A research group led by Ritsumeikan University is the first to elucidate the mechanism of cell wall pectin synthesis, a process that occurs in growing plants.

Highlights:

●Plant cells continue to grow while synthesizing their cell wall. Pectin is a major component of the plant cell wall.
●This new study discovered that glycosyltransferases are responsible for the synthesis of pectic backbone, thereby elucidating the mechanism of pectin synthesis.
●These glycosyltransferases belong to a novel gene family.
●This gene family emerged during the process of terrestrialization in plants. Pectin synthesis is a crucial step in terrestrialization.

Research Outline

A Ritsumeikan University research group led by Associate Professor Takeshi Ishimizu of the College of Life Sciences, in collaboration with researchers at Nagoya University, Konan University, and Tohoku University, have, for the first time, elucidated the mechanism by which plant cell wall¹ pectin² is synthesized.

The Ritsumeikan group included graduate student Kohei Kato, also of the College of Life Sciences, and postdoctoral researcher Yuto Takenaka of the Ritsumeikan Global Innovation Research Organization, among others.

Pectin, which is synthesized in growing plants, consists of sugars that are connected in a chain. The research group discovered that glycosyltransferases³ are responsible for synthesizing the pectic backbone. They also found that these enzymes belong to a novel gene family⁴. This gene family appears to have emerged during the terrestrialization of plants⁵. The research suggested that pectin synthesis is a key factor that contributed to successful terrestrial adaptation by plants.

This discovery has helped elucidate part of the developmental mechanism in plants. The results of the group’s study are applicable to the breeding of fast-growing crops. Pectin has been used as a gelling agent (thickening agent) in food additives. The newly discovered enzymes may also help develop a new type of gelling agent with novel functions.

This discovery was published in the September 2018 issue of Nature Plants, a sister journal to the British scientific journal Nature. The News & Views section of Nature Plants will cover this research project. This study was supported by Grants-in-Aid for Scientific Research (KAKENHI) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and the Japan Society for the Promotion of Science (JSPS), as well as the Ritsumeikan Global Innovation Research Organization (R-GIRO).

Pectin is a highly complex polysaccharide consisting of many kinds of sugars. Its structure was first identified about 30 years ago. However, the detection of pectin synthase has been a challenge for researchers. Research thus far has failed to identify the enzymes responsible for synthesis of the pectic backbone.

The Ritsumeikan group, together with colleagues and collaborators at other institutions, have established methods for detecting the activities of the enzymes involved in pectin synthesis over the course of years.

This study, in particular, was focused on a seed coat polysaccharide called mucilage, where pectin accumulates. Comprehensive analysis of genes expressed in the process of seed coat polysaccharide synthesis during seed development found the gene candidates for the enzyme. By applying an established method for detecting specific enzymatic activities, the Ritsumeikan group discovered four novel genes encoding the glycosyltransferase involved in synthesizing the pectic backbone in Arabidopsis thaliana.

Simultaneously, the same group also found that these enzyme-encoding genes belong to a novel gene family. Since this gene family can only be found in terrestrial, and not aquatic, plants, the Ritsumeikan group deduced that upward growth against gravity, a trait of terrestrial plants, was acquired because plants developed the ability to synthesize pectin. The acquisition of this gene family by an aquatic ancestor of terrestrial plants was a key process that contributed to the adaptation to life on land (Fig. 2).

This gene family is believed to be large, and to contain many other unidentified genes involved in the synthesis of pectic side chains. In other words, this discovery has opened the door to the elucidation of the complete mechanism of pectin synthesis, which has otherwise remained elusive for many years.

Since pectin synthesis is closely related to plant growth, breeding focused on the genes encoding pectin synthesis enzymes has the potential to generate fast-growing plant breeds. Increasing crop production is one of the promising future applications of this discovery. This discovery will also help elucidate the terrestrialization of plants in the evolutionary process; it will help answer a question of how pectin helps plant cells stick to each other and become tougher, allowing plants to defy gravity as they grow vertically.

Hence, this discovery will help us explore the fundamental nature of plants. Furthermore, since pectin is widely used in the food industry as a gelling agent, modifications of pectin’s structure will help in the development of a new gelling agent with novel functions.

Glossary