Localization of cell wall polysaccharides in nonarticulated laticifers of Asclepias speciosa Torr.

Asclepias speciosa Torr. has latex-containing cells known as nonarticulated laticifers. In stem sections of this species, we have analyzed the cell wa lls of nonarticulated laticifers and surrounding cells with various stains, lectins, and monoclonal antibodies. These analyses revealed that laticifer walls are rich in (1 -->4) beta -D-glucans and pectin polymers. Immunoloca lization of pectic epitopes with the antihomogalacturonan antibodies JIM5 a nd JIM7 produced distinct labeling patterns. JIM7 labeled all cells includi ng laticifers, while JIM5 only labeled mature epidermal cells and xylem ele ments. Two antibodies, LM5 and LM6, which recognize rhamnogalacturonan I ep itopes distinctly labeled laticifer walls. LM6, which binds to a (1 -->5) a lpha -arabinan epitope, labeled laticifer walls more intensely than walls o f other cells. LM5, which recognizes a (1 -->4) beta -galactan epitope, did not label laticifer segments at the shoot apex but labeled more mature por tions of laticifers. Also the LM5 antibody did not label cells at the shoot apical meristem, but as cells grew and matured the LM5 epitope was express ed in all cells. LM2, a monoclonal antibody that binds to beta -glucuronic acid residues in arabinogalactan proteins, did not label laticifers but spe cifically labeled sieve tubes. Sieve tubes were also specifically labeled b y Ricinus communis agglutinin, a lectin that binds to terminal beta -D-gala ctosyl residues. Taken together, the analyses conducted showed that laticif er walls have distinctive cytochemical properties and that these properties change along the length of laticifers. In addition, this study revealed di fferences in the expression of pectin and arabinogalactan protein epitopes during shoot development or among different cell types.