title: The gelatinous extracellular matrix facilitates transport studies in kelp: visualization of pressure-induced flow reversal across sieve plates.
abstract: OBJECTIVE
  In vascular plants, important questions regarding phloem function remain unanswered due to problems with invasive experimental procedures in this highly sensitive tissue. Certain brown algae (kelps; Laminariales) also possess sieve tubes for photoassimilate transport, but these are embedded in large volumes of a gelatinous extracellular matrix which isolates them from neighbouring cells. Therefore, we hypothesized that kelp sieve tubes might tolerate invasive experimentation better than their analogues in higher plants, and sought to establish Nereocystis luetkeana as an experimental system.
  
  
  METHODS
  The predominant localization of cellulose and the gelatinous extracellular matrix in N. luetkeana was verified using specific fluorescent markers and confocal laser scanning microscopy. Sieve tubes in intact specimens were loaded with fluorescent dyes, either passively (carboxyfluorescein diacetate; CFDA) or by microinjection (rhodamine B), and the movement of the dyes was monitored by fluorescence microscopy.
  
  
  RESULTS
  Application of CFDA demonstrated source to sink bulk flow in N. luetkeana sieve tubes, and revealed the complexity of sieve tube structure, with branches, junctions and lateral connections. Microinjection into sieve elements proved comparatively easy. Pulsed rhodamine B injection enabled the determination of flow velocity in individual sieve elements, and the direct visualization of pressure-induced reversals of flow direction across sieve plates.
  
  
  CONCLUSIONS
  The reversal of flow direction across sieve plates by pressurizing the downstream sieve element conclusively demonstrates that a critical requirement of the Münch theory is satisfied in kelp; no such evidence exists for tracheophytes. Because of the high tolerance of its sieve elements to experimental manipulation, N. luetkeana is a promising alternative to vascular plants for studying the fluid mechanics of sieve tube networks.
authors.affiliation: School of Biological Sciences, Washington State University, PO Box 644236, Pullman, WA 99164, USA.
authors.forename: Jan
authors.initials: J
authors.lastname: Knoblauch
authors.affiliation: School of Biological Sciences, Washington State University, PO Box 644236, Pullman, WA 99164, USA.
authors.forename: Winfried S
authors.initials: WS
authors.lastname: Peters
authors.affiliation: School of Biological Sciences, Washington State University, PO Box 644236, Pullman, WA 99164, USA knoblauch@wsu.edu.
authors.forename: Michael
authors.initials: M
authors.lastname: Knoblauch
mesh_terms: D001692:Biological Transport; D005109:Extracellular Matrix / Q000378:metabolism*; D044423:Kelp / Q000378:metabolism*; D052585:Phloem / Q000378:metabolism*; D011312:Pressure; D012212:Rheology
keywords: Alginate; Laminariales; Münch flow; Nereocystis luetkeana; Phaeophyceae; brown algae; long-distance transport; microinjection; phloem; photoassimilate transport; sieve plate; sieve tube