Abstract
Homogalacturonan (HG) is a multifunctional pectic polysaccharide of the primary cell wall matrix of all land plants. HG is thought to be deposited in cell walls in a highly methyl-esterified form but can be subsequently de-esterified by wall-based pectin methyl esterases (PMEs) that have the capacity to remove methyl ester groups from HG. Plant PMEs typically occur in multigene families/isoforms, but the precise details of the functions of PMEs are far from clear. Most are thought to act in a processive or blockwise fashion resulting in domains of contiguous de-esterified galacturonic acid residues. Such de-esterified blocks of HG can be cross-linked by calcium resulting in gel formation and can contribute to intercellular adhesion. We demonstrate that, in addition to blockwise de-esterification, HG with a non-blockwise distribution of methyl esters is also an abundant feature of HG in primary plant cell walls. A partially methyl-esterified epitope of HG that is generated in greatest abundance by non-blockwise de-esterification is spatially regulated within the cell wall matrix and occurs at points of cell separation at intercellular spaces in parenchymatous tissues of pea and other angiosperms. Analysis of the properties of calcium-mediated gels formed from pectins containing HG domains with differing degrees and patterns of methyl-esterification indicated that HG with a non-blockwise pattern of methyl ester group distribution is likely to contribute distinct mechanical and porosity properties to the cell wall matrix. These findings have important implications for our understanding of both the action of pectin methyl esterases on matrix properties and mechanisms of intercellular adhesion and its loss in plants.
Highlights
Homogalacturonan (HG) is a multifunctional pectic polysaccharide of the primary cell wall matrix of all land plants
The binding profile of LM7 indicates that it is specific for a partially methyl-esterified domain of HG and that its epitope is most readily produced by the non-blockwise deesterification processes such as that produced by fungal PMEs (fPMEs) action and base catalysis
Un-esterified blocks are produced if enough nonblockwise de-esterification occurs and the PAM1 epitope is produced by extensive de-esterification by fPME or base catalysis as indicated by binding to F11 and B15, respectively (Fig. 1b)
Summary
HG, homogalacturonan; IDA, immuno-dot assay; PME, pectin methyl esterase; RG, rhamnogalacturonan; N, newton(s); pPME, plant pectin methyl esterase; fPME, fungal pectin methyl esterase; DE, degree of methyl-esterification; TBS, Tris-buffered saline; BSA, bovine serum albumin; PBS, phosphate-buffered saline; PL, endopectin lyase; PG II, endo-polygalacturonase II; ELISA, enzyme-linked immunosorbent assay; ciELISA, competitive inhibition enzyme-linked immunosorbent assay; PIPES, 1,4-piperazinediethanesulfonic acid; CDTA, cyclohexanediamine N,N,NЈ,NЈ-tetraacetic acid. The results indicate that wall-based pPMEs with a range of action patterns can produce HG with non-blockwise and blockwise distributions of methyl esters at discrete cell wall microdomains, resulting in distinct spatially regulated matrix properties
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