Download Sieving for genes: Developmental regulation of important

Sieving for genes: Developmental regulation
of important plant phloem components
discovered
1 August 2014
far very little has been known about their
differentiation," says Professor Ykä Helariutta from
the Institute of Biotechnology, University of
Helsinki, Finland. "We've identified several genes
regulating the process and characterized it with
unprecedented precision."
The results of the collaboration between the labs of
Professor Ykä Helariutta and principal investigator
Eija Jokitalo are published in a pair of papers
appearing in Science and Nature Communications.
"Understanding how the phloem network develops
is a significant aspect of plant development and
could have important applications in biotechnology
and synthetic biology," describes Helariutta.
Programmed nuclear degradation during
phloem development
Cell file undergoing sieve element differentiation in root
as captured by msPI staining
Sieve elements are a key component of phloem,
the conductive tissue through which plants
transport carbohydrates and a wide range of
signalling molecules. Elongated cylindrical cells are
capped at one end by a sieve plate and arranged
end-to-end to form sieve tubes which in turn form a
network throughout a plant's body.
"Sieve elements are very special cells which play
an important role in carbon sequestration, yet so
Sieve elements lose their nucleus during the course
of normal development. In the first paper, the team
described how that happens and identified several
genetic factors controlling the process. They used
serial block-face scanning electron microscopy to
reconstruct a 3D model of developing sieve
elements from ultra-thin sections, enabling them to
track enucleation of these cells. The nucleus first
deforms from a smooth sphere to a crumpled
structure, before shrinking and loosing its contents
into the cytoplasm, where they are degraded. This
is coupled with the degradation of some organelles
and shape changes of others.
The researchers identified two transcription factors,
NAC045 and NAC086, which are expressed in
sieve element cells before enucleation. Plants
lacking both genes have defective sieve element
formation and die at the seedling stage. Serial
block-face scanning electron microscopy showed
that sieve elements in the double mutant do not
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undergo enucleation. Furthermore, by expressing
NAC45 in cells where it isn't normally found, the
researchers showed that it is sufficient to start the
degradation of the nucleus and cytoplasm.
Provided by University of Helsinki
The researchers also identified a family of genes,
dubbed NEN1-4, which act downstream of NAC045
and NAC086. Although the enucleation process
starts in plants with mutations in these genes, it
doesn't complete properly.
Control of choline transport essential in phloem
A genetic screen identified the CHER1 gene, which
encodes a choline transporter, as a crucial player in
phloem development. In mutant plants, a
fluorescent marker transported through the phloem
failed to unload in the root tip, demonstrating
defects in phloem transport. Further analysis
revealed that the phloem strands are not
continuous in the cher1 mutant, which also has
short roots, abnormal roots hairs, and changes in
the arrangement of the water-conducting xylem
tissues.
CHER1 accumulates at one end of sieve element
cells, collecting at the centre of the forming sieve
plate. Examination with serial block-face scanning
electron microscopy showed that mutant plants
have smaller sieve plates with fewer, structurallyaltered pores, inhibiting long-distance transport via
the phloem. "Control of choline transport is
essential to form continuously connected phloem
with proper sieve plates, but we still have to
uncover the exact cellular processes involved,"
says Helariutta.
More information: "Arabidopsis NAC45/86 direct
sieve element morphogenesis culminating in
enucleation." Kaori Miyashima Furuta, Shri Ram
Yadav, Satu Lehesranta, Ilya Belevich, Shunsuke
Miyashima, Jung-ok Heo, Anne Vatén, Ove
Lindgren, Bert De Rybel, Gert Van Isterdael, Panu
Somervuo, Raffael Lichtenberger, Raquel
Rocha,Siripong Thitamadee, Sari Tähtiharju, Petri
Auvinen, Tom Beeckman,Eija Jokitalo and Ykä
Helariutta. Science 1253736 Published online 31
July 2014. DOI: 10.1126/science.1253736
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APA citation: Sieving for genes: Developmental regulation of important plant phloem components
discovered (2014, August 1) retrieved 17 June 2017 from https://phys.org/news/2014-08-sieving-genesdevelopmental-important-phloem.html
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