Stability of the Borate-ester Cross-link in Rhamnogalacturonan II at Low pH and Calcium Activity in muro and in vivo
The discovery by Matoh and colleagues (Matoh et al. 1993, Kobayashi et al. 1996) that most of the boron bound to higher plant cell walls is present as a borate-ester that covalently cross links two chains of the pectic polysaccharide rhamnogalacturonan II (dRG-II-B), became central for our present understanding of the physiological functions of this microelement. Many, if not all, of the known functions of apoplastic boron may be related to the covalent cross-linking of pectin by RG-II-dimers. The stability of this cross-link may also provide an explanation of some of the physiologically essential effects of apoplastic calcium ions including the well-known antagonism of extracellular calcium and protons. Isolated dRG-II-B is stable at pH > 3 and its formation in vitro is accelerated by polyvalent cations (O’Neill et al. 1996, Ishii et al. 1999). In muro the stability of borate cross link is believed to be increased by the presence of firmly complexed calcium ions (Matoh et al. 1996, Matoh and Kobayashi 1998, Kobayashi et al. 1999).
KeywordsBoric Acid Chenopodium Album Pectic Polysaccharide Pectin Content Borate Ester
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