Body of Abstract: Analysis of pectin mutant phenotypes provides information on gene function in the plant. Interpreting phenotypes of pectin-related mutants, however, is challenging due to lack of validated models depicting the number and type of pectic polymers in the wall and how they are connected to each other and to other wall polymers. Over 100 years of research [1] has demonstrated that all pectic glycans are built on a homogalacturonan (HG) or rhamnogalacturonan backbone. HG can be synthesized as a homopolymer in vitro although the amount of HG that exists as a separate homopolymer in the wall is unknown. The substituted HGs rhamnogalacturonan II (RG-II), xylogalacturonan and apiogalacturonan exist within an HG chain, but whether they exist in walls as isolated glycans or covalently linked pectic heteroglycans, pectic proteoglycans, or both, is unclear. In some cell types rhamnogalacturonan I (RG-I) exists covalently linked to arabinogalactan protein (AGP) [2], but whether all RG-I is present as pectic AGPs, or rather as HG-RG-I-HG heteroglycans or an RG-I-HG-RG-II-HG linked network is unknown. This uncertainty challenges the depiction of pectin structure in plant cell wall models and interpretations of pectin function. Based on pectin structure data from wild type and mutant/transgenic plants [1,3,4] and informed by analyses of pectin biosynthetic enzymes [5-6], two hypothetic testable models for pectin structure in the plant cell wall will be presented: the “Cross-linked Pectic Glycoconjugate Model” and the “Pectic Heteroglycan and Glycoconjugate Model”.
1. Mohnen, D. et al. 2023. In The Plant Cell Wall – a Historical Perspective, Ed. Anja Geitmann, CRC Press/Taylor & Francis Group (in press).
2. Tan et al., 2023. Carbohydr. Polymers 301:120340