Locating p-coumaric acid in the cell walls of commelinid monocotyledons

Abstract

p-Coumarate is known to be esterified to the cell walls of commelinid monocotyledons. In the Poaceae, it is known that most of the p-coumarate is attached to lignin with a small proportion attached to the noncellulosic polysaccharide glucronoarabinoxylan. Hence, most of the esterified p-coumarate is present in cell walls that are lignified. However, in other families of the commelinid monocotyledons, it is not known which cell wall molecules are esterified by p-coumarate. Two approaches were used aimed at identifying which cell walls contained p-coumarate. Firstly, a monoclonal antibody specific to p-coumarate (INRA-COU1) was used in conjunction with immunofluorescence microscopy to locate p-coumarate in the walls of different organs of 10 species from 8 families of commelinid monocotyledons. Maize (Zea mays) stem was examined at several stages of development. Histochemical colour reagents were also used to determine which cell walls contained lignin. Secondly, microdissection was used to isolate the walls of specific cell types from maize and pineapple (Ananas comosus, Bromeliaceae). These walls were saponified and the hydroxycinnamic acids extracted and analysed by HPLC. INRA-COU1 stongly labeled the epidermal cell walls of the stems of maize and Cyperus papyrus (Cyperaceae) and these walls contained lignin. Sclerenchyma fibre walls from the stems of these species also stained for lignin but were not labeled by INRA-COU1. Both labeling and lignin staining were weaker in the epidermal walls of young maize than of mature maize. Labeling occurred in the epidermal walls of pineapple fruit, but it was difficult to determine whether these walls were lignified. INRA-COU1 also labeled non-lignified parenchyma walls in Cyperus stem and pineapple fruit. In all other samples either very weak or no labeling was observed. Using the second approach, p-coumarate was identified in wall preparations from maize stem from epidermis and one layer of sclerenchyma fibre cells, and in wall preparations from mainly sclerenchyma fibres but not the epidermis. This showed that there were cell walls in maize that contained p-coumarate but were not identified by INRA-COU1. This may be because of the type of lignin present in these walls, which may prevent the antibody binding. p-Coumarate was also identified in a preparation from pineapple fruit containing epidermal cell walls and some subepidermal cell walls.