From the sap's perspective: The nature of vessel surfaces in angiosperm xylem

Abstract

Plant xylem is a unique evolutionary invention: It functions as a negative pressure hydraulic system that can move vast quantities of water and solutes over distances longer than 100 m. No other kind of organism transports liquids under negative pressure, and the most successful attempts to build an artificial negative pressure hydraulic system succeeded only to move a small amount of water over a few centimeters in a single microchannel (Wheeler and Stroock, 2008). That experiment proved that it is possible to move water under negative pressure, but it left the question unanswered how plants can do this so effectively and over such long distances. Somehow plants manage to move large volumes of water in heterogeneous channels that range in diameter from nanometers to a few hundred micrometers, that is, in nanofluidic and microfluidic systems where water moves very close to surfaces of other phases, both solid and gas (Eijkel and van den Berg, 2005;Squires and Quake, 2005). Plants move all this water efficiently along these phase surfaces without constantly creating gas bubbles in the system . What do we actually know about these surfaces in xylem conduits (i.e., vessels and tracheids)? INVITED SPECIAL ARTICLE PREMISE OF THE STUDY:Xylem sap in angiosperms moves under negative pressure in conduits and cell wall pores that are nanometers to micrometers in diameter, so sap is always very close to surfaces. Surfaces matter for water transport because hydrophobic ones favor nucleation of bubbles, and surface chemistry can have strong effects on flow. Vessel walls contain cellulose, hemicellulose, lignin, pectins, proteins, and possibly lipids, but what is the nature of the inner, lumen-facing surface that is in contact with sap?METHODS: Vessel lumen surfaces of five angiosperms from different lineages were examined via transmission electron microscopy and confocal and fluorescence microscopy, using fluorophores and autofluorescence to detect cell wall components. Elemental composition was studied by energy-dispersive X-ray spectroscopy, and treatments with phospholipase C (PLC) were used to test for phospholipids.KEY RESULTS: Vessel surfaces consisted mainly of lignin, with strong cellulose signals confined to pit membranes. Proteins were found mainly in inter-vessel pits and pectins only on outer rims of pit membranes and in vessel-parenchyma pits. Continuous layers of lipids were detected on most vessel surfaces and on most pit membranes and were shown by PLC treatment to consist at least partly of phospholipids. CONCLUSIONS:Vessel surfaces appear to be wettable because lignin is not strongly hydrophobic and a coating with amphiphilic lipids would render any surface hydrophilic. New questions arise about these lipids and their possible origins from living xylem cells, especially about their effects on surface tension, surface bubble nucleation, and pit membrane function.

High porosity with tiny pore constrictions and unbending pathways characterize the 3D structure of intervessel pit membranes in angiosperm xylem

“…Despite the striking agreement between the shrinkage model and gold perfusion results, some caution is required for the interpretation of our pore constriction size dimensions. Observation of intervessel pit membranes with confocal laser scanning microscopy, which requires no dehydration or any chemical treatment, showed pillow‐shaped structures with a much thicker appearance than TEM images of freshly embedded material (Schenk et al, ). Therefore, it is possible that sample dehydration by alcohol during TEM preparation may cause some artificial shrinkage.…”

“…According to the Young Laplace equation, the pressure difference forcing a bubble through a 20 nm pore, assuming a contact angle of zero (Caupin, Cole, Balibar, & Treiner, ; Meyra, Kuz, & Zarragoicoechea, ), and a pore shape correction factor of 0.5 (Schenk et al, ), would be 7.2 MPa in pure water. Because surface‐active substances, such as phospholipids, are known to occur in xylem sap and to be associated with pits (Jansen et al, ; Schenk et al, ; Schenk et al, ), the surface tension inside pores is likely to be much reduced. If the surface tension of an air‐water meniscus is reduced to 24 mJ m −2 , which is a typical equilibrium surface tension for phospholipid monolayers (Lee, Kim, & Needham, ), a meniscus could pass through a 20 nm pore with a shape correction factor of 0.5 under a pressure difference of 2.4 MPa.…”

“…Sample preparation for TEM was performed as described above, but without applying OsO 4 treatment. Since no OsO 4 was used as post‐fixative, pit membranes were highly transparent (Jansen et al, ; Schenk et al, ; Schenk, Espino, Rich‐Cavazos, & Jansen, ), and individual gold particles of all sizes could easily be observed as circular, electron dense structures. OsO 4 treatment, however, results in binding of Os to unsaturated fatty acid chains of lipids (Riemersma, ), which results in dark, electron dense particles associated with pit membranes.…”

Wood: Biology of a living tissue

“…This layer therefore plays an important role in any exchange of water, ions, and other molecules between the vessel and vessel-associated cells. By focusing on the surface chemistry of angiosperm vessels, Schenk et al (2018) propose a third possible function for these vessel-associated cells: the production of amphiphilic lipids that coat many vessel surfaces, especially the pits connecting vessels. These surface-active lipids may play a role in preventing xylem embolisms below a critical pressure threshold by coating hydrophobic surfaces and gas bubbles in xylem sap (Schenk et al, 2017).…”

“…One should not underestimate the psychology of images, especially when teaching about plants-every botany instructor has encountered students who think that plants are barely alive. Images of dye tracers in wood (e.g., Jacobsen et al, 2018) and confocal microscopy of living wood (e.g., Schenk et al, 2018) provide very different images of wood as a living tissue. The contributions in this special section provide new mental images by telling different stories of living wood.…”