Homogalacturonans and Hemicelluloses in the External Glands of Utricularia dichotoma Traps

The <i>Utricularia</i> (bladderworts) species are carnivorous plants that prey mainly on invertebrates using traps (bladders) of leaf origin. On the outer surfaces of the trap, there are dome-shaped glands (capitate trichomes). Each such trichome consists of a basal cell, a pedestal cell, and a terminal cell. During the maturation of these external glands, there are changes in the cell wall of the terminal cell of the gland (deposited layers of secondary wall material). Thus, due to changes in the cell wall, these glands are excellent models for studying the specialization of cell walls. The main aim of this study was to check whether different cell wall layers in terminal gland cells have a different composition in the case of homogalacturonans (low-methylesterified HGs, fully de-esterified HGs, and galactan) and hemicelluloses (galactoxyloglucan, xyloglucan, and xylan). The antibodies were used against cell wall components (anti-pectins JIM5, JIM7, LM19, CCRC-M38, and LM5 and anti-hemicelluloses LM25, LM15, CCRC-M1, and CCRC-M138). The localization of the examined compounds was determined using immunohistochemistry techniques, Carbotrace 680, and Calcofluor White. Our study showed the presence of various components in the cell walls of external gland cells: methylesterified and demethylesterified homogalacturonans, galactan, xylan, galactoxyloglucan, and xyloglucan. In the terminal cell, the primary cell wall contains different pectins in contrast to the secondary wall material, which is rich in cellulose and hemicelluloses. We also found that the basal cell differs from the other gland cells by the presence of galactan in the cell wall, which resembles the epidermal cells and parenchyma of traps. A particularly noteworthy part of the cell wall functions as a Casparian strip in the pedestal cell. Here, we found no labeling with Carbotrace 680, possibly due to cell wall modification or cell wall chemical composition variation. We have shown that the apoplastic space formed by the cell walls of the terminal cell is mainly composed of cellulose and hemicelluloses (galactoxyloglucan and xyloglucan). This composition of the cell walls allows the easy uptake of components from the external environment. Our research supports the external glands' function as hydropotens.