Immunocytochemical characterization of early-developing flax fiber cell walls

The deposition and formation of a thick secondary wall is a major event in the differentiation of flax (Linum usitatissimum) fibers. This wall is cell ulose-rich; but it also contains significant amounts of other matrix polyme rs which are noncellulosic such as pectins. We have used immunocytochemical techniques with antibodies specific for various epitopes associated with e ither pectins or arabinogalactan proteins (AGPs) to investigate the distrib ution of these polymers within the walls of differentiating young fibers of 1- and 2-week-old plants. Our results show that different epitopes exhibit distinct distribution patterns within fiber walls. Unesterified pectins re cognized by polygalacturonic acid-rhamnogalacturonan I (PGA/RG-I) antibodie s and rhamnogalacturonan II recognized by anti-RG-II-borate complex antibod ies are localized all over the secondary wall of fibers. PGA/RG-I epitopes, but not RG-II epitopes, are also present in the middle lamellae and cell j unctions. In marked contrast, beta-(1-->4) galactans recognized by the LM5 monoclonal antibody and AGP epitopes recognized by anti-beta-(1-->6) galact an and LM2 antibodies are primarily located in the half of the secondary wa ll nearest the plasma membrane. LM2 epitopes, present in 1-week-old fibers, are undetectable later in development, suggesting a regulation of the expr ession of certain AGP epitopes. In addition, localization of cellulose with the cellobiohydrolase I-gold probe reveals distinct subdomains within the secondary walls of young fibers. These findings indicate that, in addition to cellulose, early-developing flax fibers synthesize and secrete different pectin and AGP molecules.