There is a picture in fig 1. The root tip shows the position of phloem cells. Credit: Pawel.
A small area at the root tip is responsible for orchestrating the growth and development of the complex network of tissues that transport sugars through plant roots.
A team of international scientists have published a paper in Science detailing how plants build their phloem cells, the tissue that transports sugars and starches to the parts of the plant that we harvest.
The research shows how global signals in root meristems coordinate different phases of phloem tissue.
A plant's phloem is a specialized tissue that forms a network of strands throughout the body. It transports sugars, nutrients and a range of signaling molecule between leaves, roots, flowers and fruits.
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 better transport this sugar energy to where it is needed.
How do plants build a road?
The plant's roots grow throughout its life. Incremental growth means roots continually grow as they add new tissues to the tip of the root. A continuous file of specialized phloem cells running the length of roots is what delivers the primary nutrition to the parts of the plant where it is needed for growth. To fulfill this vital role, phloem tissue needs to grow quickly so it can supply sugars to surrounding tissues, like building a service lane that needs to be completed in the first stage of constructing a multi-lane highway.
There is a fig 2. The phloem is at single-cell resolution. Credit: Pawel.
The problem that has puzzled plant scientists is how a single gradient of proteins can stage the construction phases across all the different cell files that are present in roots. Plant scientists are trying to figure out how one cell type can read the same gradient as its neighbors, but interprets it differently to stage its own specialized development.
The central role of cell-to-cell communication and complex feedback-mechanisms have been uncovered by researchers over the past 15 years. This new research shows how the single lane of phloem cells is constructed.
The phloem cell file in the model plant Arabidopsis was analysed by the group using single-cell RNA-seq and live images. Their work showed how the genes that control phloem development interact with the genes that control root maturation.
The phloem cell file appears to be helped by this mechanism to fast-track maturation using its own machinery. The study shows how global signals in the root meristem interact with the cell type specific factors to determine distinct phases of phloem development. A "seesaw" mechanism of genetic repression that leads to rapid developmental transitions was revealed by using cell sorting and single-cell sequencing.
The group showed how phloem development is staged over time, with early genetic programs disrupting late genetic programs and vice versa, just as road asphalt-laying work crews' hand over construction to lane painters in the latter stages of highway construction. They showed how early phloem regulators instructed specific genes to split the phloem cells into two different types, like the construction of a fork in the road.
Professor Yrj Helariutta said that his teams' reconstruction of the steps from birth to terminal differentiation exposed the steps. Helariutta said that phloem development requires broad maturation gradients with cell-type specific transcriptional regulators.
We have mapped the cellular events from the birth of the phloem cell to its terminal differentiation into phloem sieve element cells by combining single-cell transcriptomics with live images. We were able to uncover genetic mechanisms that coordinate cellular maturation and connect the timing of the genetic cascade to broadly expressed master regulators of meristem maturation. The precise timing of development mechanisms was important for proper phloem development.
The evolution of these mechanisms will be explored by the researchers, as well as whether these steps can be replicated in other regions of plants.
There is more information about the Cell-by-cell dissection of phloem development. www.science.org/doi/10.1126/science.aba5531
Science journal information.
The sugar transport lane is revealed in the blueprints of plants retrieved from thephys.org.
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