It’s Not Sunlight That Grows Leaves… Signals from the Phloem [Reading Science]

POSTECH Team Identifies Key Regulatory Mechanism Synchronizing Leaf and Vascular Bundle Development

The process by which plant leaves spread out broadly and form evenly distributed veins is not merely the result of simple growth, but rather the product of sophisticated signal regulation initiated within internal transport tissues, recent findings have revealed.

On February 2, the National Research Foundation of Korea announced that a research team led by Professor Hwang Ildoo at Pohang University of Science and Technology has identified a key mechanism by which protein signals originating from the phloem, the plant’s nutrient transport tissue, simultaneously regulate the overall leaf morphology and vascular bundle development.

Integrated Model of Leaf-Vascular Bundle Development by Vascular Bundle-Derived RNA Binding Protein through Regulation of miR165 Biogenesis.
Figure a. The JUL1 gene is expressed in the vascular bundles of young leaves, and the protein moves throughout the leaf, coexisting with pri-miR165a in the abaxial side.
b. JUL1 protein binds to the G-rich region of pri-miR165a, inhibiting secondary structure formation and thereby suppressing miR165 production.
c. In JUL mutants, miR165/166 is overproduced, leading to decreased HD-ZIP III activity and increased abaxial development.
Figure provided and described by Soyoung Park, Graduate Student at Pohang University of Science and Technology

The research team confirmed that the protein 'JUL1 (JULGI)', produced in the phloem, precisely regulates the production of the microRNAs 'miR165/166', which determine the upper and lower structure of leaves. Through this signal regulation, the shape of the leaf and the distribution of veins (vascular bundles) are balanced and properly formed.

Plants transport nutrients produced via photosynthesis in their leaves throughout the body via the phloem. Because of this, it has been expected that the development of leaves and vascular bundles would be closely linked, but the molecular mechanisms by which these two tissues are integrated have not been clearly elucidated until now.

Focusing on JUL1, previously known as a protein that suppresses phloem development, the research team discovered that this protein directly binds to the precursor RNA of miR165/166 on the lower side of the leaf, inhibiting its maturation. In effect, it acts as a 'traffic controller', simultaneously coordinating the vertical balance of the leaf and the arrangement of vascular bundles.

Experimental results showed that when JUL1 protein does not function normally, leaves curl downward severely or vascular bundles develop abnormally. Conversely, even when JUL1 is expressed in only specific tissues, abnormal plant morphology is restored, demonstrating that this protein exerts a 'non-cell-autonomous' effect, regulating not only the cells where it is produced but also surrounding tissues.

The research team explained that, through these findings, they have experimentally demonstrated for the first time that phloem-derived signals serve as a 'structural axis' determining leaf morphology. In particular, by presenting a new molecular regulatory mechanism in which a protein modulates the structure of RNA itself to suppress microRNA production, they have provided a solution to the long-standing challenge in plant biology of understanding 'integrated development between tissues'.

Professor Hwang Ildoo stated, "An integrated understanding of leaves, where photosynthesis occurs, and the phloem, through which photosynthetic products are transported, is essential for improving crop productivity. Based on this research, it will be possible to simultaneously optimize leaf morphology and transport capacity, paving the way for the development of customized crops resilient to environmental changes."

This research was supported by the Leader Research Program and domestic and international postdoctoral fellowships from the Ministry of Science and ICT and the National Research Foundation of Korea. The results were published online in the international journal Nature Communications on January 10.