Intersection of transfer cells with phloem biology—broad evolutionary trends, function, and induction
- Department of Biological Sciences, School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW, Australia
Transfer cells (TCs) are ubiquitous throughout the plant kingdom. Their unique ingrowth wall labyrinths, supporting a plasma membrane enriched in transporter proteins, provides these cells with an enhanced membrane transport capacity for resources. In certain plant species, TCs have been shown to function to facilitate phloem loading and/or unloading at cellular sites of intense resource exchange between symplasmic/apoplasmic compartments. Within the phloem, the key cellular locations of TCs are leaf minor veins of collection phloem and stem nodes of transport phloem. In these locations, companion and phloem parenchyma cells trans-differentiate to a TC morphology consistent with facilitating loading and re-distribution of resources, respectively. At a species level, occurrence of TCs is significantly higher in transport than in collection phloem. TCs are absent from release phloem, but occur within post-sieve element unloading pathways and particularly at interfaces between generations of developing Angiosperm seeds. Experimental accessibility of seed TCs has provided opportunities to investigate their inductive signaling, regulation of ingrowth wall formation and membrane transport function. This review uses this information base to explore current knowledge of phloem transport function and inductive signaling for phloem-associated TCs. The functional role of collection phloem and seed TCs is supported by definitive evidence, but no such information is available for stem node TCs that present an almost intractable experimental challenge. There is an emerging understanding of inductive signals and signaling pathways responsible for initiating trans-differentiation to a TC morphology in developing seeds. However, scant information is available to comment on a potential role for inductive signals (auxin, ethylene and reactive oxygen species) that induce seed TCs, in regulating induction of phloem-associated TCs. Biotic phloem invaders have been used as a model to speculate on involvement of these signals.
Keywords: transfer cell, ingrowth wall architecture, phloem transport, inductive signals
Citation: Andriunas FA, Zhang H-M, Xia X, Patrick JW, and Offler CE (2013) Intersection of transfer cells with phloem biology—broad evolutionary trends, function, and induction. Front. Plant Sci. 4:221. doi: 10.3389/fpls.2013.00221
Received: 25 March 2013; Accepted: 07 June 2013;
Published online: 01 July 2013.
Edited by:Aart V. Bel, Justus-Liebig-University Giessen, Germany
Reviewed by:Ykä Helariutta, University of Helsinki, Finland
Thomas L. Slewinski, Cornell Univeristy, USA
Copyright © 2013 Andriunas, Zhang, Xia, Patrick, and Offler. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.
*Correspondence: Christina E. Offler, Department of Biological Sciences, School of Environmental and Life Sciences, The University of Newcastle, University Drive, Callaghan, NSW 2308, Australia e-mail: email@example.com