The Protective Layer as an Extension of the Apoplast

Abstract

The protective layer between the cell wall and plasmalemma of xylem parenchyma cells has variously been suggested to be involved in protection of the protoplast from attack by autolytic enzymes from neighbouring, dying cells, tylose formation, deep supercooling of xylem, and strengthening of the pit. None of these ideas has universal application to all species in which parenchyma cells possess a protective layer. It is proposed instead, that the protective layer is primarily laid down in order to preserve apoplastic continuity around the protoplast of a lignified cell, bringing the entire plasmalemma surface, and not just that part of it in contact with the porous pit membrane, into contact with the apoplast. If this is so, then other functions may be coincidental, or have arisen secondarily.

Immunogold localization of pectins and glycoproteins in tissues of peach with reference to deep supercooling

“…The authors have suggested that the JIM14 epitope may be a signaling molecule that is small enough to easily traverse the cell wall. The amorphous layer, which in our study labeled heavily with JIM14, has been reported to play a role in host defense against vascular pathogens via tylose formation (Mueller and Beckman 1984), solute exchange (Sauter 1972;Sauter et al 1973;van Bel and van der Schoot 1988), freezing response (Wisniewski et al 1993), and as a vehicle to provide apoplastic continuity around the protoplasts of lignified cells (Barnett et al 1993).…”

The Role of Xylem Parenchyma in the Storage and Utilization of Nonstructural Carbohydrates

“…In addition, a specialized cell wall layer is deposited underneath the pit membrane, lining the entire conduit-parenchyma interface between the plasmalemma and the lignifi ed wall. This so-called amorphous or protective layer may enlarge the actual area available for the exchange of substances (Barnett et al 1993 ); however, other functions such as providing a buffer against xylem pressure oscillations were also proposed (Van Bel and Van der Schoot 1988 ). Both the pit membrane and the amorphous layer are rich in pectins (Wisniewski and Davis 1995 ;Plavcová and Hacke 2011 ).…”

Submicroscopical Features of Leaves of Xyris Species

“…In X. hymenachne (Fig. 4), the walls of the stomata guard cells were covered internally by a special layer, called protective layer (O'Brien, 1970) or secondary cellulosic layer (Czaninskiy, 1973), which was reported for the parenchymatous cells of the secondary xylem by Barnett et al (1993), Chaffe (1974), Foster (1967 and Wisniewski & Davis (1989). Such a protective layer formed electron dense ramified clusters (Fig.…”

“…4, arrow) which were permeated with pectic compounds. As the protective layer is supposed to increase the water/nutrients movement via apoplast (Barnett et al, 1993) in Xyris hymenachne it may facilitate the hydroactive function of the stomatal cells.…”