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. 2018 Aug 20;9:1202. doi: 10.3389/fpls.2018.01202

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Copyright © 2018 Rui, Chen, Kandemir, Yi, Wang, Puri and Anderson.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Homogalacturonan (HG) is synthesized in the Golgi, and is de-methyl-esterified and degraded in the apoplast. In the Golgi, galacturonosyltransferases (GAUTs) transfer galacturonic acid (GaiA) residues onto existing a-1,4-linked GalA chains. Pectin methyltransferases (PMTs) add methyl groups onto GalA residues. Although it is currently unknown whether PMTs function after GAUTs or PMTs and GAUTs act as a protein complex, the first scenario is shown in the figure. Highly methyl-esterified HG is then exocytosed to the apoplast, where it is de-methyl-esterified by pectin methyl-esterases (PMEs). De-methyl-esterified HG can be crosslinked by Ca²⁺, or subject to degradation by polygalacturonases (PGs) and pectate lyases (PLs).