Abstract
BackgroundPectins are a group of structurally complex plant cell wall polysaccharides whose biosynthesis and function remain poorly understood. The pectic polysaccharide rhamnogalacturonan-I (RG-I) has two types of arabinogalactan side chains, type-I and type-II arabinogalactans. To date few enzymes involved in the biosynthesis of pectin have been described. Here we report the identification of a highly conserved putative glycosyltransferase encoding gene, Pectic ArabinoGalactan synthesis-Related (PAGR), affecting the biosynthesis of RG-I arabinogalactans and critical for pollen tube growth.ResultsT-DNA insertions in PAGR were identified in Arabidopsis thaliana and were found to segregate at a 1:1 ratio of heterozygotes to wild type. We were unable to isolate homozygous pagr mutants as pagr mutant alleles were not transmitted via pollen. In vitro pollen germination assays revealed reduced rates of pollen tube formation in pollen from pagr heterozygotes. To characterize a loss-of-function phenotype for PAGR, the Nicotiana benthamiana orthologs, NbPAGR-A and B, were transiently silenced using Virus Induced Gene Silencing. NbPAGR-silenced plants exhibited reduced internode and petiole expansion. Cell wall materials from NbPAGR-silenced plants had reduced galactose content compared to the control. Immunological and linkage analyses support that RG-I has reduced type-I arabinogalactan content and reduced branching of the RG-I backbone in NbPAGR-silenced plants. Arabidopsis lines overexpressing PAGR exhibit pleiotropic developmental phenotypes and the loss of apical dominance as well as an increase in RG-I type-II arabinogalactan content.ConclusionsTogether, results support a function for PAGR in the biosynthesis of RG-I arabinogalactans and illustrate the essential roles of these polysaccharides in vegetative and reproductive plant growth.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0780-x) contains supplementary material, which is available to authorized users.
Highlights
Pectins are a group of structurally complex plant cell wall polysaccharides whose biosynthesis and function remain poorly understood
Pectic ArabinoGalactan synthesis-Related (PAGR) is a highly conserved DUF-246 domain containing protein Land plants have an expanded group of genes encoding proteins related to GT family 65, the DUF246 family of GT65-like proteins with 39 members in Arabidopsis [22]
Following sequential extraction of cell wall material from PAGR-overexpressing plants, we found that the CDTA cell wall extract contained an increased arabinose content whereas the subsequent 4 M KOH extract and the residual material showed no differences in monosaccharide composition compared to the wild type (Additional file 8: Figure S8)
Summary
Pectins are a group of structurally complex plant cell wall polysaccharides whose biosynthesis and function remain poorly understood. The pectic polysaccharide rhamnogalacturonan-I (RG-I) has two types of arabinogalactan side chains, type-I and type-II arabinogalactans. Pectins are an important group of structural cell wall polysaccharides in plants and are major constituents of primary cell walls. Pectins are important components of many foods and food products. Pectins comprise several types of acidic polysaccharide domains: homogalacturonan, Stonebloom et al BMC Plant Biology (2016) 16:90 a b c d. RG-II forms dimers through boron di-ester bonds and is a ubiquitous component of plant cell walls, highlighting its importance. As many as 67 different transferase activities are thought to be necessary for the biosynthesis of pectin [1]
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