An immunohistochemical approach to cell wall polysaccharide specialization in maritime pine (Pinus pinaster) needles

Pine needles are specialized leaves bearing distinctive anatomical features whose function is to minimize water loss. At the molecular level, needle specialization is expected to lead to heterogeneity in cell wall (CW) composition. By immunohistochemical analysis of CW epitopes in maritime pine (Pinus pinaster) needles, we described the variability of CW composition in all tissues, discussing its possible relationship with the cell function. Cells specialized in water transport, tracheids, transfusion tracheids, and endodermis, together with epi/hypodermal and stomatal cells, had similar CW composition, where epitopes for pectins (homogalacturonan and rhamnogalacturonan-I), xyloglucan, and heteroxylans were abundant. Heteroxylans differ between the dermis, enriched in arabinoxylan, and endodermis, enriched in glucuronoxylan. Xylem CWs were very rich in (arabino)xylans and homogalacturonan and distinctively lacked immunolabelling for rhamnogalacturonan-I. CWs from phloem, transfusion phloem, mesophyll, and resin ducts were enriched in homogalacturonan, rhamnogalacturonan-I, and xyloglucan, showing a low content for heteroxylan epitopes. Arabinogalactan proteins were detected in mesophyll and in cells specialized in solute transport such as phloem, duct sheath, and guard cells. These results show that tissues with similar functions have similar polysaccharide composition, shedding light on the role of CW components through different tissues.