Differing structures of galactoglucomannan in eudicots and non-eudicot angiosperms

Area of Science:

• Plant Biology
• Plant Cell Wall Biochemistry
• Evolutionary Botany

Background:

• Cell wall mannan hemicelluloses exhibit structural diversity across plant evolution.
• Beta-galactoglucomannan (β-GGM), a novel structure with specific β-galactose branches, was recently identified in eudicots.
• The evolutionary origin and acquisition of β-GGM remain unclear.

Purpose of the Study:

• To investigate the evolutionary history of β-GGM and its associated enzymes.
• To determine when and how plants acquired β-GGM.
• To identify candidate mannan β-galactosyltransferases (MBGTs) in non-eudicot species.

Main Methods:

• Comparative analysis of mannan structures in plant groups sister to eudicots.
• Bioinformatic search for orthologues of key mannan-modifying enzymes (e.g., AtMBGT1) in non-eudicot angiosperms.
• In vivo functional characterization of candidate enzymes using complementation assays in Arabidopsis and reverse genetics in rice.

Main Results:

• Glucomannan structures in sister groups to eudicots were distinct from β-GGM.
• Orthologues of AtMBGT1 from rice (OsGT47A-VII) and Amborella (AtrGT47A-VII) lacked direct MBGT activity.
• The rice orthologue (OsGT47A-VII) exhibited MUR3-like xyloglucan galactosyltransferase activity and was crucial for root growth in rice.
• Gene duplication and diversification within the GT47A-VII family in eudicots likely facilitated the acquisition of mannan β-galactosyltransferase activity.

Conclusions:

• Beta-galactoglucomannan (β-GGM) is likely a eudicot-specific cell wall polysaccharide.
• The evolution of the GT47A-VII gene family played a role in the acquisition of β-GGM.
• Understanding β-GGM evolution provides insights into plant cell wall diversification and adaptation.