Abstract
Plant vascular systems are constructed by specific cell wall modifications through which cells are highly specialized to make conduits for water and nutrients. Xylem vessels are formed by thickened cell walls that remain after programmed cell death, and serve as water conduits from the root to the shoot. In contrast, phloem tissues consist of a complex of living cells, including sieve tube elements and their neighboring companion cells, and translocate photosynthetic assimilates from mature leaves to developing young tissues. Intensive studies on the content of vascular flow fluids have unveiled that plant vascular tissues transport various types of gene product, and the transport of some provides the molecular basis for the long-distance communications. Analysis of xylem sap has demonstrated the presence of proteins in the xylem transpiration stream. Recent studies have revealed that CLE and CEP peptides secreted in the roots are transported to above ground via the xylem in response to plant–microbe interaction and soil nitrogen starvation, respectively. Their leucine-rich repeat transmembrane receptors localized in the shoot phloem are required for relaying the signal from the shoot to the root. These findings well-fit to the current scenario of root-to-shoot-to-root feedback signaling, where peptide transport achieves the root-to-shoot signaling, the first half of the signaling process. Meanwhile, it is now well-evidenced that proteins and a range of RNAs are transported via the phloem translocation system, and some of those can exert their physiological functions at their destinations, including roots. Thus, plant vascular systems may serve not only as conduits for the translocation of essential substances but also as long-distance communication pathways that allow plants to adapt to changes in internal and external environments at the whole plant level.
Highlights
Plant vascular tissues are formed through highly specialized cell wall modifications to achieve their roles as conduits of water and nutrients
Direct evidence for the biological relevance of long-distance transport of mRNA is still missing. These findings collectively suggested that the phloem translocation system is highly specialized for systemic signaling where a range of molecules are delivered as signal agents
Analyses of GUS reporter lines for the receptors indicated that HAR1, a CLE-RS receptor, is preferentially expressed in the phloem (Nontachaiyapoom et al, 2007) and xylem intermixed with phloem1 (XIP1)/CEPR1, a CEP1 receptor, is expressed in the phloem (Bryan et al, 2012). These findings suggest that the phloem is the site where the root-derived peptides are converted to another secondary signal, and that such secondary messengers can be transported on the phloem sap flow toward “shoot-to-root.”
Summary
Plant vascular tissues are formed through highly specialized cell wall modifications to achieve their roles as conduits of water and nutrients. Long-distance signaling via vascular tissues of such genetic components, xylem vessels, and phloem sieve tubes are formed running through the entire plant body Such structures are well-designed to play their roles of conduits for water and nutrients. Secreted oligopeptides belonging to the CLE peptide or the CEP family have been shown to be translocated from the roots to the shoots to act as long-distance signaling factors in systemic suppression of nodule formation or in nitrogen starvation response of root systems, respectively (Okamoto et al, 2013; Tabata et al, 2014; Figure 1A). Many receptors have been found from the transcriptomic analyses of phloem tissues (e.g., Deeken et al, 2008) These imply general importance of secreted peptide transport via xylem on plant long-distance signaling. Future challenges are to investigate their roles of remains and to reveal their aspects of how each molecular species is used for systemic signaling
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