| Abstract Detail
Wood: Biology of a Living Tissue Morris, Hugh [1], Schiele, Sandra [2], Klepsch, Matthias [2], Schenk, H. Jochen [3], Jansen, Steven [2]. The structure and multiple functions of vessel-associated cells in xylem. Vessel-associated cells (VAC) occur in the xylem of woody and herbaceous angiosperms and are in direct contact with a vessel wall via half-bordered pits. VAC can be ray or axial parenchyma cells and characterised by a high amount of mitochondria and ribosomes, a well-developed endoplasmic reticulum, few plastids, and various small vacuoles. These organelles indicate a high metabolic activity. Angiosperm vessels show considerable variation in the amount of VAC, although there is a continuum between vessel contact cells and distant cells. Interestingly, we find a positive correlation between vessel diameter and axial parenchyma fraction, which is also reflected in the amount of paratracheal axial parenchyma. This relationship is supported at the interspecific level and within growth rings of ring-porous species.
VACs show multiple functions, including: (1) storage of water, while starch is typically absent, with possibly functional consequences for hydraulic capacitance, embolism avoidance and/or refilling, and positive root/stem pressure, (2) xylem-phloem interactions via symplastic connectivity between parenchyma cells across ray initials in the vascular cambium, and (3) defence and resilience to frost, wound response, and pathogens. Several of these functions are likely affected by the unique, “protective” or “amorphous” layer that is most pronounced between the plasmalemma of the VAC and the parenchyma-vessel pit membrane. Unlike vessel-vessel pit membranes, the pit membrane and amorphous layer of VAC show pectic polysaccharides, which play a major role in the production of gels and tyloses in response to aging processes (heartwood formation), wounds, and infections. Additional features of VAC include a highly efficient osmoregulation to obtain hydraulic equilibrium with xylem sap in adjacent vessels, and super-cooling to avoid frost damage.
A novel, functional role of VAC is the secretion of surface active molecules that can lower surface tension (i.e., surfactants) from vesicles that cross the VAC plasmalemma and secrete their contents via the amorphous layer and pit membrane into the xylem sap. The release of these insoluble lipid based surfactants explains not only their origin in xylem sap, but also the occurrence of an electron dense “black cap” on the outermost layer of the parenchyma-vessel pit membrane, micelles in pores of intervessel pit membranes, and a deposition on inner vessel walls. Xylem sap surfactants are suggested to play a crucial role for water transport under tension by coating hydrophobic surfaces of vessel walls and nanobubbles, which enables them to remain below the critical size at which bubbles would expand to create embolism.
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1 - Ulm, Institute of Systematic Botany and Ecology, Albert-Einstein-Allee 11, Ulm, 89081, Germany
2 - Ulm University, Institute of Systematic Botany and Ecology, Albert-Einstein-Allee 11, Ulm, 89081, Germany
3 - California State University Fullerton, Department Of Biological Science, PO Box 6850, Fullerton, CA, 92834-6850, USA
Keywords:
xylem
axial parenchyma
radial parenchyma
wood anatomy
surfactants
amorphous layer
pit membrane
transmission electron microscopy
water relations.
Presentation Type: Symposium Presentation
Session: SY09, Wood: Biology of a Living Tissue
Location: Oglethrope Auditorium/Savannah International Trade and Convention Center
Date: Wednesday, August 3rd, 2016
Time: 10:45 AM
Number: SY09007
Abstract ID:265
Candidate for Awards:None |
