, which are known to move in only one direction along an established timeline as they mature. For example, transit-amplifying hair follicle cells never revert to their original stem cell state. This helps explain in part why hair can keep growing even while its pigmentation fails, says Sun.

Earlier work by the same research team at NYU showed that WNT signaling was needed to stimulate the McSCs to mature and produce pigment. That study had also shown that McSCs were many trillions of times less exposed to WNT signaling in the hair follicle bulge than in the hair germ compartment, which is situated directly below the bulge.

In the latest experiments in mice whose hair was physically aged by plucking and forced regrowth, the number of hair follicles with McSCs lodged in the follicle bulge increased from 15% before plucking to nearly half after forced aging. These cells remained incapable of regenerating or maturing into pigment-producing melanocytes.

The stuck McSCs, the researchers found, ceased their regenerative behavior as they were no longer exposed to much WNT signaling and hence their ability to produce pigment in new hair follicles, which continued to grow.

By contrast, other McSCs that continued to move back and forth between the follicle bulge and hair germ retained their ability to regenerate as McSCs, mature into melanocytes, and produce pigment over the entire study period of two years.

"It is the loss of chameleon-like function in melanocyte stem cells that may be responsible for graying and loss of hair color," said study senior investigator Mayumi Ito, PhD, a professor in the Ronald O. Perelman Department of Dermatology and the Department of Cell Biology at NYU Langone Health.

"These findings suggest that melanocyte stem cell motility and reversible differentiation are key to keeping hair healthy and colored," said Ito, who is also a professor in the Department of Cell Biology at NYU Langone.

Ito says the team has plans to investigate means of restoring motility of McSCs or of physically moving them back to their germ compartment, where they can produce pigment.

For the study, researchers used recent 3D-intravital-imaging and scRNA-seq techniques to track cells in almost real time as they aged and moved within each hair follicle.

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Hair-raising research: Scientists find surprising link between immune system, hair growth

Salk scientists have uncovered an unexpected molecular target of a common treatment for alopecia, a condition in which a person's immune system attacks their own hair follicles, causing hair loss. The findings, published in Nature Immunology on June 23, 2022, describe how immune cells called regulatory T cells interact with skin cells using a hormone as a messenger to generate new hair follicles and hair growth.

"For the longest time, regulatory T cells have been studied for how they decrease excessive immune reactions in autoimmune diseases," says corresponding author Ye Zheng, associate professor in Salk's NOMIS Center for Immunobiology and Microbial Pathogenesis. "Now we've identified the upstream hormonal signal and downstream growth factor that actually promote hair growth and regeneration completely separate from suppressing immune response."

The scientists didn't begin by studying hair loss. They were interested in researching the roles of regulatory T cells and glucocorticoid hormones in autoimmune diseases. (Glucocorticoid hormones are cholesterol-derived steroid hormones produced by the adrenal gland and other tissues.) They first investigated how these immune components functioned in multiple sclerosis, Crohn's disease and asthma.

They found that glucocorticoids and regulatory T cells did not function together to play a significant role in any of these conditions. So, they thought they'd have more luck looking at environments where regulatory T cells expressed particularly high levels of glucocorticoid receptors (which respond to glucocorticoid hormones), such as in skin tissue. The scientists induced hair loss in normal mice and mice lacking glucocorticoid receptors in their regulatory T cells.

"After two weeks, we saw a noticeable difference between the mice -- the normal mice grew back their hair, but the mice without glucocorticoid receptors barely could," says first author Zhi Liu, a postdoctoral fellow in the Zheng lab. "It was very striking, and it showed us the right direction for moving forward."

The findings suggested that some sort of communication must be occurring between regulatory T cells and hair follicle stem cells to allow for hair regeneration.

Using a variety of techniques for monitoring multicellular communication, the scientists then investigated how the regulatory T cells and glucocorticoid receptors behaved in skin tissue samples. They found that glucocorticoids instruct the regulatory T cells to activate hair follicle stem cells, which leads to hair growth. This crosstalk between the T cells and the stem cells depends on a mechanism whereby glucocorticoid receptors induce production of the protein TGF-beta3, all within the regulatory T cells. TGF-beta3 then activates the hair follicle stem cells to differentiate into new hair follicles, promoting hair growth. Additional analysis confirmed that this pathway was completely independent of regulatory T cells' ability to maintain immune balance.

However, regulatory T cells don't normally produce TGF-beta3, as they did here. When the scientists scanned databases, they found that this phenomenon occurs in injured muscle and heart tissue, similar to how hair removal simulated a skin tissue injury in this study.

"In acute cases of alopecia, immune cells attack the skin tissue, causing hair loss. The usual remedy is to use glucocorticoids to inhibit the immune reaction in the skin, so they don't keep attacking the hair follicles," says Zheng. "Applying glucocorticoids has the double benefit of triggering the regulatory T cells in the skin to produce TGF-beta3, stimulating the activation of the hair follicle stem cells."

This study revealed that regulatory T cells and glucocorticoid hormones are not just immunosuppressants but also have a regenerative function. Next, the scientists will look at other injury models and isolate regulatory T cells from injured tissues to monitor increased levels of TGF-beta3 and other growth factors.

