Wildflower Cells Reveal Mystery of Leaf's Structure

Dec 22, 2022

Confocal microscopic images of the developing spongy mesophyll in Arabidopsis thaliana taken at (a) 0, (b) 24 and (c) 72 hours of development. (See methods for details.) The black scale bar in each frame represents 50 μm. (d) Mesophyll tissue observed in a microcomputed tomography (microCT) scan of a mature Arabdidopsis leaf. The leaf has three orthogonal axes, the basal–apical (BA), medial–lateral (ML) and adaxial–abaxial (AdAb) axes. Leaf images are in the three planes orthogonal to these axes, i.e. the transverse (yellow), longitudinal (red) and paradermal (purple) planes, respectively. The paradermal slice is taken at the location of the dashed white lines drawn on the other slices, and the location of the transverse (longitudinal) slices are indicated by yellow (red) dashed lines on the paradermal slice.

In plants, the cells that form the internal structure of leaves start out as tightly compacted spheres in the early stages of leaf development. As the leaf develops and expands, these cells take on new shapes and loosen up. Yet the leaf's microstructure remains robust and intact.

A team of researchers—including a mechanical engineer, plant biologist, and applied physicist—has figured out how this happens. Doing so not only answers questions that have long baffled the plant world, but it could lead to the manufacturing of energy-producing photosynthetic materials. The results of their work appear in the Journal of the Royal Society Interface.

The middle layer of plant leaves is known as the spongy mesophyll, which is a porous network of cells where photosynthesis happens. In this process, carbon dioxide (CO₂) comes up through the bottom of the leaf, sunlight comes in through the top, and then the two interact within the middle layer of cells. In a leaf's early stages, the cells in this layer are nearly spherical and tightly packed together. However, if the cells stay this way, the light and the carbon dioxide have no room to interact. So the cells loosen up to make room to allow photosynthesis to happen. But in doing so, why doesn't the leaf lose its structure and break apart?

"The spongy mesophyll is able to develop into a very porous material, yet retain the properties of a solid," said Corey O'Hern, professor of mechanical engineering & materials science. "That's the paradox, that the leaf needs to create this labyrinthian structure of air space to allow diffusion of CO₂—but the leaf still has to remain mechanically stable."

To understand this counterintuitive process, O'Hern and the other researchers used images made with confocal microscopy of the cells in different phases of the leaf's development.

"We created a computational model to describe the shapes of individual cells and how much they stick to each other," O'Hern said. "Then we modeled the development of the spongy mesophyll by pulling on the tissue on all sides."

These studies included measuring the shapes of all cells and the porosity of the mesophyll (that is, how much of the material is made up of cells and how much is made up of air). The researchers charted the course of the cells' development from early to late stages of development and observed how the cells morph from tightly packed spheres to elongated and multi-lobed shapes.

They found that, rather than causing the leaf structure to break down, the cells spreading out maintained the leaf's structure. "What's happening is that the cells in the spongy mesophyll are still pushing outward, while the epidermal tissue in the leaf is keeping it inside," O'Hern said.

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Video: How a Desire to Lead Brought This Wheat Breeder to Canada

Gurcharn Singh Brar is a wheat breeder whose path meandered from the breadbaskets of Punjab, India, to the sprawling fields of the Prairies. In a candid conversation, Brar shared insights into his journey, the challenges faced, and the undying passion that fuels his quest for better crops.

It all began with a childhood rooted in the wheat fields of Punjab, where agriculture isn’t just a livelihood but a way of life. His fascination with wheat and its potential led him to pursue a bachelor’s degree in agricultural sciences at Punjab Agricultural University. It was during this time that he encountered the spectre of rust diseases, particularly stripe rust, which plagued the region’s wheat crops. Determined to combat this menace, he set his sights on a journey that would take him across continents.

Venturing abroad for his graduate studies, he found himself in Saskatchewan at the Crop Development Centre (CDC), working under the mentorship of renowned researchers like Randy Kutcher and Pierre Hucl. Here, he delved deep into the world of wheat genetics, focusing on stripe rust resistance — a quest that would shape his academic pursuits for years to come.

After completing his master’s and Ph.D. in six and a half years, he embarked on a professional journey that would see him traverse academia and research. From brief stints as a research officer to landing his dream faculty position at the University of British Columbia’s Plant Science program, his career trajectory was marked by a strong drive to make a difference in the world of wheat.

Despite the allure of British Columbia’s unique agricultural landscape, he found himself wanting to return to the vast expanses of the Prairies, where wheat reigns supreme. He recently returned to the Prairies and is the new wheat breeder at the University of Alberta in Edmonton.

“The opportunity to lead an established wheat breeding program at the University of Alberta was a dream come true. With the necessary resources and infrastructure in place, I’m excited to drive innovation and develop high-yielding wheat varieties tailored to the unique conditions of northern Canada,” he says.

Brar, one of Seed World Canada‘s 2024 Next-Gen Leaders, has become known for identifying novel sources of resistance to priority diseases and his efforts in developing wheat germplasm with multiple disease-resistant traits.

In addition to his groundbreaking research, Brar is committed to mentoring the next generation of agricultural scientists.

“I believe in nurturing talent and empowering students to pursue their passions,” he says. “Watching my students grow and thrive in their research endeavours is hugely rewarding.”

As he looks ahead, Brar’s vision for the future of wheat breeding is clear: “My number one target is to develop high-yielding wheat varieties adapted to the northern climates of Canada. By focusing on early maturity and strong straw traits, we can maximize yield potential while ensuring resilience to environmental challenges.”

His decision to also join the Prairie Recommending Committee for Wheat, Rye, and Triticale (PGDC) executive as member-at-large came from a desire to play an even more important role in the world of Canadian cereals.