Immunolocalization of homogalacturonans in the apex of the long-day plant Sinapis alba at floral transition.: The pectin content drops dramatically in the first hours of this transition

The pectin content in the meristem of Sinapis alba at flowering transition was assessed in electron microscopy using immunocytochemistry and the monoclonal antibody 2F4 that recognizes a Ca2+-induced supramolecular conformation of homogalacturonans (Liners et al. Plant Physiol 91: 1419-1424, 1989; Liners et al. Plant Physiol 99: 1099-1104, 1992). The meristem's pectins were not recognized at all by the 2F4 monoclonal antibody unless they were enzymatically (PME: pectin methylesterase) or chemically (NaOH) de-esterified, indicating that native pectin must be largely esterified in the apical meristem. A striking but transient decrease of the homopolygalacturonic content of the meristem's cell walls occurred between 20 and 24 h after start of the inductive long day. These changes suggest a role for pectin in floral transition.