How do plants build a sugar transport route?

The root of the plant grows indefinitely. The continued birth and maturation of cells in a gradient along the longitudinal root axis requires tissue-wide coordination of cell division with cell differentiation. Within the root, a single developing protophloem cell strand is surrounded by other tissues, each cell type differentiating at its characteristic rate.

Despite the communication of the protophloem cells with the surrounding environment, the development program of the phloem within the plant vascular system is accelerated compared to certain types of surrounding cells.

The phloem is a highly specialized vascular tissue that forms an interconnected network of continuous strands throughout the body of a plant. It carries sugars, nutrients, and a range of signaling molecules between leaves, roots, flowers, and fruits.

As a result, the phloem is central to plant function. Understanding how the phloem network is initiated and developed is essential for future agricultural, forestry and biotechnology applications. This could reveal how best to transport this sugary energy to need.

How do factories build a sugar alley in a multi-lane highway?

The problem that has long intrigued plant scientists is how a single informative gradient of proteins can stage the phases of construction through all the different specialized cellular files (roadways) present in the roots. Plant scientists have worked on how one type of cell reads the same gradient as its neighbors, but interprets it differently to stage its specialized development.

In an article published in Science, an international team of scientists presents a detailed diagram of how plants build phloem cells, the tissue responsible for transporting and accumulating sugars and starch in the parts of the plant we harvest (seeds , fruits and storage tubers) to feed much of the world.

This pivotal research reveals how global signals in root meristems coordinate distinct maturation phases of phloem tissue.

Diagram of the root of the plant, showing the different cell subtypes of phloem cells, which carry sucrose around the plant and ultimately give rise to starch in food. Phloem tissue arises from stem cells down the tapered end of the root and is then pushed upward as they mature along an assembly line. The researchers dissected the developmental trajectory of 19 cells of the phloem cells from birth to maturation in a path of 19 cell positions. The graph on the right shows a computational analysis of the single-celled states along the trajectory, where the “pinch points” mark dramatic developmental transitions. The research consisted of finding the genetic mechanisms that control developmental transitions and the regulatory “code” for phloem construction. Image credit: Science and Pawel Roszak

Over the past 15 years, researchers from Yrjö Helariutta’s teams at the University of Cambridge and the University of Helsinki have discovered the central role of cell-to-cell communication and the complex feedback mechanisms involved in vascular structuring. This new research, undertaken with collaborators at New York University and North Carolina State University, reveals how this single phloem cell pathway is constructed independently of surrounding cells.

The Sainsbury / Helsinki group dissected each step of the cell file construction of the phloem (the sugar transport pathway) in the model plant Arabidopsis thaliana using single-celled RNA-seq and live imaging. Their work showed how the proteins that control the broad gradient of root maturation interact with the genetic machinery that explicitly controls phloem development.

This mechanism appears to help the phloem cell file speed up maturation by using its machinery to interpret maturation signals. Pawel Roszak, co-first author of the study and researcher at Sainsbury Laboratory Cambridge University (SLCU), explains: “We have shown how the global signals in the root meristem interact with cell type specific factors to determine the distinct phases of phloem development at cell resolution. The use of cell sorting followed by deep, high-resolution single-cell sequencing of the underlying gene regulatory network has revealed a “flip-flop” mechanism of reciprocal genetic repression that triggers rapid developmental transitions.

The group also showed how phloem development is staggered over time, with early genetic programs inhibiting late genetic programs and vice versa, just as road asphalt laying crews hand over construction to track painters in later stages. stages of road construction. Additionally, they showed how the early phloem regulators instructed specific genes to divide phloem cells into two different subtypes, such as building a fork in the road leading to two separate destinations.

Co-head of the work, Yrjö Helariutta, said his teams’ reconstruction of the stages from birth to terminal differentiation of the protophloem in the Arabidopsis root laid out the stages. Helariutta said: “Wide maturation gradients interfacing with cell type specific transcriptional regulators to stage cell differentiation are necessary for phloem development.”

“By combining single-cell transcriptomics with live imaging, here we have mapped cellular events from the birth of the phloem cell to its terminal differentiation into phloem sieve element cells. This has enabled us to discover genetic mechanisms that coordinate cell maturation and relate the timing of the genetic cascade to the major widely expressed regulators of meristem maturation. The precise timing of the development mechanisms was essential for proper phloem development, with apparent “fail-safe” mechanisms to ensure the transitions. “

New York University researchers provided several key technical tools for the project. Kenneth Birnbaum, professor in the Department of Biology and the Center for Genomics and Systems Biology, and Dennis Shasha, professor of computer science at the Courant Institute of Mathematical Sciences, contributed to the analysis of single-celled RNA-seq data, developing the computational models that revealed genetic networks involved in the change of development programs, that is, team changes in the construction of highways. Birnbaum’s lab also shared long-term live imaging techniques that allowed the research team to “watch” phloem cells progress from birth to full maturation for days under the microscope. Long-term microscopy was able to show the coordination of the timing of “pathway division” and phloem enucleation events with the molecular state of cells.

“This is a golden age for discovery in biology with new tools that help us connect the molecular state of individual cells with real-time observations of the cell”, said Birnbaum. “These single-cell measurements and live imaging techniques are to the cell what satellite imagery is to the earth. Our phloem project shows the potential of these tools to understand the development of plant cells, especially those that are so crucial for human nutrition.

The researchers plan to further explore the evolution of these mechanisms and whether these steps are replicated in other regions of plants and other plant species.

Journal reference

  1. Pawel Roszak, et al., Cell-by-cell dissection of phloem development relates a maturation gradient to cell specialization. Science 24 December 2021 Vol 374, number 6575 DOI: 10.1126 / science.aba5531

Rachel J. Bradford