Crucial breakthrough in pectin biosynthesis

 Most people know pectin as a common household gelling agent in making jams and jellies, but its uses are vast. It has anticancer properties, for instance, and may have a role in important biological functions including plant growth and development and defense against disease.

Despite the importance of pectin as a major component in the primary walls of plants, scientists have known relatively little about how this family of complex polysaccharides is made. Especially perplexing has been how the synthesis of the three different classes of pectic polysaccharides is coordinated to produce the pectin matrix in cell walls.

Now, in what has been described as a "crucial breakthrough in pectin biosynthesis," a team of researchers at the University of Georgia has discovered a gene that encodes one of the proteins responsible for pectin synthesis. This is a powerful new molecular tool that could help researchers understand--and potentially manipulate--pectins.

The result, which arose from the use of biochemistry and bioinformatics to discover gene sequences, could be genetically altered pectins that might dramatically improve plant species' ability to fight disease and new pectins that could be specifically targeted to fight cancers in humans. (It has also been demonstrated that pectin lowers serum cholesterol levels.) While not the breakthrough that will allow immediate manipulation of total pectin biosynthesis, it is, one of the researchers involved said, "the first word of the Rosetta Stone that will show us the blueprint for pectin biosynthesis."

The project, led by Debra Mohnen of the UGA department of biochemistry and molecular biology and the Complex Carbohydrate Research Center, was just published in the Proceedings of the National Academy of Sciences (PNAS). Other authors of the paper include Jason Sterling, Melani Atmodjo, Sarah Inwood, V.S. Kumar Kolli, Heather Quigley and Michael Hahn.

"Numerous studies show that pectins contribute to the physical and biochemical properties of the plant cell wall," said Mohnen. "We know they are required for normal plant growth and development, but to really understand pectin function, we need to identify and be able to manipulate the biosynthetic enzymes and corresponding genes."

The breakthrough came in the identification, for the first time, of a gene sequence encoding a pectin biosynthetic enzyme, which the team named galacturonosyltransferase-1 or GAUT1. The researchers discovered the gene and the protein it encodes while searching the genetic map database for a common laboratory plant in the mustard family, Arabidopsis thaliana.

The identification of GAUT1 as a galacturonosyltransferase that synthesizes pectin, a family of complex polysaccharides present in the cell walls of all land plants, means that the ability to manipulate pectin synthesis and thereby improve pectin's plant-helping or cancer-fighting properties is now on the horizon.

"We knew that the enzymes we were looking for were membrane-bound, so we took advantage of our understanding of the enzyme's biochemistry to identify the genes," said Mohnen.

 While this first step may well be a crucial one in elucidating pectin biosynthetic genes, Hahn said the research is still at the bottom rung of the ladder, but the team has for the first time genetic tools that should help to identify multiple genes encoding enzymes involved in pectin biosynthesis.

Pectic polymers appear to have multiple roles in growth, development and disease resistance, and the new tools will open new areas of inquiry for researchers.

"We could, for instance, modify a pectic structure to get a specific biological effect," said Mohnen. "The ability to modify pectin synthesis could have huge ramifications."

In an accompanying commentary on the research, to be published later in PNAS, Antony Bacic of the Australian Centre for Plant Functional Genomics at the University of Melbourne said "the description of the biosynthetic processes involved in the synthesis of the non-cellulosic and pectic polysaccharides of the cell wall has, until the 21st century, been slow to unfold."

He points out the large number of pectin uses, from food production to cancer prevention and treatment, and notes that more information on the process of manipulating the quality and quantity of wall polysaccharides is crucial and badly needed.

"The work [described in the UGA paper] represents a significant advance, as it is the first functional identification of an Arabidopsis pectin homogalacturonan galacturonosyltransferase [GAUT1] using biochemical and functional genomic approaches."

While questions remain regarding pectin biosynthesis, "the identification of GAUT1 and the GAUT1-related gene family provides the molecular tools to begin to break through the bottleneck of our understanding of pectin synthesis," Bacic said.