Life science
Agriculture

Mechanisms of phloem formation in plants
Department of Biological Sciences, Graduate School of Science
Professor Tatsuo Kakimoto			

https://researchmap.jp/read0079052?lang=en

Assistant Professor Pingping Qian

https://researchmap.jp/qianpingping2013?lang=en

Life science

Abstract
Phloem is an essential tissue for plants, transporting products ofphotosynthesis and other substances. However, the mechanisms
controlling its formation were not well understood. We found thatmultiple Dof-type transcription factors expressed in phloem precursor cells are necessary and sufficient to form phloem cells. Phloem
Dofs not only induce genes necessary for phloem formation, but also
induce secretory peptide molecules CLE25, 26, and 45, which inhibit the formation of constituent cells of the phloem. We revealed that
CLE25, 26, 45 are perceived by the BAM receptor-CIK co-receptor
complex, reducing phloem-Dof and preventing the cells surrounding
the intended phloem formation site from becoming phloem.

▍ Background & Results
The phloem consists of a pathway for transporting photosynthetic
products called sieve tubes and companion cells, which assist in
the metabolic activities of the sieve tubes. The sieve tubes are comprised of cells called sieve elements, which have lost their nuclei andare connected in a row through small pores called sieve pores. Despite lacking nuclei, these cells remain alive and transport nutrients
through the sieve pores. Companion cells are attached to the sieve
tubes and are involved not only in loading and unloading nutrients
but also support the metabolic activities of sieve elements lacking nuclei through small pores known as plasmodesmata. How phloem formation is regulated, including master transcription factor for phloem
formation and regulatory mechanisms for phloem pattern formation,
were not known well. Initially, we selected candidate transcription
factors that were specifically expressed in the phloem. We overexpressed candidate genes and screened for those that could induce
phloem ectopically and identified a group of Dof-type transcription
factors (phloem-Dof) capable of inducing phloem even in otherwise
non-phloem locations. Phloem-Dofs could induce both sieve tubes
and companion cells, but the fate determination of whether they become sieve elements or companion cells remains a future challenge.
Subsequently, we analyzed the changes in the overall gene expression pattern when phloem-Dof genes were expressed throughout
the entire plant using microarray technology. The overexpression not
only increased the expression levels of genes known to be involvedin phloem formation but also induced peptide signaling molecules
CLE25, 26, and 45, which inhibit phloem formation. Disrupting theCLE25, 26, 45 genes or their receptor genes expanded the phloem
region. Additionally, it was discovered that the receptor activated by
CLE destabilizes the phloem-Dof proteins, exerting lateral inhibition
to prevent the surrounding cells from differentiating into phloem.

▍ Significance of the research and Future perspective
This study demonstrated the coordinated action of transcriptional regulation and intercellular communication creates the phloem
pattern. This study clarified the regulatory mechanisms of phloem
formation, enabling the manipulation of cells to increase or decrease the formation of phloem through mutations or genetic modifications. In the future, this could lead to technologies allowing forthe artificial control of phloem formation, potentially influencing thedistribution of nutrients in crops and other plants.
P a t e n t
T r e a t i s e Qian, Pingping et al. A Dof-CLE circuit controls phloem organization. Nature Plants 2022, 8, 817–827. doi: 10.1038/s41477-022-01176-0
URL https://kakimoto0.wixsite.com/kakimoto-lab/en
K e y w o r d phloem, Dof, transcription factor, peptide