Elucidation of the Mechanism of Cell Wall Pectin Synthesis in Growing Plants is critical to understanding Plant Terrestrialization and Evolution
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.
●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.
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).
One of the most characteristic features of plant cells is the presence of cell walls. Cell walls support the growth and shaping of plant cells, and have two important properties, namely flexibility and toughness. Plant cell walls are flexible as they stretch during cellular growth. They are also tough when you consider the direction of plant growth, since plants grow their shoots vertically against the pull of gravity. Plant cell walls are composed of various kinds of polysaccharides, such as cellulose, pectin, and hemicellulose.
Among these, pectin is actively synthesized during cellular growth. Since pectin is abundant in intercellular spaces, it has been proposed to play a role in cell adhesion. Pectin is also used as a gelling agent in food. It is a familiar, plant-derived staple ingredient that supports our daily life, such as in the manufacture of jams.
To elucidate the function of pectin, which is important in both plants and humans, researchers have long sought to identify the enzyme involved in pectin synthesis, in order to further elucidate the mechanism of pectin synthesis.
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.
1. Plant cell wall: A structure surrounding plant cells, mainly composed of polysaccharides, including cellulose, hemicellulose, and pectin, as well as lignin, a complex organic polymer. The cell wall is divided into two types, namely primary and secondary cell walls. The primary cell wall is rich in pectin and is synthesized while the cell is growing. The secondary cell wall is rich in cellulose and lignin, and thickens after the cell has stopped expanding.
2. Pectin: Abundant in the primary cell wall and the intercellular spaces. Pectin plays a key role in cellular growth and cell adhesion. It has a complex structure consisting of 13 different types of saccharides, including galacturonic acid and rhamnose. Pectin is made up of three domains: rhamnogalacturonan-I, rhamnogalacturonan-II, and homogalacturonan. Pectin forms a gel under specific conditions.
3. Glycosyltransferase: A group of enzymes that transfers a sugar moiety from one molecule to another to synthesize carbohydrate compounds such as polysaccharides. The structure of the sugar chain is determined by a specific glycosyltransferase.
4. Gene family: A set of genes that are similar in their nucleotide sequence and function. Gene families emerge in the evolutionary process. The number of gene families also increases due to gene duplications.
5. Terrestrialization of plants: The colonization of land by ancestral aquatic plants in the evolutionary process. About 500 million years ago, green algae evolved into charophycean green algae, which are considered the most primitive plant that adapted to life on land.
Title of Paper
Title: Pectin RG-I rhamnosyltransferases represent a novel plant-specific glycosyltransferase family
Authors: Yuto Takenaka (Ritsumeikan University), Kohei Kato (Ritsumeikan University), Mari Ogawa-Ohnishi (Nagoya University), Kana Tsuruhama (Ritsumeikan University), Hiroyuki Kajiura (Ritsumeikan University), Kenta Yagyu (Ritsumeikan University), Atsushi Takeda (Ritsumeikan University), Yoichi Takeda (Ritsumeikan University), Tadashi Kunieda (Konan University), Ikuko Hara-Nishimura (Konan University), Takeshi Kuroha (Tohoku University), Kazuhiko Nishitani (Tohoku University), Yoshikatsu Matsubayashi (Nagoya University), and Takeshi Ishimizu (Ritsumeikan University)
Journal: Nature Plants 4, 669-676 (2018)