Researchers obtain a better understanding of the most structurally complex polysaccharide found in nature
The Science: Rhamnogalacturonan II (RG-II) is a component in the cell walls of most plant species. It is the most structurally complex polysaccharide yet to be identified in nature. RG-II is crucial for keeping plant cell walls intact. It forms dimers in the cell wall, which are cross-linked by boron. It is the major reason why boron is required for plant growth, and often is added to fields as a soil amendment. Changes to RG-II can hinder plant growth or even be lethal, making it difficult for researchers to study. As a result, there is limited knowledge of how RG-II is made. To better understand RG-II, researchers studied a specific gene that is crucial for plant survival. They found that when plants have a mutated type of this gene, RG-II doesn’t function properly because it does not have an acid that the researchers believe is made by this gene.
The Impact: Despite being a crucial part of plant cell walls, RG-II has been poorly understood. The findings of this study answer some questions about how RG-II functions, providing researchers with a better understanding of this complex polysaccharide. In addition, the method the researchers developed to study these interactions could help find more proteins involved in making RG-II. This will be useful for efforts to engineer plants to have desirable traits for bioenergy purposes.
Summary: In order to understand RG-II, researchers needed to identify the glycosyltransferases (GT) involved in its formation. Traditional methods to study RG-IIs GTs, a type of enzyme, have not been very successful. Changes to RG-II can be lethal in plants, so it is difficult for researchers to be able to study them.
Researchers developed a method for modifying plants so that they don’t have functional GTs. Using CRISPR/Cas-9 gene editing, they generated knockout mutants in plant callus tissue. This allowed the successful production of functionally null material, without requiring the plant to produce viable seed. They combined this with a candidate gene approach to study the RCKT1, a predicted GT of previously unknown function. They showed that in these callus mutant cell walls, RG-II does not function normally because it lacks 3-deoxy-D-manno-octulosonic acid (Kdo). The researchers show that RCKT1 adds this monosaccharide sugar to a specific position on RG-II, meaning it is necessary for the normal function of RG-II. This is, to their knowledge, the first Kdo GT identified in higher organisms – previous examples had only been found in bacteria.
Publication: Zhang, Y., Sharma, D., Liang, Y., et al. Putative rhamnogalacturonan-II glycosyltransferase identified through callus gene editing which bypasses embryo lethality. Plant Physiology (2024). [DOI: 10.1093/plphys/kiae259]
Written by Emily Nelson