Blueprint reveals how plants build a sugar transport route

Fig 1. Arabidopsis root tip showing the position of phloem cells along the length of the root. Credit: Pawel Roszak

A tiny region at the root tip has been found to be responsible for orchestrating the growth and development of the intricate network of vascular tissues that carry sugars through plant roots.

In an article published in Science today, an international team of scientists presents a detailed diagram of how plants build phloem cells, the tissue responsible for the transport and accumulation of sugars and starch in the parts of the plant that we collect (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.

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 develops is important for future applications in agriculture, forestry, and biotechnology, as it could reveal how to best transport this sugary energy to where it’s needed.

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

Plant roots continue to grow throughout the life of a plant. This phenomenon, known as indeterminate growth, means that the roots continually lengthen as they add new tissue to the end of the root, as if building an endless highway. A continuous file of specialized phloem cells running the length of the roots (analogous to a lane on a highway) delivers the main nutrient, sucrose, to the parts of the plant where it is needed for growth. To fulfill this vital role, the phloem tissue must grow and mature quickly so that it can provide sugars to the surrounding tissue, much like building a service road which must be completed in the first stage of building a ‘a multi-lane highway.

Blueprint reveals how plants build a sugar transport route

Fig 2. Development of the phloem to unicellular resolution. Credit: Pawel Roszak

The problem that has long puzzled plant scientists is how a single informative gradient of proteins is able to stage the phases of construction through all of the different specialized cell files (roadways) that are present in the roots. How a cell type reads the same gradient as its neighbors, but interprets it differently to stage its own specialized development is a question that plant scientists have strived to resolve.

Over the past 15 years, researchers from the Yrjö Helariutta teams at the University of Cambridge and the University of Helsinkidiscovered 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 building the file of phloem cells (the sugar transport pathway) in the model factory 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 specifically controls phloem development.

It is a mechanism that appears to help the phloem cell file speed up maturation by using its own machinery to interpret maturation signals. Dr Pawel Roszak, co-first author of the study and researcher at the Sainsbury Laboratory, University of Cambridge (SLCU), explains: “We have shown how the global signals in the root meristem interact with the type-specific factors. cell 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 scales over time, with early genetic programs inhibiting late genetic programs and vice versa, just as asphalt laying crews hand over construction to track painters in later ones. stages of highway 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.

The co-leader of the work, Prof. Yrjö Helariutta, said that his teams’ reconstruction of the stages from birth to terminal differentiation of the protophloem in the Arabidopsis root exposed the steps. 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 mapped cellular events from the birth of the phloem cell to its terminal differentiation into phloem sieve element cells. the genetic mechanisms that coordinate cell maturation and relate the timing of the genetic cascade to the major regulators of maturation in widely expressed meristems. The precise timing of the developmental mechanisms was critical for proper phloem development, with apparent “fail-safe” mechanisms integrated “to ensure transitions.”

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.

Complete characterization of the vascular structure of plants

More information:
Pawel Roszak et al, Cell-by-cell dissection of phloem development links a maturation gradient to cell specialization., Science (2021). DOI: 10.1126 / science.aba5531.

Provided by the University of Cambridge

Quote: Blueprint Reveals How Factories Are Building Sugar Transport Lane (2021, December 23) retrieved December 23, 2021 from

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