Mung Bean Hypocotyl Homogalacturonan: Localization, Organization and Origin

Abstract

Specific antibodies were used to localize both pectic structures and pectinmethylesterases (PME) along the mung bean hypocotyl. Calcium ions were also detected and estimated in both young, plastic and mature, stiffened cell walls. Highly methylesterified pectins were present in all cell walls but decreased from the hypocotyl hook downwards. Expanded cell walls were characterized by a high content of calcium ions and acidic pectins, although the latter's cross-reactivity to JIM 5 antibodies was partly lost. Co-localization of acidic homogalacturonan and calcium ions suggests the presence of egg-box structures that might participate in the cell wall stiffening process which develops along the hypocotyl. Acidic polymers could originate from the activity of the pectinmethylesterases present in precise wall areas but direct export of acidic polygalacturonan through Golgi vesicles was also observed.