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Scaling new heights with new research showing how plants can grow at altitude

A new study has found that plant species are adapted to the altitude where they grow by 'sensing' the oxygen levels that surround them.

Altitude is an important part of plant ecology with at least 30% of plant species diversity contained in mountains and climate change is leading to the retreat of alpine species and some crops to higher altitudes.

Research led by scientists at the University of Nottingham has identified a mechanism through which plants can sense atmospheric oxygen levels (that decrease with altitude) that will help to understand how plants live at high altitude. The work was carried out in collaboration with scientists in Spain and Ecuador and was funded by the Leverhulme Trust. Their findings have been published today in Nature.

Researchers analysed plants growing at low and high-altitude locations. The team, working in Nottingham, Ecuador and Spain was able to identify how oxygen-sensing controls the pathway of chlorophyll synthesis, permitting plants to match the levels of a key toxic chemical to surrounding oxygen levels.

Climate change is leading to the displacement of wild species and crops (for example coffee) to higher altitudes, this research offers new insights into the underlying genetic mechanisms controlling their ability to survive at different altitudes. This new understanding of the genetic changes plants go through at altitude could lead to approaches to help plant breeders enhance the capacity of crops to grow at higher altitudes.

The research was led by Professor Michael Holdsworth from the University of Nottingham in collaboration with Professor Karina Proaño at ESPE University in Sangolquí, Ecuador and Professor Carlos Alonso Blanco from the Spanish National Centre for Biotechnology CSIC.

Professor Holdsworth commented: "Altitude is a key component of ecology with different altitudes subjecting plants to changing environments, some components of which are fixed by altitude and others that are not. For life at high altitude, it was previously considered that plants need to adapt to many variables, including high UV light and lower temperatures usually present at high altitude but this study is the first time that perception of atmospheric oxygen levels has been shown to be a key determinant of altitude adaptation in plants. "

He continues: "Exploring this novel finding allowed us to show that atmospheric oxygen level is the key determinant of altitude perception. We define the molecular pathway through which oxygen-sensing results in an adapted phenotype and we find that distinct species of flowering plants are adapted to absolute altitude through conserved oxygen-sensing control of chlorophyll synthesis and hypoxia gene expression. Showing that this mechanism works in diverse species provides a new paradigm for plant ecology."

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Scientists solve 50-year-old mystery behind plant growth

A team of researchers led by UC Riverside has demonstrated for the first time one way that a small molecule turns a single cell into something as large as a tree. For half a century, scientists have known that all plants depend on this molecule, auxin, to grow. Until now, they didn't understand exactly how auxin sets growth in motion.

The word auxin is derived from the Greek word "auxein," meaning "to grow." There are two main pathways that auxin uses to orchestrate plant growth, and one of them is now described in a new Nature journal article.

Plant cells are encased in shell-like cell walls, whose primary layer has three major components: cellulose, hemicellulose, and pectin.

"Cellulose works like rebar in a high rise, providing a broad base of strength. It's reinforced by hemicellulose chains and sealed in by pectin," said UCR botany professor and research team leader Zhenbiao Yang.

These components define the shape of plant cells, resulting in sometimes-surprising formations like the puzzle-piece-shaped leaf epidermis cells that Yang has been studying for the last two decades. These shapes help tightly glue cells together and provide physical strength for plants against elements such as the wind. With everything locked so tightly by the cell walls, how is movement and growth possible?

One theory posits that when plants are ready to grow, auxin causes their cells to become acidic, loosening the bonds between components and allowing the walls to soften and expand. This theory was proposed half a century ago, but how auxin activates the acidification remained a mystery until now.

Yang's team discovered auxin creates that acidity by triggering the pumping of protons into the cell walls, lowering their pH levels. The lower pH activates a protein, expansin, appropriately named because it breaks down links between cellulose and hemicellulose, allowing the cells to expand.

The pumping of protons into the cell wall also drives water uptake into the cell, building inner pressure. If the cell wall is loose enough and there is enough pressure inside the cell, it will expand.

"Like a balloon, expansion depends on how thick the outsides are, versus how much air you're blowing in," Yang explained. "Lowering the pH in a cell wall can allow water outside of a cell to move in, fueling turgor pressure and expansion."

There are two known mechanisms by which auxin regulates growth. One is the pH lowering that Yang's team described. Another is auxin's ability to turn on gene expression in the nucleus of the plants' cells, which in turn increases the amount of expansion and other growth-regulating factors in the cell.

The latter mechanism also lowers the pH of the cell and facilitates growth. UC San Diego professor of cell biology Mark Estelle is a leading authority in this field. He discovered and researches this other mechanism.

"Dr. Yang's recent work represents a significant advance in our understanding of how auxin regulates cell expansion. It's been known that acidification of the extracellular space promotes cell expansion but it wasn't known how this happens," Estelle said. "It's exciting to see an old problem being solved."

It is an understatement to say that auxin simply "contributes" to plant growth. It is essential to nearly every aspect of a plant's growth and development, including aspects that are important to agriculture such as fruit, seed and root development, shoot branching, and leaf formation. Even the plant's correct responses to gravity and light depend on auxin to ensure roots head down while the shoots grow up toward light.

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